Misfire after servicing W202

[scorpie]

Hello All,
I recently serviced my W202, I tried to start it and it was very rough, it feels like a misfire in one cylinder. The OB Diagnostic tool cannot find the computer to diagnose the problem. It did start eventually but behaved as if it was on three cylinders, low power etc. I tried putting back the old spark plugs, replaced the coils, check the wiring for any obvious faults or breaks, I checked for loose pipes etc. It is a C180 Auto, with 90,000 mls.
I have used this site often and has yet to let me down, but this problem seems unusual, any ideas would be really welcome,

Thanks.

 

[grober]

There can be issues with the insulation of the low voltage supply wires to the coils on these cars or the coils themselves or the HT leads. Sounds as if you have disturbed something when servicing the car- I assume you changed the plugs? You could also have a defective new spark plug but this is very unusual. What make of plugs did you us? and what is the age of the car? does it have the ht coils mounted underneath the inlet manifold or the cam cover plate? Don't drive the car much since if the low voltage wires/coils are faulty they will eventually blow the engine ECU ignition output transistors
ps did you get the HT lead firing order correct?

 

[scorpie]

I think you might be right, I used Genuine MB plugs, the coils are on top with the HT leads tailored to each coil so mixing them up is not possible to do, the car is a 2001 Saloon. I have read that the wiring is quite brittle, but I can see no obvious sign of this.
I don't understand how this has happened, I have serviced the car twice before with no adverse affects,

Thanks for your help on this!

 

[grober]

Remember when testing this is a "lost spark" system where 2 plugs supplied from the one coil fire together. the operative one is on the compression>power stroke the other "lost" one is on the exhaust>induction stroke so doesn't effectively do anything.

 

[Dieselman]

After checking the HT and spark power next stop would be the MAF for this fault.
Meter pin5 WRT Gnd: Idle ~2v-dc, full revs ~4.8v-dc.

 

[scorpie]

Excuse my ignorance Grober, but I don't understand, if there are four plugs and four coils how are two plugs supplied form one coil? (This may be a bit beyond my competence, so speak slowly).
I did check the MAF, replaced it and even tried removing it while testing, no effect, but thanks for the suggestion Dieselman!

 

[Dieselman]

grober is correct about the coils, they do fire two plugs simultaneously employing wasted spark technology.
There should be two coils, each with a downward facing HT connection for one plug and a separate HT lead and cap for the other.
Both plugs then spark together but as only one is on the compression stoke only that one has an effect.
Disconnecting the MAF isn't a satisfactory test, you need to meter it.

Another possible fault is an air leak into the intake somewhere.

 

[scorpie]

Thank you Grober and Dieselman, it's great to have such heavyweights replying to this problem, my suspicion is that, as Grober suggested, I have damaged the HT connection or the wiring connected to it, either way it looks like a trip to the stealership for me to pay heavily for my mistake.

Thanks again.
I'll update this post with result.

 

[kth286]

I think you have the 111952 engine in that car.

If so, then you do have 4 coils (one above each plug).

It is likely one of the extenders down to the plug from the coil is not connecting properly.

I would be inclined to replace those extenders each time you replace the plugs, as the wire inside them will be very brittle.

Approx the same price as the plugs. Ask for discount.

 

[grober]

Didn't realise they changed the set up on the later engines in which case then its possibly item 46 you need to change? Êàòàëîã

 

[scorpie]

Solution: Oil had gotten into the ECU and this was upsetting the computer. The ECU was removed, cleaned and had it's seals replaced. Four new plugs were fitted just for good measure, it's running perfectly now, thanks to all who tried to help.

 

[kth286]

scorpie

Do you understand what you have written - 'cause I don't.

Any chance of a more detailed explanation ?

Where are the items you mentioned located, and how did the oil get into them as a result of you changing the spark plugs ?

 

[momomo]

Didn't realise they changed the set up on the later engines in which case then its possibly item 46 you need to change? Êàòàëîã

I didnt know that either. Im sure all w202 4 cylinder engines had 2 coil packs, a pack per two plugs. The OP did say it was a 202 but then said it was a 2001, maybe its not a 202 hence the initial confusion

 

[Iain]

202's can be found with a 2001 'X' plate

 

[scorpie]

Yes, I didn't understand the solution either, but that was what the dealership told me. I didn't have time to go into more detail from the garage yet, but it does add up in the sense that the computer was disrupted and wasn't working properly, ESP warning coming on, not able to read oil level through dashboard view. The year of my car is 2001, our license plates change every year and this may have caused confusion with the W202 tag.
How did oil get in, I don't know, it may have been an unlikely coincidence but either way the seal around the Electronic Control Unit was replaced and the car is working perfectly since.

 

[Neebo]

JUST A thought........On some 111 engines in a 203 the oil leaks from the variable valve timing device into the loom,usually damages the o2 sensor first then makes its way to the air mass and ECU,Never seen it on a 202 though ?????????????

 

[scorpie]

Latest update: It was not oil that leaked into the ECU as reported but water because a large black plastic cover for deflecting water safely was missing, the water go in to the ECU while the car was being serviced as it was outside while intermittent showers came and went.