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?
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.
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.
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?
You could (carefully) left-foot brake to sustain a load condition where it's occurring, or use a gradient to help you.
Given your description at idle, it could be fuelling related and possibly caused by a slow lambda sensor increasing the rich-lean amplitude too near the lean limit of one or more cylinders. You could disconnect the lambda sensor and see if that eliminates the problem - it may cause other issues, depending on how well adapted it is.
Given your description at idle, it could be fuelling related and possibly caused by a slow lambda sensor increasing the rich-lean amplitude too near the lean limit of one or more cylinders. You could disconnect the lambda sensor and see if that eliminates the problem - it may cause other issues, depending on how well adapted it is.
Unfortunately the plugs - esp if recent - may show little or no difference.
Just had a song & a dance with a 2.0 petrol 307. Misfire at cold but fine once warmed up & usually OK for rest of the day. On a warm day usually no problem even from cold. Took a while to put the EML on, so initially no code.
My general reader gave P0300 random misfire. Plugs looked fine but changed as cheap option. OK for a week & then same issue with P0300. These are known for cam follower issues & I began to wonder if that might be the problem.
Checked by a mechanic friend with a much better diagnostic tool & narrowed to P0302 which is cylinder No2. Moving plugs didn't move the fault to another cylinder. These don't have plug leads or again they would have been moved to see if the fault followed a lead. Coil pack changed & (So far!) all OK.
Just had a song & a dance with a 2.0 petrol 307. Misfire at cold but fine once warmed up & usually OK for rest of the day. On a warm day usually no problem even from cold. Took a while to put the EML on, so initially no code.
My general reader gave P0300 random misfire. Plugs looked fine but changed as cheap option. OK for a week & then same issue with P0300. These are known for cam follower issues & I began to wonder if that might be the problem.
Checked by a mechanic friend with a much better diagnostic tool & narrowed to P0302 which is cylinder No2. Moving plugs didn't move the fault to another cylinder. These don't have plug leads or again they would have been moved to see if the fault followed a lead. Coil pack changed & (So far!) all OK.
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.
Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff