Loss of power on full throttle = limp mode

[SteveX]

Hi Guys,

Well I am nearing the end of my initial list of things to do to the CLK I purchsed.

The one I am stumped with at the moment is the power loss when at running temp then putting on full throttle (or close to it).

Here are the symptoms:

- Drives perfect from cold start (I wouldn't know if the problem is there from cold as I do not 'floor it' from cold

- Drives fine at temperature until you need to overtake on a dual carriage way for example and push the throttle quite far down

- I tested the kick down, doesn't really work, and the problem comes on shortly after so I initially thought a problem with the kickdown and box

- Pulling over and reving the car in neutral seems like it has gone into 'limp mode' Very poor throttle response, car reluctant to rev, stamping the throttle has no effect (throttle has to be open quite far and for a few second before it even starts to pick up)

- When driving the car like this it has nearly no power and just stays around 2-3k at speed, will go higher if in 1st or second when pulling away, but will take a long time, there is no power and it is not responsive.

- Restarting the car immediatly cures the issue, throttle response is back, acceleration is crisp

- I've identified that flooring the car again to overtake brings the fault back, sometimes as the car tries to go to say 5k, it then goes into 'limp mode' I guess until switching ignition on and off

- No fault lights on dash

- Unplugged MAF while the problem was occuring and engine running - no difference

- Ran with the MAF unplugged, car runs the same, seems to run ok, flooring and getting high up the revs at WOT puts the car in 'limp mode'

- Engine fan running for quite a long time, but this may not be related, I was up and down the dual carriage way working the car to get this fault to occur. Engine temp say at 80


So that's it really, the seconday cats are making a lowish pitch clonk, could at tickover, could it be the cats be heating up too much at speed, or blocked and causing an issue? If so though wouldn't a management light come on? And why would it be able to accelerate hard again after restarting a couple of time until the failt re-occurs?

The car runs identical without the MAF plugged in, I'm not sure if that proves anything or not, since the issue is at high RPM. Could it be the program that runs without the MAF causes an issue when the engine is loaded or get's higher up the RPM, then causing 'limp mode'.

Gear box changes gear fine from pulling away, but at speed just won't kick down, I guess because of lack of power.

No other symptoms, car runs smooth either way, just total loss of power as described above, I really need to get it fixed as there is a pretty big hill on my commute and I would not be confident if the error occured when I needed to give it some throttle that it would get up the hill. I also would not be happy with anyone else driving it in it's current state.


Thanks,

 

[SteveX]

Sorry should have added, it's a 1999 W208 CLK 320.

I was planning on ordering a MAF anyway before today, but not sure what my fault finding has proved, Im now confused and tired, so your expertise and opinion of my findings would be appreciated chaps

Thanks,

 

[Steve@Avantgarde]

Have you run any form of diagnostic check and have any fault codes? If not I suggest you do that first before ordering parts.

 

[SteveX]

Thanks, I suspect it's going to come up with MAF codes anyway seeing as I have unplugged and plugged it in, I can run it to a friend who runs a garage to see what codes it pulls.

I've read the following:

http://forums.mercedesclub.org.uk/showthread.php?t=69804&highlight=maf+failure+symptoms

http://forums.mercedesclub.org.uk/showthread.php?t=12659&highlight=maf+failure+symptoms

Sounding very similar, runs same with or without MAF unplugged and goes into limp mode at full throttle / speed until turned on and off.

I'm busy working at the moment, but will pop out and get the codes read later, but any other opinions or advice in the mean time appreciated.


Thanks,

 

[Corned]

If it runs the same with/without the MAF plugged in, then it certainly points towards a duff MAF.

However it is still best to get the codes checked, just in case something else is causing similar symptoms.

 

[SteveX]

Thanks, I'm going to do volt test on the MAF as well, but I am uncertain of the 5 pin outs and which to check - I can't find any wiring diagram that tells me which should feed the signal back to the ECU for airflow measurement.

I pulled the cover, and left to right there is a yellow and green, red and blue, brown, brown and yellow, white and yellow.

If anyone knows it would be appreciated.

Thanks,

 

[Corned]

I think it's pin 5, regardless of which colour wire is attached.

You can check without doing any damage. IIRC, you should see something like 1.7VDC at idle, climbing towards 4VDC at full throttle.

 

[SteveX]

Ok found them.

Here's the results:

At idle on cold start it was everywhere, output was from .3 bouncing up to 1.5.

Rev'd steadily, output from 1-3volts steadily.

Took her for a spin replicated the problem, limp mode on so I came back:

0.95 - 1.5 bouncing around at tickover

steady at 2.5 when engine held at 3k rpm, but then would randomly bounce around as low as 0.8 up to 3, would have a bit of a fit, then go back steady for a bit, then throw another fit

Engine off, ignition on, a steady 1.0, but keep it on there it then has moments of dropping as low as .1 and bounces to and from there to .8 then settles at 1.

Mercedes tech I called said could be AFM or gearbox putting it into limp mode, but seems the power output to me, why would it be working fine for a bit then go into limp mode?

 

[SteveX]

Yes you are correct it was 3 ground and 5 output thanks

My wife gave it full throttle for a bit and it shot down as low as 0. something.....

 

[SteveX]

Just did another check with ignition on only, intially 1.01 for about 5 seconds, then dropped to 0.3 bouncing around to 0.8 then back to 1.00.

Does any of this sound normal? I suspect it shouldn't have the fits of bouncing all over the place, when I saw it give an output of under 1 at full throttle it scared me!

 

[Corned]

I would expect the output to match the input. I.e. if the incoming air flow is at a constant (steady RPM) then the output should be equally steady. Readings which are erratic and jumping around do not sound right.

I think that would be enough of a smoking gun for me, but I can't tell you what to do! Or rather, you shouldn't just take my word for it.

 

[SteveX]

Hmm, did another re-test, seemed more stable, didn't run it around the block and get the problem though, just did it from a restart and tickover after a burn-off so not as much air going past or rubbish on it I assume.

It was holding failry stable this time at around 2.4 at 3.5k rpm.

Switched off and sat rock solid on 1.01

Confused.com

 

[mancman]

I have a coupe which went into limp mode no faults noticed, took it into mercedes who diagnosed it was a due to a valve in the gearbox they changed and has been ok since.

 

[SteveX]

I have a coupe which went into limp mode no faults noticed, took it into mercedes who diagnosed it was a due to a valve in the gearbox they changed and has been ok since.

Great info thanks, any other symptoms and details on how it occured would really be appreciated.

Mine is only if I try and kick down a few times and build up revs.

Light / part or medium throttle seems fine.

Seems odd I have got some erratic readings from the MAF though, having not dealt with the bosch unit on the Mercedes before I am not sure if it is erratic at idle / revs rather than when moving.

 

[SteveX]

I recall when driving home when the issue occured and the car had no power I pressed the speed limit button in error, it came up with 'speedtronic visit workshop' not surprising if it was in limp mode I guess (as it wouldn't kick down etc and I had severe engine power loss).

I hear that Mercedes speedtronic and distronic systems are disengaged when in limp home mode which makes sense.

I popped to my friends Garage today, it was the older diag port, so not OBD2 compliant inside so we could not read the codes. The genuine bosch MAF retails at £200, Mercedes want £250.00, Eurocar parts want £170.00, my friend can obtain one for £160.00 but is happy to absorb the VAT for me as a favour so it will be £135 for a genuine Bosch item and only 1 hour delivery time.

My local indy wants £70.00 + VAT for a diagnostic check, not bad, but I'm going to take a punt on the MAF first I think, it's readings have varied all evening between stable and damn right erratic.

If the MAF is not faulty, then I will ebay my old functioning one for £50-60 (bargain price for a genuine item ), I don't mind a loss of £70.00 for peace of mind the MAF is replaced and has been ruled out. I hear from most people and the techs that I know that MAF's are hard to diagnose unless you actually have the kit plugged in when the problem occurs, as they can sometimes not log a code (if malfunctioning), and limp home could throw other errors that whilst they do not specifically state the MAF, they show a whole host of other systems around which have been faulty as a result. For the sake of a few pounds I am going to get it and rule that out, but fingers crossed it will fix it.

If the issue persists, on to the gearbox and then a specialist I guess.

As always, any comments and advice on this thread are welcomed

 

[mancman]

mine went into limp mode as i was driving it felt like the car just jerked literally for a second and that was it, i could,nt change gear with the paddle shift it stayed in d, swithed it off and started it up and it was fine, happened twice in 3 weeks, since the valve has been changed it appears to be ok, hope this helps in some small way.

 

[SteveX]

mine went into limp mode as i was driving it felt like the car just jerked literally for a second and that was it, i could,nt change gear with the paddle shift it stayed in d, swithed it off and started it up and it was fine, happened twice in 3 weeks, since the valve has been changed it appears to be ok, hope this helps in some small way.

Interesting, thanks very much. The car doesn't jerk as you described, it's still very smooth, but I understand that all symptoms can vary

Mine is when I wind the car up at full throttle, as it gets higher in revs and more power comes on or if I try to kick down a few times, it then goes into limp home mode and speedtronic also fails. The gears remain though, I counted the changes earlier, it just seriously looses power.

I guess at this stage the likely culprits seem either:

A) MAF
B) Gearbox problem

The car has FSH and the engine has been maintained, I just carried out a full service this week and if I don't accelerate hard / build revs or kick down hard the car runs smooth and perfectly, although on tickover when I left it running for some time it did smell quite rich - more than I would expect.

I'll replace the MAF first and take it from there, I appreciate your input


Cheers,

 

[SteveX]

Hmm, would the pedal accelerator sensor cause this issue?

I wouldn't think it would cause the speedtronic light to come on, just a thought.

 

[SteveX]

Was just cleaning the interior and warmed the car up, noticed that the exhaust fumes smell very very rich even at warm, as though the car is still on a cold start program.

Not sure if these smell rich at idle, but it doesn't seem normal to me.

 

[initialbdriftracer]

my car did this for a while,
i replaced the 2 O2 sensors, and the airflow meter.

problem solved