Misfire/backfire/revs problems - help!

[lordluvitt]

Hello,

My 1998 W202 C180 has slowly been developing a fault over the last couple of weeks. It began with a slight decrease in power intermittantly with what sounds like the odd backfire in the engine now and then. The revs seem to stutter a little as well. And then this morning whilst pulling away from a junction, the revs froze at a very low level so that I could not accellerate and was just coasting (in gear) at around 10mph regardless of how much I pumped the gas pedal. Thankfully there was layby within 100m so I pulled over. I switched the engine off, started it again and put my foot on the gas pedal increasing the revs slowly. Rather than a smooth increase in revs over the full range, the revs just pulsed instead but with more power than when I was coasting. I switched the ignition off and on a couple of times and tested again with the same results occurring. I decided to get back on the road at which point the engine seemed to run fine and I was able to continue the journey.

Any ideas what may be at fault here? Obviously as this is an intermittant problem it is difficult to go to a garage as I know full well that the fault will typically not occur when a mechanic looks at it.

Thanks in advance.

 

[colechelle]

sounds like a air mass sensor, you need some fault codes for a better idea tho!

 

[lordluvitt]

Thanks. Is it only a MB service centre that can run a diagnostic on this to produce the code or can any garage do it?

 

[colechelle]

anyone with a good hand held tester or star machine will do!!

 

[lordluvitt]

Just ran a search on previous threads on this forum and similar problems listed have generally had the same advice given - the air mass sensor is at fault. As these units cost around £150-180 to replace from Euro/GSF I would welcome any more responses to my query from experienced MB owners before purchasing such a unit (I don't want to waste money if something else is causing the problem).

Is there a way of testing the air mass sensor to be sure that this is the fault?

 

[MB TECH WARRINGTON]

Just ran a search on previous threads on this forum and similar problems listed have generally had the same advice given - the air mass sensor is at fault. As these units cost around £150-180 to replace from Euro/GSF I would welcome any more responses to my query from experienced MB owners before purchasing such a unit (I don't want to waste money if something else is causing the problem).

Is there a way of testing the air mass sensor to be sure that this is the fault?

the only sure way is to get an expert to test it on a good computer,

 

[colechelle]

as mb tech and myself both said, get the fault codes read, then your know what and where to go!!!!

 

[lordluvitt]

Great. Thanks for all of your help chaps.

 

[colechelle]

give us some fault codes and we should beable to help some more,thats what the sites about!!!

 

[lordluvitt]

I have just returned from my local garage where they tried to carry out a diagnostic check and recover the fault codes. However, it seems that my ECU is not providing any information at all to the diagnostics computer! The mechanic checked his machine with a different system in the car (the airbag one I think) and a fault was relayed almost immediately regarding the child safety switch (or something irrelevant like that). So the mechanic summised that there may be an ECU fault which is preventing it from passing a reading onto the diagnostics computer. This may mean that the ECU is causing the fault on the engine performance.

He asked whether any warning lights have appeared on the dash which there hasn't and so he suggested that the first port of call would be to check all of the wiring into the ECU to see whether they are clean or if there is any visible damage to them. When I described the symptoms that the car suffered (backfire, sluggish running) he also suggetsed that the CAT may be clogged up and in need of replacement.

Any thoughts on these results would be most welcome.

 

[colechelle]

CAT blocking would cause loss of power and a hissing noise through the exchaust joints, not mis-firing, backfiring, merc are funny things to read without the correct system, have you got a indi where you are with a star machine???

 

[lordluvitt]

It was an independant that I went to in my village (trying to avoid MB dealer costs initailly). I've used him for many years and I do trust him. I think it was the Star machine that he was using (yellow and black portable device that plugs into the diagnostics port and has electrical 'card keys' that he had to plug into the lead). It was this machine that could not get the reading from the ECU but worked on a different circuit to confirm that the machine was working.

His CAT suggestion was simply a suggestion but he said that they only tend to last about 3 years. I have not replaced it within the 18 months that I have owned the car and I have no idea when it was last done (service history does not include receipts/details).

But as the ECU does not provide a reading then I can only go by the suggestions that I am given rather than the car telling me the exact culprit. Tricky one this.

 

[English Teacher]

take a look at my postings

Your engine problem seems very similar to my car.

I had loss of power, severe backfiring and no engine management fault light on my 1999 C200. My solution was to very carefully clean my MAF sensor with Isopropyl Alcohol (£6.99 from Maplin's).

Despite advice to take it to a dealer or independant to interogate the ECU, I opted to thoroughly clean the MAF filter first, almost certainly saving me a great deal of money. I was motivated partly by having no engine management light lit.

You can see my posting on this forum, which is titled something like 'engine stalls after normal start and returning to idle'.

It has details on how to clean the sensor thoroughly, especially cleaning the sensor plate and backfushing the bottom element of the sensor.

 

[English Teacher]

OOPS, I should have stated that my posting 'engine stalls after normal start and returning to idle' is in the 'Electrics,vacum, ignition and ECU' forum.

 

[lordluvitt]

Excellent. Thank you.

I am trying to avoid the route of dumping the car at the garage as the problem is an intermittent one and I know full well that it will be difficult to trace (ie. lots of new parts fitted but nothing cured). Better to carefully tackle each possibility in a logical manner but to get the ECU working to provide a fault diagnosis as a priority.

The cleaning of the MAF has to be my next step. Just wondering whether this was the cure to your issue? Is there anything else to clean on the ECU do you know?

 

[lordluvitt]

Had the same issue this evening at the same road junction. Car would only pull away at a crawl despite gas pedal pushed to the floor. When I got to the layby and switched off the ignition and started it again, the revs didn't want to rise too high although the needle on the rev dial went to the top (strange). Got the car on the road again after switching ignition off and on a couple of times.

1. I could sense this happening before it did - car was stuttering a lot (misfire? incorrect air/fuel mixture?)

2. Does the engine have a 'safe mode'? The fact that the power is not being generated but the rev dial disagrees.

3. Problem seems to be corrected after switching ignition off and on. If electrical problem, am I 'resetting' the system?

 

[lordluvitt]

Forgive me but I'm a little confused

Is the MAF the same as the air mass sensor (which Eurocarparts also call an Air Mass Meter)? Too many names for the same item????????

 

[MB TECH WARRINGTON]

Forgive me but I'm a little confused

Is the MAF the same as the air mass sensor (which Eurocarparts also call an Air Mass Meter)? Too many names for the same item????????

yes, and to stop you continuously posting get the car looked at on a proper machine, sounds like the machine the guy has used is not a star machine, that is honestly the best advice i can give, some faults which may appear as electric faults may in fact be internal mechanical faults, we had a c class yesterday with airmass like symptoms, after a few hours of invest it was found that no1 cyl had very low compression, bear these things in mind also,

 

[lordluvitt]

Just thought I would post a final message to put this thread to bed and assist any future readers.

Having had a diagnostic test carried out, the garage confirmed that the Air Mass Sensor was faulty. A new Bosch one was bought (£150) and fitted in about ten minutes (very easy diy job). The car now runs perfectly, better than ever.

Just to reiterate what the symptons were:
The car initially displayed a slight loss of power in higher gears that over time gradually got worse. Hesitation on acceleration was felt and the occasional backfire in both the exhaust and the airbox could be heard. Later on, starting the engine would result in it cutting out immediately (wouldn't hold revs at idle). Revs would flutter when engine was running. Occasionally the engine would resort to 'Limp Mode' when pulling away from standstill. And finally, over time, the engine had absolutely no power to drive when cold and low power when warm.

The air mass sensor feeds data to the ECU to control the fuel/air mixture and consequently as the sensor had failed, the mixture was running too lean. I hope that this thread is of use to other MB owners in future. Thanks to all members that responded and helped with this matter.

 

[mightymerc]

your diagnostics showing no faults stored. Its not your ecu doesnt go on this model car I bet you its your plug caps replace all four along with spark plugs see the results for yourself. Buy the plug caps from dealer. I did this and vrooom!!!! car drives beautifully like new never seen anything like it. My car did exactly what you described at the beginning, serious misfiring.backfiring etc. Its definately not your cat either or anything silly like throttle body I assure you cost to have all the wrong diagnosis done including scary wrong assumptions from dealer tech at cost of £436 onc vat and no change. All that time plug caps but wont show on diagnostics. However cat would if faulty along with ecu etc. But if you are told no faults dont touch the ecu or cat. get those plug caps and see for yourself. The coils dont go on this model either unless you tamper with them. Trust me go for new plug caps with spark plugs. The air mass meter cant be faulty as it doesnt show up on your diagnostics. It would say so. Let me save you from the heart ache i went through with the dealership.