Choked inlet manifold on ML270

[dieselman]

Wondering if the extra volume of oxygen through the EGR has had an effect on mixture. We always talk about air/fuel mixture but I wonder if it should be called oxygen/fuel mixture. With the quite high volume of oxygen now being introduced through the EGR at idle and low revs, perhaps things have leaned off substantially.

I suspect so as the MAF will be seeing less air pass to will reduce fuelling accordingly.
This is why I didn't do mine this way.

 

[oigle]

Think that is the go so will look at restricting the amount of air entering through the EGR - bit of a suck and see I guess.
If it doesn't work, I'll have a go at your way, dieselman. Wish you had mentioned it earlier.

 

[Alex Crow]

no such thing as a lean mixture in this situation. as dieselman has pointed out many times, diesels are not throttled, and so output is controlled by fuelling alone. whereas a petrol car is kept close to stoichiometric in all conditions, a diesel is not.

consider when the egr is opened at idle, the air mass measured is halved, but the fuelling remains the same (to maintain that idle speed) - the engine's ecu is not really interested in the actual value at idle for fuelling calculations at all.

the air mass sensor is nowhere near as vital to the diesels management as it is on a petrol engine. this goes part way to explaining why the petrol engined MBs have different air mass sensors, and why they cost so much more. on a petrol engine the intake is throttled, severely so at idle. this means idle air mass is extremely small compared to air mass at full throttle. so the petrol engine air mass meter has to work over a much wider range of values, and it has to work to a great deal of accuracy to enable accurate fuelling close to stoichiometric in all transient situations.

on the diesel, the air mass values only really become vital at or near full throttle. it is also obviously useful for detecting problems with egr valves and blocked cats etc. we do find MB cdi engines that have poor pick up and performance when air mass sensors have failed, but this is more to do with MB's chosen path re fault condition running. note that on earlier 606 turbo diesels the maf can be way out of spec and no-one ever notices! also note that many cdi engines in vans do not bother to use a maf sensor at all, just relying on the manifold pressure sensor.

moving on, the function of egr as described by MB is complex, but it is claimed to assist combustion by the introduction of whatever products in the exhaust gas are beneficial. this improved combustion is really how emissions are cleaned up, the egr does not recycle a large percentage of the exhaust gas - perhaps a third at idle? perhaps less. bear in mind that the volume of gas exiting an engine will always be higher than the volume entering, and the ratio will increase as output increases.

so perhaps there is a power loss under zero boost conditions due to not having the positive effects of egr on combustion, but i honestly doubt that. checking fault codes and live data would be interesting.

 

[oigle]

no such thing as a lean mixture in this situation. as dieselman has pointed out many times, diesels are not throttled, and so output is controlled by fuelling alone. whereas a petrol car is kept close to stoichiometric in all conditions, a diesel is not.

consider when the egr is opened at idle, the air mass measured is halved, but the fuelling remains the same (to maintain that idle speed) - the engine's ecu is not really interested in the actual value at idle for fuelling calculations at all.

the air mass sensor is nowhere near as vital to the diesels management as it is on a petrol engine. this goes part way to explaining why the petrol engined MBs have different air mass sensors, and why they cost so much more. on a petrol engine the intake is throttled, severely so at idle. this means idle air mass is extremely small compared to air mass at full throttle. so the petrol engine air mass meter has to work over a much wider range of values, and it has to work to a great deal of accuracy to enable accurate fuelling close to stoichiometric in all transient situations.

on the diesel, the air mass values only really become vital at or near full throttle. it is also obviously useful for detecting problems with egr valves and blocked cats etc. we do find MB cdi engines that have poor pick up and performance when air mass sensors have failed, but this is more to do with MB's chosen path re fault condition running. note that on earlier 606 turbo diesels the maf can be way out of spec and no-one ever notices! also note that many cdi engines in vans do not bother to use a maf sensor at all, just relying on the manifold pressure sensor.

moving on, the function of egr as described by MB is complex, but it is claimed to assist combustion by the introduction of whatever products in the exhaust gas are beneficial. this improved combustion is really how emissions are cleaned up, the egr does not recycle a large percentage of the exhaust gas - perhaps a third at idle? perhaps less. bear in mind that the volume of gas exiting an engine will always be higher than the volume entering, and the ratio will increase as output increases.

so perhaps there is a power loss under zero boost conditions due to not having the positive effects of egr on combustion, but i honestly doubt that. checking fault codes and live data would be interesting.

I hear you Alex but there are so many conflicting issues here. If air volume is not to be considered, then why does blocking off the EGR result in excessively high mixtures - black smoke etc - and a resultant limphome mode as described by others? I understand the idle remaining constant as the fuel input is set. However, at higher engine speeds, the fuel should be regulated by engine revs, throttle position and maf sensing. There are of course other minor sensors providing input. Given the engine revs cannot be altered, throttle position and maf remain the only variables. Of these, the maf is greatly affected by egr openings at lower throttle settings/revs. Perhaps, the "open" EGR may be allowing a higher than normal ingress of air which in turn reduces the air through the maf and thus the fuelling reduces to suit. I have not looked inside the exhaust gas cooler which precedes the EGR normally, but it could have quite a high restriction of gas flow in it.
Guess I can "prove" the issue quite easily by playing around with various restrictors in the EGR air entry to see if it makes any difference.
Would appreciate your thoughts on that.

 

[oigle]

Followup to above

Well, I have put my thoughts into action and played with the volume of air being allowed to enter through the EGR.
I initially put a plug with a 12mm hole in it inside the entry to the EGR and road tested it. Results - better than before but not as good as it used to be.
Removed plug and inserted one with a 6mm hole - quite small. Results were immediate. Back to its old self and no apparent excessive smoke from the exhaust - haven't followed the vehicle but none obvious from drivers seat. Will do a "follow" when I get a chance.
Interestingly, using a small air filter on the EGR, the operation of the EGR can be heard. Not too loud but surprisingly obvious. Bit like a turbo blowoff valve. Seems it sucks quite hard at times. With the smaller restrictor, the noise diminished noticeably.
It would appear from the above that the normal operation of the EGR does not permit much volume of exhaust gas to recirculate. Maybe the cooler has a restrictive matrix in it? Don't know, but it seems obvious that it doesn't flow a lot of air if a 6mm hole allows a suitable amount into the manifold. Didn't try any other sizes of hole as I was limited in the plugs at my disposal.
Have ceased experimenting with it now and will drive for a longer term evaluation - towing etc. Hopefully I won't be plagued with ecu lights or limp home modes.
Will advise further in a month or so after full evaluation.
Thanks for all your help. Hope this info helps others with a similar issue.

As an aside, I wonder if one were to place a suitably sized restrictor in the EGR entry on a normally setup EGR to further reduce the volume, if it would maybe help to richen up the off idle mixture and help with initial throttle response and reduce lag. Just conjecture.

Ian.

 

[dieselman]

As an aside, I wonder if one were to place a suitably sized restrictor in the EGR entry on a normally setup EGR to further reduce the volume, if it would maybe help to richen up the off idle mixture and help with initial throttle response and reduce lag. Just conjecture.

Ian.

Yes it would. That's the point where EGR is most felt and leads to tardy torque build up and poor economy.
The problem with restricting the EGR in this fashion is that the MAF will detect it if the restriction is too small. The EGR valve is modulated to create a restriction anyway.

 

[dieselman]

consider when the egr is opened at idle, the air mass measured is halved, but the fuelling remains the same (to maintain that idle speed) - the engine's ecu is not really interested in the actual value at idle for fuelling calculations at all.

the air mass sensor is nowhere near as vital to the diesels management as it is on a petrol engine. this goes part way to explaining why the petrol engined MBs have different air mass sensors, and why they cost so much more. on a petrol engine the intake is throttled, severely so at idle. this means idle air mass is extremely small compared to air mass at full throttle. so the petrol engine air mass meter has to work over a much wider range of values, and it has to work to a great deal of accuracy to enable accurate fuelling close to stoichiometric in all transient situations.

on the diesel, the air mass values only really become vital at or near full throttle. it is also obviously useful for detecting problems with egr valves and blocked cats etc. we do find MB cdi engines that have poor pick up and performance when air mass sensors have failed, but this is more to do with MB's chosen path re fault condition running. note that on earlier 606 turbo diesels the maf can be way out of spec and no-one ever notices! also note that many cdi engines in vans do not bother to use a maf sensor at all, just relying on the manifold pressure sensor.

I disagree partially with this.

At idle the MAF input isn't used for fuelling because the accelerator potentiometer has a switched idle position as an over-ride to the MAF, however once off the stop the MAF plays a vital role in determining the fuelling.
The reason people don't notice MAF sensor failure until it's severe is mainly because it doesn't cause rough running as it does in a petrol, just a reduction of torque, which also is mainly at the upper end of the MAF output...the MAF generally continues to read fairly accurately upto about 3V with the drop off becoming greater the greater the air flow/mass above that figure.
This means a car driver using only up to about 2500rpm will hardly notice any difference, but above that the torque won't increase so full on acceleration becomes slow.

Here are readings I took of a failing MAF and a new one in the same 606TD.

 

[oigle]

If one could get the restrictor size right, it would make for a cheap "chip" job. The reason I chipped mine initially was to get quicker throttle response. The 6mm restrictor looks good using fresh air. Might have to be a little different using exhaust gas although I understand that diesel exhausts are still fairly oxygenated compared to petrol ones. Would expect to need a slightly bigger restrictor. Could be worth a try for someone with a standard EGR setup. No use to me now with exhaust blanked off. Wouldn't fix the fouling problem but might give a chipped effect. Been an interesting experiment that seems to have paid dividends at this stage. Time will tell. Next step is to remove primary cat and see if that further improves performance. Will wait until I'm sure I have the EGR properly sorted. That will be another story... Thanks for your valued input.

 

[dieselman]

If one could get the restrictor size right, it would make for a cheap "chip" job. The reason I chipped mine initially was to get quicker throttle response.

Restricting the EGR won't increase performance because the MAF will see a reduction of EGR gas so will open the valve more to compensate. If the MAF reading is still too high the ECU will trip limp home and throw an EGR code.

In your case you will know if the restrictor is the smallest it can be by measuring approx 0.4v drop at idle when the EGR is operating.

A side effect of deleting EGR is the output combustion gasses will be cooler, due to the cooler intake gas. The cooler output gas will be less expanded than normal so will have slightly less pressure to drive the turbine of the turbo.
I measured the gasses to be ~50c cooler, so I think about 1-2lb less pressure.
I lagged the exhaust manifold to keep the temperature up thus provide full boost pressure again.
A positive aspect of cooler intake is that overall EGT goes down which is beneficial for a 'chipped' engine, to avoid the chance of engine damage.

If you want maximum output you must install an EGT gauge, then keep working on air-flow (not necessarily boost pressure), then fuelling.
Anything that improves airflow will cause EGT to fall, anything that increases fuelling will cause it to rise.

Forget replacement air filters, they are a huge con as they don't create a noticeable restriction anyway and paper filters offer much better filtration.

 

[Alex Crow]

my posts above were perhaps a little too long, i will try to sum up.

petrol vs diesel, and the role of the maf sensor.

petrol engined cars need to maintain air/fuel ratio within a tightly controlled range, at all operating conditions. to achieve this they are utterly reliant on the maf sensor, usually in conjunction with an exhaust oxygen sensor. the maf sensor is particularly useful in transient load conditions. if a petrol engine runs rich it may missfire and foul plugs and cats, if it runs lean it may missfire and overheat, possibly melting pistons!

conversely the diesel engine does not control air/fuel ratio with a tight limit. yes there is a maximum value for 'richness' (the point where black smoke is formed from excess fuelling), but no limit to how 'lean' the ratio can be. diesel engines are not throttled at all, and on simple engines without turbo or egr valve the ONLY control of power output is the fuel injected.

the petrol engine needs a throttle valve to regulate its output. as we have seen it needs a carefully measured a/f ratio, so needs a drastic method of restricting air flow at idle! the diesel has no throttle at all, and can run as lean as it likes (as long as enough fuel is injected to maintain idle as a minimum).

so the engine management of the petrol engine has to synchronise throttle and fuelling, whereas the diesel does not, this can be seen to be very significant when considering the role and accuracy of the maf sensor. i have already said in an above post that the restriction to power output on a diesel with failing maf sensor is fairly arbitrary - all down to the nuances of the ecu's mapping really. in many ways the maf value is used as a safety check to prevent overfuelling, especially useful on forced induction diesels.
the diesels ecu does not need the values from the maf sensor if you are doing 50 mph, or if you pull away at a junction at 1/3 throttle. it is not required for fuelling calculations in such conditions.

to sum up the summing up, in the usual understanding of mixture, and the concepts of weak or rich mixture, diesels are not the same as petrols! on a diesel it is very much the case that power output = fuel injected per stroke x rpm. this is why it is almost impossible for a simple remap to improve mpg by the claimed 15%, if you are cruising at say 50mph then there will be x amount of fuel injected plain and simple - no remap can magically deliver more calorific value from this fuel!

was that shorter? not really. and i went a bit off topic at the end, sorry.

 

[dieselman]

Any chance of a précis of the précis Alex?

For info for anyone that doesn't know. Diesel engine mixture strength can range from a maximum of about 20:1 for full power with smoke down to 100:1 at idle, cruise is typically about 50:1.
I'm pleased you mentioned about remapping and cruise economy, some people seem to get confused about this.

 

[oigle]

Restricting the EGR won't increase performance because the MAF will see a reduction of EGR gas so will open the valve more to compensate. If the MAF reading is still too high the ECU will trip limp home and throw an EGR code.

In your case you will know if the restrictor is the smallest it can be by measuring approx 0.4v drop at idle when the EGR is operating.

A side effect of deleting EGR is the output combustion gasses will be cooler, due to the cooler intake gas. The cooler output gas will be less expanded than normal so will have slightly less pressure to drive the turbine of the turbo.I measured the gasses to be ~50c cooler, so I think about 1-2lb less pressure.
I lagged the exhaust manifold to keep the temperature up thus provide full boost pressure again.
A positive aspect of cooler intake is that overall EGT goes down which is beneficial for a 'chipped' engine, to avoid the chance of engine damage.

If you want maximum output you must install an EGT gauge, then keep working on air-flow (not necessarily boost pressure), then fuelling.
Anything that improves airflow will cause EGT to fall, anything that increases fuelling will cause it to rise.

Forget replacement air filters, they are a huge con as they don't create a noticeable restriction anyway and paper filters offer much better filtration.

Interesting thought - there is the corollary to this - if inlet gases are cooler, they will be more dense and thus should expand further during combustion, providing a greater volume of exhaust gas. One might offset the other.
Seems that the EGT is higher on light throttle without boost as the EGR gases tend to reduce combustion temps from what I have read. See below in red.

The purpose of the EGR system is to reduce emissions. By mixing exhaust air back into the engine, it lowers peak combustion temperatures under partial load and reduces NOx emissions anywhere from 50%-75%. The reason why putting exhaust gas (hotter than ambient air) into the intake stream reduces NOx is because the free oxygen would otherwise turn into NOx. Because a diesel runs very lean (efficient), there's also considerably more oxygen in the exhaust vs. a gasoline engine which would otherwise turn into NOx emissions during combustion. The MAF air intake sensor also sees a reduction in air intake and adjusts fueling. Everything else being equal and within limits, in a diesel, more fuel = more power = higher temperatures. In a gasoline car, more fuel at the right ratios = lower temperatures.

Don't think that is an issue tho' as temps off boost would be low anyway. EGT under boost should not alter as the EGR doesn't work under those conditions.
I have noticed one unwelcome side effect. The engine is taking much longer to warm up without the "cooler" in the system. It doubles as an engine coolant warmer.
How do I go about measuring the voltage drop - presumably at the MAF ? I wouldn't mind doing that check.

 

[oigle]

my posts above were perhaps a little too long, i will try to sum up.

petrol vs diesel, and the role of the maf sensor.

petrol engined cars need to maintain air/fuel ratio within a tightly controlled range, at all operating conditions. to achieve this they are utterly reliant on the maf sensor, usually in conjunction with an exhaust oxygen sensor. the maf sensor is particularly useful in transient load conditions. if a petrol engine runs rich it may missfire and foul plugs and cats, if it runs lean it may missfire and overheat, possibly melting pistons!

conversely the diesel engine does not control air/fuel ratio with a tight limit. yes there is a maximum value for 'richness' (the point where black smoke is formed from excess fuelling), but no limit to how 'lean' the ratio can be. diesel engines are not throttled at all, and on simple engines without turbo or egr valve the ONLY control of power output is the fuel injected.

the petrol engine needs a throttle valve to regulate its output. as we have seen it needs a carefully measured a/f ratio, so needs a drastic method of restricting air flow at idle! the diesel has no throttle at all, and can run as lean as it likes (as long as enough fuel is injected to maintain idle as a minimum).

so the engine management of the petrol engine has to synchronise throttle and fuelling, whereas the diesel does not, this can be seen to be very significant when considering the role and accuracy of the maf sensor. i have already said in an above post that the restriction to power output on a diesel with failing maf sensor is fairly arbitrary - all down to the nuances of the ecu's mapping really. in many ways the maf value is used as a safety check to prevent overfuelling, especially useful on forced induction diesels.
the diesels ecu does not need the values from the maf sensor if you are doing 50 mph, or if you pull away at a junction at 1/3 throttle. it is not required for fuelling calculations in such conditions.

to sum up the summing up, in the usual understanding of mixture, and the concepts of weak or rich mixture, diesels are not the same as petrols! on a diesel it is very much the case that power output = fuel injected per stroke x rpm. this is why it is almost impossible for a simple remap to improve mpg by the claimed 15%, if you are cruising at say 50mph then there will be x amount of fuel injected plain and simple - no remap can magically deliver more calorific value from this fuel!

was that shorter? not really. and i went a bit off topic at the end, sorry.

Love the bit re the remap and improved economy...
Re your - no limit to how 'lean' the ratio can be - I agree totally. However, as you have stated, power is a product of fuel input in a diesel engine and, if excess air is entering through the EGR for example, the resultant loss of airflow through the MAF will reduce the fuel input and thus the power available. You say the MAF is not very sensitive on a diesel but my experimenting seems to indicate otherwise. Restricting the EGR flow has definitely (no doubt at all) increased the responsiveness and power of the engine at light throttle. With the EGR unrestricted and open to atmosphere, the motor was very dull and lacking in response. From my experimenting, I'd say you are right re no difference at 50mph cruise where the motor would be running light boost anyway and the EGR not operating, but wrong re pulling away from an intersection on lightish throttle/no boost with the EGR in use.

 

[Alex Crow]

Love the bit re the remap and improved economy...
Re your - no limit to how 'lean' the ratio can be - I agree totally. However, as you have stated, power is a product of fuel input in a diesel engine and, if excess air is entering through the EGR for example, the resultant loss of airflow through the MAF will reduce the fuel input and thus the power available.

not so, for instance at idle the fuel injected is the same whether the egr is open or closed. the ecu does not use the maf to DICTATE fuelling in the same way as a petrol engine. indeed there are no maf sensors on mercedes vans at all, but they still achieve good driveablility, economy and power out puts.

one point i am trying hard to stress here is that the subtle nuances of the ecu's mapping/programming dictate just how much effect maf values have on performance. see above point about vans, if your engine happened to have the same management as the vans i suspect egr deletion would be much easier.

and yes, the maf sensor is a sensitive device whether fitted to a diesel or petrol engine, i am not disputing this, but i AM saying that it does not need to be anywhere as accurate on a diesel, nor does it have to operate over such a wide range of values. it is no coincidence that MB charge £70 for the maf on your ML cdi, but charge £280 for one for the ML 320 petrol...

one thing that i would like to understand better, how does egr raise egt when it lowers peak combustion temp?

 

[dieselman]

it is no coincidence that MB charge £70 for the maf on your ML cdi, but charge £280 for one for the ML 320 petrol...

That's because they are made by Pierberg, not Bosch. Bosch MAF are the same price for either petrol or diesel.
I don't understand your reasoning behind saying the diesel MAF works over a narrower range. It doesn't they both work exactly the same, but may have different output curves depending on specific application. The part that is significantly different is the size of the tube depending on engine speed and capacity.

one thing that i would like to understand better, how does egr raise egt when it lowers peak combustion temp?

The incoming charge air is a lot cooler so cools the combustion when running on fresh air as opposed to hot recirculated gas.
Does EGR actually cool peak combustion temperature or does it simply remove the free oxygen so reduces NOX as a result? My understanding was the latter.

 

[oigle]

not so, for instance at idle the fuel injected is the same whether the egr is open or closed. the ecu does not use the maf to DICTATE fuelling in the same way as a petrol engine. indeed there are no maf sensors on mercedes vans at all, but they still achieve good driveablility, economy and power out puts.

one point i am trying hard to stress here is that the subtle nuances of the ecu's mapping/programming dictate just how much effect maf values have on performance. see above point about vans, if your engine happened to have the same management as the vans i suspect egr deletion would be much easier.

and yes, the maf sensor is a sensitive device whether fitted to a diesel or petrol engine, i am not disputing this, but i AM saying that it does not need to be anywhere as accurate on a diesel, nor does it have to operate over such a wide range of values. it is no coincidence that MB charge £70 for the maf on your ML cdi, but charge £280 for one for the ML 320 petrol...

one thing that i would like to understand better, how does egr raise egt when it lowers peak combustion temp?

Yes, I have previously agreed that idle fuel input is set and cannot be altered by air ingress through the EGR. Found that myself as idle didn't change with the EGR open to atmosphere, I was referring to low engine speeds off boost ie. when accelerating from rest, when I made my comment re MAF readings varying as air is allowed through the EGR. I really think I have proved that anyway with a very noticeable performance (and thus increase in fuelling) difference when I restricted the air available to the EGR inlet. It seems the MAF is quite sensitive but, as you say, not subject to the same requirements that a petrol engine has, with varying vacuum etc. I would think that the diesel engines fitted with MAF sensors would be more efficient than ones without - just an extra fine level of control of fueling.
Is it purely engine speed and throttle setting that controls the fueling of the vans? As you said, opening or closing of the EGR wouldn't matter at all with such a basic setup.
I won't comment too much on EGT's and peak compustion temps other than to say EGT's can be awfully much higher in petrol engines - then one needs to be careful. Peak combustion temps are to be watched but the ecu seems to have safeguards there with reduction in fueling/boost and oil sprayers inside the motor to prevent damage occuring. This only happens under full load/boost anyway, so is irrelevant in this EGR discussion. It will be closed.
Do appreciate all your comments - I have learned from them.

 

[Number_Cruncher]

>>Does EGR actually cool peak combustion temperature or does it simply remove the free oxygen so reduces NOX as a result?

It's very much the former. If free oxygen were reduced enough to affect NOx, in a diesel you would be well into the mixture regime where excessive black smoke would be produced.

The way the chemical reactions work is that above a critical temperature Nitrogen and oxygen can combine to form NOx, and also the reverse reaction can also take place [dissociation is the big word that I'll try not to use!]. However, once the temperature falls below this critical value, the reactions stop, and the amount of NOx is then fixed. EGR acts to dilute the combustion reactions such that this critical temperature isn't passed. However, this reduction of the *peak* may also be accompanied by an increase in the bulk temperature of the exhaust gases - there isn't necessarily a contradiction there.

 

[dieselman]

However, this reduction of the *peak* may also be accompanied by an increase in the bulk temperature of the exhaust gases - there isn't necessarily a contradiction there.

I'm happy to accept that. My observations of the EGT gauge indicate the overall bulk temperature to be lower.

 

[Alex Crow]

thanks guys, that makes sense then. with egr overall intake temp is higher. dilution of intake air reduces peak temp, but we still have the same work done by combustion. therefore higher intake temp + same energy release = higher exhaust temp.

and yes, egr does lower peak combustion temps in order to reduce mono nitrogen oxides (NO and NO2) which can react with other airbourne compounds (including volatile organic compounds) to produce harmful products to us, and to the ozone layer. this according to wikipedia.

one other thing barely touched on is that egr running is less fuel efficient, and produces more particulates.

and as for the maf working over a narrower range, an unthrottled diesel will typically consume 25 kg/h at idle while the same sized petrol engine uses 12 kg/h. but i now see that this difference is carried on up the rpm range where at max power the typically forced induction diesel will intake proportionally more air than the normally aspirated petrol - so typically proportionally the same range. therefore i retract my misguided claims of diesel mafs being used over a narrower air flow range - but reiterate that they do not NEED to be as accurate as those fitted on petrol engines. i accept that they may well be fairly equal in design and function. one thing i can be absolutely sure of though, as we have covered, the diesels management does not use maf values in the same way the petrol engine does, and the notion of mixture in this context is always going to be misleading - the ecu does not need to even consider maf values, except close to max fuelling and for fault monitoring.

and the vans, yes throttle position and rpm are used, but it is the MAP sensor that is vital. map values can be used in much the same way as maf values. broadly speaking map x rpm = maf. obviously map values would not help with monitoring egr at idle, but then it is less likely to be giving incorrect values as the maf in a fault situation such as a split hose - we see time and again engines with slight leaks that have not triggered use of default mapping (aka limp home mode) that throw out black smoke at max rpm. this is because the maf meters the air before the leak, therefore true air mass at intake is significantly lower than measured. the ecu monitors maf and map and expects them to be within a certain range of each other (given rpm, intake temp, atmospheric pressure, fuel injected etc) and will trigger fault status when they drift from this range - hence the usefulness of having both maf and map values for fault monitoring.

 

[dieselman]

but we still have the same work done by combustion. therefore higher intake temp + same energy release = higher exhaust temp.

Extrapolating this further, with a lower intake temperature the engine should be more efficient due to the greater oxygen content as a result of greater air mass.
In itself this will allow greater fuelling before critical temperatures are reached.
I wonder if this means that the turbo becomes more efficient as well because the reduced expansion if intake gas allows more gas to enter the cylinder so reduces intake back pressure for a given mass of air.

You can feel the difference in temperature of the alloy intake manifold on a OM606, I wonder if a phenolic gasket would be worth fitting as well as most heat now seems to be that conducted from the engine.

one other thing barely touched on is that egr running is less fuel efficient, and produces more particulates.

Which is where we started..