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'95 S124 E250 D / '98 W202 C180
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Discussion Starter #1
Hi there, for a few weeks now my car has been stumbling during the first 30 seconds after starting it, it sounds like it is misfiring with a loud sound in the exhaust like a backfire when rpm is higher during warm up. It almost stalls sometimes.
It gets smooth once rpm drops after warm up. Any idea ? Sparkplugs and coils were replaced 13k miles ago. The car is a 1998 model with the M111 engine. 94k miles on the clock.
 

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1995 C280 (165K mi)
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1,618 Posts
This is a hard problem to diagnose. Check the codes. Backfires and misses cause codes to come up.

Check you secondary air pump.
 

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'95 S124 E250 D / '98 W202 C180
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70 Posts
Discussion Starter #3
Thanks, unfortunately i can't check the codes because euro W202s are not equiped with an OBD2 port. Im starting to think that the flexfuel conversation is interfering with things. If it was a faulty MAF or o2 sensor i would have other symptoms, am i right ?
 

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W163 and General Mercedes Gremlin
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3,615 Posts
Thanks, unfortunately i can't check the codes because euro W202s are not equiped with an OBD2 port. Im starting to think that the flexfuel conversation is interfering with things. If it was a faulty MAF or o2 sensor i would have other symptoms, am i right ?
Unplug the MAF and test drive it to feel how it runs.
For a faulty O2 sensor, try taking some live readings at the OBD1 port (38pin) of the STFT and LTFT and pot here.
 

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W202 230K Sport manual & a few non Merc's
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504 Posts
Thanks, unfortunately i can't check the codes because euro W202s are not equiped with an OBD2 port.
Your car has an OBD2 port. It's 32 pin rather than the later 16 pin. You'll need a 32 round plug to 16 connector, loads on eBay for small money. The port is located in the engine bay beside the ECU housing, look for a large round black cover, the port it's under that.
 

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'95 S124 E250 D / '98 W202 C180
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70 Posts
Discussion Starter #6
Thanks guys ! I will try starting without the MAF to see if it behaves the same. I must say the car runs very well past the first 30 seconds. I will probably order the adapter too.
 

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'95 S124 E250 D / '98 W202 C180
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Discussion Starter #7
I'm back. I tried starting the car without the MAF and nothing changed. I'm really starting to think it is a fuel supply related issue. Do you know anything about the symtoms of a failing fuel pump and/or fuel pressure regulator ? I ordered a 38 pin to OBD cable and think about ordering a fuel pressure gauge as well. What kind of pressure should i expect ? i've heard things ranging from 50 to 65 psi during idle and load.
 

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W163 and General Mercedes Gremlin
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3,615 Posts
I'm back. I tried starting the car without the MAF and nothing changed. I'm really starting to think it is a fuel supply related issue. Do you know anything about the symtoms of a failing fuel pump and/or fuel pressure regulator ? I ordered a 38 pin to OBD cable and think about ordering a fuel pressure gauge as well. What kind of pressure should i expect ? i've heard things ranging from 50 to 65 psi during idle and load.
What scanner are you using?
Not all 38 pin adapters mated to the typical scanner will scan correctly.
And not all 38 pin adapters are created equal either.
 

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'95 S124 E250 D / '98 W202 C180
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70 Posts
Discussion Starter #9
I will be using a cheap bluetooth OBD2 adapter i have laying around. It is called ELM327 i think. If it does not work (my hopes are low) i will probably get my hand on something better.
 

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W202 230K Sport manual & a few non Merc's
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504 Posts
The ELM should work o.k..

As for your issue I'm leaning towards a failing MAF sensor but obviously that's a bad guess at best.
 

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'95 S124 E250 D / '98 W202 C180
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Discussion Starter #11 (Edited)
I tried running the car without the MAF and it behaved the same if not worse. I will check for codes whenever i can but i'm pretty sure it has something to do with fuel supply. On E85 it requires around 5 tries (used to require 2/3 tries and then it used to run rock solid once started) and now when it finally starts it dies after a few seconds with popping noise likely in the intake. I checked with a tank of gas/petrol and it always starts right up but there's is a perceptible stumble during a few seconds which wasn't there before. I think E85 exacerbates an existing problem caused by somthing weak in the fuel supply system. I was always on the verge of not starting on E85 and maybe it's juste because of a few PSI drop for whatever reason (fuel pump, fuel pressure regularor, an injector)...
 

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W163 and General Mercedes Gremlin
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3,615 Posts
David,

Some more info for you to digest and check
 

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'95 S124 E250 D / '98 W202 C180
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Discussion Starter #13 (Edited)
Interesting. Do you think that it could be a clogged fuel filter, even if i changed mine like 9 months ago ? Again, hard to check without a pressure gauge but ethanol might have done more cleaning in my fuel tank and i don't have much to lose changing it again. However i dont believe i have a carbon build up problem, I dont use the car for a lot of short trips, i mainly do 300 to 500 kms of highway every week and E85 produces close to zero carbon build up.
 

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W163 and General Mercedes Gremlin
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Interesting. Do you think that it could be a clogged fuel filter, even if i changed mine like 9 months ago ? Again, hard to check without a pressure gauge but ethanol might have done more cleaning in my fuel and i don't have much to lose changing it again.
I doubt it's the FF, if you changed it recently. But this also depends on what gas grade you are using and the supplier (from what Ive read about other members using a subpar gas supply and lower grade can, overtime, cause issues).
Mixed opinions from members on this topic.
As for FPG, they are cheap to invest into,
or go rent A FPG at your local auto shop; and test the pressure at the rails.

Another suggestion - I give our engines some cheap TLC once a year - either seafoam top end or berryman b12 to give the pipes a good cleaning. -
 

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'95 S124 E250 D / '98 W202 C180
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Discussion Starter #15
I only use E85, i never use gas by the way.
 

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W163 and General Mercedes Gremlin
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I only use E85, i never use gas by the way.
Just so you know for your own knowledge.
It's your ride, so you decide. :)


 

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'95 S124 E250 D / '98 W202 C180
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Discussion Starter #17 (Edited)
I unplugged the vacuum hose from the fuel pressure regulator and there's a strong gas smell so i think i found the culprit.
 

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'95 S124 E250 D / '98 W202 C180
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Discussion Starter #18
I replaced the leaking FPR without any improvement but it was bad anyway so i didn't bother with testing. Next thing will be checking fuel pressure, i'm thinking about the fuel pump going bad too it is starting to make a whirring noise when i turn the key.
Since the issue happens only when the car is cold (at leats 4 hours of sitting i don't think it has anything to do with something electrical like the alternator. Now i think it is either a bad fuel pump or a bad coolant temp sensor.
 

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W163 and General Mercedes Gremlin
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3,615 Posts
I replaced the leaking FPR without any improvement but it was bad anyway so i didn't bother with testing. Next thing will be checking fuel pressure, i'm thinking about the fuel pump going bad too it is starting to make a whirring noise when i turn the key.
Since the issue happens only when the car is cold (at leats 4 hours of sitting i don't think it has anything to do with something electrical like the alternator. Now i think it is either a bad fuel pump or a bad coolant temp sensor.
Whirring noise of the FP is normal when you turn the ignition key to position 2.
It doubt it's the FP - you said it's running fine after 30secs. You can test the FP at the rail.

To test the coolant temp sensor. Ohm it out when it's cold, use a hair dryer to it to heat it up, read the resistance again and see if it changes. When you startup the car in cold weather does the engine fan kick on at all? (Indication of not signal getting to the CTS )
 

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'95 S124 E250 D / '98 W202 C180
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Discussion Starter #20
Thanks for the reply. The reason why i mention the FP is because it did not make that noise before. I will test the sensor as you say. The fan behaves like it should, it only starts when sitting in traffic or when i turn the AC on.
 
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