Stall at start issue

MICHAEL LYNCH2

MEMBER
2005 MERCEDES BENZ C200
1.8L
4 CYL
2WD
AUTOMATIC
100 MILES

I have purchased the car listed above Coupe. It had been sitting under a Tarpaulin for 3 years with a skipped timing chain. When I pulled it apart it had worn down 4 teeth on the inlet sprocket which I have now all fixed. I renewed both the chain guides, chain and sprockets all at the same time. Did a leak down with the valves all in the shut position with the cams removed and the cylinders were all well sealed so no bent valves fortunately. Anyway everything is running well. Have no DTC issues on scan and the car runs extremely well. But I have an annoying issue whereby when the car is starting it will either run really poor for a couple of seconds or will stall altogether. It will restart straight away but will have the same issue. Will run really lumpy/poor for a few seconds then even out and run totally okay. Very odd that it is not throwing any codes. So I have been looking for a vacuum leak but everything seems fine with no split or damaged hoses. I have cleaned the throttle body and the MAF and it is still doing the same thing. I have checked the fuel pressure and its running about 56 psi and there is no leak down in the rail overnight. Also when driving it can have a very small hiccup here and there. Not regular and not often but it is there occasionally. The injectors are showing okay on the scan tool and as I say no faults coming up at all. Seems to be better when it is already warmed up. BTW, it has fresh fuel. All suggestions gratefully taken on board. Cheers, Guys

W203 M271

7 Replies

KASEKENNY

MECHANIC
18,907 POSTS

Hi Michael,

If you have a scan tool can monitor the o2 sensor data when this is acting up? Also, short term fuel trim because that will tell us what the air/fuel is doing and how the PCM is trying to adjust.

Let me know this information and we can go from there because I am thinking it may be something with your emission control system. Thanks

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Saturday, July 13th, 2019 AT 5:20 PM

MICHAEL LYNCH2

MEMBER
23 POSTS

Hi, Thank you for your quick response. Yes I should have checked the fuel trim numbers and 02 sensor voltage first. When I did as you suggested the STFT was ranging from -.8 at idle to -4.9 at 2500 rpm the LTFT was -3.9 so obviously a rich condition which eliminated a vacuum leak possibility. The 02 was at.45V normal at idle so normal range. Did some digging and pulled the Intake Air Temperature sender which was a little dirty. Not too bad but cleaned it and put it back in place and started the car. STFT numbers went to +2% and the LTFT started climbing back towards 0% so it seems to have solved the issue. Will know more when I start it from cold. But so far so good will let you know.
Cheers for the advice

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Sunday, July 14th, 2019 AT 1:06 AM

KASEKENNY

MECHANIC
18,907 POSTS

Awesome. Glad you are on the right path even if it is not fixed. However, it sounds like you got it. Also, thanks for the information. This will surely help someone else that is having a similar issue. Let me know if you need more assistance with this one. Thanks

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Sunday, July 14th, 2019 AT 7:25 AM

MICHAEL LYNCH2

MEMBER
23 POSTS

Hi
As an update. The car stalled immediately on initial cold startup this morning but then ran straight away upon starting again and was not as lumpy in its running as it was previously. So I think the computer has not yet had enough running time to adjust the LTFT average to where it should be. I will let it run for an hour today and LTFT should correct itself. It seems to me the computer must use the baseline fuel map its programmed with and then use the LTFT data to modify this on initial startup whilst in open loop to enable optimisation. (I hope I'm making sense) Its only in closed loop at operating temperature and getting information from the o2 sensor that it can bring up the LTFT data close to programmed baseline. Atm the computer is still using old data and trying to compensate for a previously rich condition on startup and that is the reason it still is not starting straight up. Once I have let it run in closed loop for a while it should make the LTFT % correction and then the computer will stop trying to compensate for a condition that no longer exists. Please correct me if I am not understanding this correctly. Although I am an engineer I'm not a mechanical engineer. Cheers

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Sunday, July 14th, 2019 AT 5:55 PM

KASEKENNY

MECHANIC
18,907 POSTS

You are mostly correct. However, the only portion that is slightly off is the long term during open loop. In general the only sensors that are being used during open loop are your temp sensors (coolant, intake air, ambient air), MAP/MAF sensor, and Barometric to determine best possible air/fuel until the engine is up to temp.

Once it is to operating temp it starts looking at o2 sensors and any other engine sensors to monitor how it is doing and adjusts injectors accordingly.

If you found your intake temp sensor dirty and that made an improvement, I would clean the MAF sensor as well.

Here is a step by step guide to do so but as an engineer I am sure this will be a walk in the park. Let me know what happens. Thanks.

https://www.2carpros.com/articles/mass-air-flow-service

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