SL500 around idle misfire - what next?

Alex Crow

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i have every sympathy with nick, and his specialist. i can testify to the difficulty of testing and troubleshooting the HT system on these cars, COP and misfire detection has really helped us here. herman make the approved MB test equipment for these sytems, and this is the correct machine to use (depending on which particular model it is!). i would suggest however that a beter way to test would be with a picoscope in the hands of someone with great experience using one - such as peter remmington, here is his site....http://www.remmington.plus.com/

good luck!
 

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alexander - Thanks I got the PM - I'm checking now !

number cruncher - thanks for the knock sensor test to check it is not retarding due to other noise...but how can I check it IS retarding when there is a knock...which is the way it might be...do they throw codes ???

The best way is to watch live data while driving the car.

The "back Street" approach would be to temporarily remove the sensor from the block, but, leave it electrically connected, and drive the car - see if it makes any difference.

If you do find it makes a difference, you then have the subtle question of fiding out what vibration the knock sensor is actually detecting - is it really unstable combustion?, or is it something else?

In a different time, in a different world, many Montego 2 litre engines used to run like donkeys because the knock sensor "heard" exhaust noise from a loose flange, interpreted it as knock, and fully retarded the timing. Classy!

I've also had a similar problem with Carlton and Senator 24 valve engines, when these get a bit of wear in the valves and timing chain, the knock sensors "think" it's knock and fully retard the ignition.

As for putting up fault codes, if the electrical circuit to the sensor is OK, you're very unlikely to see a knock sensor fault - although, these sensor are very low current devices, interfacing with a large gain amplifier in the ECU, which means they are not only susceptible to mechanical noise, but also electrical interference.
 

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thanks NC.

i would suggest the knock sensors are most unlikely to be causing any idling issues, and unbolting and securing them away from any vibration is great way to test them. knock sensors are, in them selves, not going to produce spurious signals, if they fail they will simply not detect any detonation. as NC says they can on occasion detect other vibration sources as detonation, but in my experience this is extremely unlikely to happen on your car - i have never seen a knock sensor problem on any mercedes in 14 years. also, i suspect that the knock controll is not enabled at a closed throttle, the ignition angle being determined simply by the mapping, with no detonation led retarding of timing.
 

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>>knock sensors are most unlikely to be causing any idling issues,

I agree fully, and posted more for the sake of completeness than to suggest it's a likely problem.

I've probably mentioned this before, but Sykes used to make a very good "laptop" engine analyzer which took the hard work out of interpreting the HT waveform by reporting burn times in milliseconds for each cylinder on screen - I've used this feature many times to home in on tricky / intermittent HT faults.
 
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Thanks guys...

Yep I can see the knock sensor is not my idle misfire issue but potentially the cause of a likely separate issue I raised...the pinking fluttery sound I sometimes get on acceleration.

I am concerned that the knock sensors might not be detecting detonation which results in the EZL ignition retarding 3 degrees or whatever. I am not concerned with false detections therefore, pulling them off the block is not a relevant test in this case...

...I read that different knock sensors work in different ways but good ones generate a signal when they detect a specific frequency emitted by detonation (not just a knock). Clearly its hard to fake this to see if they work...

Maybe someone will come up with a knock sensor test for this but for now I will focus on the idle and fast idle "pomph"...I'll meet the Indi soon in the new year and discuss the previous Herman test and consider doing another one now one problem has been solved and the situation simplified.

Another thought was I might just throw some money at cleaning the injectors anyway...whilst I haven't eliminated everything else they might benefit from a proper clean-up anyway after 90k miles. I can't stop thinking about the one-sided colouration of the insulator on plug 3...perhaps that injector dribbles out of one of the two holes instead or spraying nicely.

So...Indi first...check the LTFT etc. and codes again, and reset the ASR light (after I pulled the module out)...at least it drives "ok" again !
 

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A few tank fulls of injector cleaner can do a good job
 
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Nick-V

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I did that a few times before all this...some tell me these injectors rarely give trouble...whereas the specialist say 50% of injectors they test NEED cleaning too...

Its a gamble really...would petrol injectors benefit from a proper clean after these miles anyway...possibly they are causing a non-50% ltft reading after all this time and possibly one or more may not be spraying quite right...

Not sure what it costs to get them off and on (1 hour?) but its probably about £20 x 8 cash price for the clean...could be a good £200 tune up, may fix the misfire, may be a waste of time !
 

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The injectors can be difficult to remove with a risk of breakage. I have always followed the 500 engine post as they are of interest to me,,but for the life of me I cannot recall any of these with injector problems, even at mileages of well over 100k
 
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OK point taken about risk of breakage and I've added you to the growing list of those who say they rarely give a problem...I'll put that idea back on simmer then...
 
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Alex Crow

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add me to the 'rarely give a problem' list. we have never had to change one! the older ke-jetronic ones suffered one or two problems like leak off at shut down causing starting problems however.

may i ask, although i am sure you must have already answered this one, how do the seperate banks compare on lambda values/cycles and fuel trim? i cannot remember, but assume they do have seperate trims for each bank? and what measures have you taken to rule out manifold air draw?
 
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OK will do...the votes are piling up in that corner thanks !

I believe there is only one "long term fuel trim" value and one "short term fuel trim" value. I remember some ex-merc guys commenting it was a bit out (say 10%) but can't remember which way and my Indi also said so but he can't remember either so this will be checked again. Its possible it is now different due to the fixes I carried out - I will report back. Certainly there is only one O2 sensor so that also tells me one trim value for both banks on the 1995 model.

The Indi stated the lambda value fluctuates...I need to get more info from him and probably re-test because "I" now understand what to expect in terms of low, high and average voltage readings and cycles...its done 30k miles...might work but be lazy...was that your thought?

I do not believe we have tested inlet manifold pressure but the Indi believes he has tested for air leaks...of course this could cause a trim value...and if a significant leak in one cylinder could cause an occasional lean misfire (bad burn). I also want to discuss testing for an EGR valve leak...is that what you meant when you mentioned air draw?

I "think" the next step is to get facts from STAR again and to get facts from Herman again for myself. What are trim values, is it electrics, is it one or several cylinders...
 

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yes nick, you have my thoughts correct. shame you have only the one lambda sensor, that could have been very helpful!

i would expect typical lambda values to fluctuate between around 200mv and 600mv every 2-3 seconds, with it dropping to well below 200mv and well above 600mv every 15 seconds or so.

to check for manifold air draw, spray a solvent such as brake cleaner or carb cleaner all around the manifold in as many places as possible, idlind speed should rise a little if there is a leak. i doubt very much however that a vacuum test of the manifold would help!

also, just a thought, but it could just be an HT issue ;). good luck nick.
 
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Nick-V

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I thought I read somewhere that lamda sensors cycle 8-10 times per second when on a 'scope and that less cycles might indicate lazy? Perhaps you are indicating what I should look for if simply using a mv meter?

We did the inlet manifold spray test and checked all pipes already...nothing obvious but one or two other gadgets like EGR valve and "evac" are potentials apparently...herman first...I feel the need to find out WHAT is misfiring.

I HOPE it is HT as it would be easier to fix but this was thrown into some doubt before...again Herman should tell us this IF it can also report spark voltage and duration (I read somewhere). I'd be delighted if its simply a coil or something!

At this stage, and after having fixed a couple of things, I really think the next step is for me to assert my hands-on involvement a bit, ensure that we get good facts and do things slowly and methodically.

I appreciate the help here and I hope for a quiet hour or two with the Indi one Saturday in the next 2-3 weeks...
 
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Nick-V

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I've read up again on Lambda testing to find I'm wrong...whilst a good sensor has a transition time (from rich to lean or vice-virca) of 50-150ms the expected cycle rate with a voltmeter is 1hz (one per second) and it is regarded as lazy at >3hz.

The real-world observation of a jump every 15 seconds or so interested me...could this be when my "pomph" occurs - its about the same rate (very roughly). I wonder why this occurs.

I think another Lambda test is another priority for me! STAR (LTFT etc.), Lambda (if STAR does not show this) followed by Herman (which cylinder, HT etc.).
 

Alex Crow

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missfiring cylinders will play havoc with lambda contrtol, this is due to unburnt mixture containing 20% or so oxygen as opposed to the 2% or less of exhaust gas. and yes, lambda sensors can respond very quickly, but it is pretty much the mixture control side of closed loop that determines the cycling speed.
 

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>>but it is pretty much the mixture control side of closed loop that determines the cycling speed.

Yes, the lambda sensor can only sense rich or weak, and there's an obvious delay between fuel being injected, the intake and compression strokes completing, the power stroke, and the exhaust gas leaving the cylinder and travelling down the exhaust pipe.

The delay, and the non-linear response of the sensor means that you always get cycling, and the cycling frequency is inversely proportional to the delay. Typically, this is about 1 Hz at tickover, and faster as the engine revs up.

One practical way to test for a sluggish lambda sensor is to drive on a quiet road at about 30 with one foot *under* the throttle to hold it absolutely fixed. If you feel any low frequency surging, the switching speed of the sensor is sluggish. In this scenario, before condemning the sensor, always check that a proper heating current is flowing through the heater circuit of the sensor.
 
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Nick-V

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Thanks guys...I understand your explanations for the typical cycle time.

I don't appear to get any surging while driving (I can use cruise control for the 30mph test too) but I am interested to see if the lambda sensor cycles slower than 1hz...I can imagine how a slow response might lead to a very occasional lean misfire (my "pomph") and indeed hesitation on accelerating (I don't have this).

I am hopeful that STAR on my OBD1 system will show the live sensor values at the LH control unit to avoid the need to connect a lambda tester, scope or voltmeter. I am most interested to see if the misfire that I did fix has had an effect on the previously adrift LTFT setting and on the "fluctuating lambda reading" (???) that the Indi mentioned...

I suspect it'll be 2-3 weeks before I can get to see him and find out.
 

Number_Cruncher

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>>(I can use cruise control for the 30mph test too)

No, cruise control would compensate for the slow switching - you wouldn't feel it.
 

Alex Crow

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if you do find that the cycling is 3 or 4 secs or even a littler more then do not consider the sensor to be a suspect in regards to the occasional idle miss (pomp?). the mixture correction will still be near enough, and the intermittent description you describe does not really fit stft errors - it sounds like probably just one cylinder after all....
 

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