Help me diagnose this misfire please!
Discussion
Our 2009 petrol Merc A150 has developed a misfire under load at low revs. There are no OBD error codes stored.
Can anyone suggest a logical sequence of activities to diagnose what's wrong?
I'll check the plugs look healthy and are gapped correctly, but assuming they are fine, all I can think to do is replace them, the coil packs and the plug leads. Anything else I should be doing?
Can anyone suggest a logical sequence of activities to diagnose what's wrong?
I'll check the plugs look healthy and are gapped correctly, but assuming they are fine, all I can think to do is replace them, the coil packs and the plug leads. Anything else I should be doing?
AER said:
You probably need to run a sustained level of misfire to set a diagnostic code. If it's not cat-damage level of misfire, it won't set until at least 1000 engine cycles which, at low speed, is quite some time.
Alternatively you could pull the injector leads in pairs (1-4 or 2-3) and see if the misfire is still there. It might help narrow which cylinder pair. You might also need to pull the lambda sensor to prevent it running rich. It will be a P-code disaster, but ignore that and clear them later.
The misfire is very intermittent, only seems to do it when load, throttle position etc. is just so. I wouldn't be able to get it to do it sat still at idle I don't think. Which reminds me, another symptom the car has is a very slight stutter at idle ever 5 or 10 seconds-ish.Alternatively you could pull the injector leads in pairs (1-4 or 2-3) and see if the misfire is still there. It might help narrow which cylinder pair. You might also need to pull the lambda sensor to prevent it running rich. It will be a P-code disaster, but ignore that and clear them later.
So, if the plugs look normal (no mean feat checking them on this car), then look at unplugging the injectors to isolate which cylinder pair. Just to clarify, must that be done with the lambda sensor removed and/or unplugged?
kev b said:
I am not familiar with this engine but if it has a throttle, ie not direct injection, then it might be worth cleaning the throttle body and butterfly plate. This cured a similar fault on a relatives VW.
I will do, I've had problems with a dirty throttle and MAP sensor before, resolved by cleaning, but that threw up an check engine light and error code.
I think, the plug lead on no 3 was not pushed fully on, or water had got in it or something.
Looks like it's been arcing and it's all burnt and corroded inside the end of the lead too.
Have replaced all the plugs and put a new set of leads on. Got to love Euro Car Parts on a Saturday afternoon! For anyone that hasn't worked on a W169 A Class, I can't describe how difficult swapping plugs and leads is on this car. I've changed gearboxes more easily!
Anyway, misfire cured. Result.
Looks like it's been arcing and it's all burnt and corroded inside the end of the lead too.
Have replaced all the plugs and put a new set of leads on. Got to love Euro Car Parts on a Saturday afternoon! For anyone that hasn't worked on a W169 A Class, I can't describe how difficult swapping plugs and leads is on this car. I've changed gearboxes more easily!
Anyway, misfire cured. Result.
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