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Misfiring upon cold startup

J

JRW

New member
Hi there - your advice would be greatly appreciated please!
I have a MY03 'S' with 28k miles. When I start from cold I sometimes find that the engine appears to misfire - almost as though the firing is in the wrong sequence. The engine is rough and lumpy and will not idle smoothly. I usually switch off immediately and wait a few minutes. More often than not, when I come to restart all is fine! The car has not long had a major service with new plugs, and apart from this intermittent problem, otherwise runs fine. Any ideas what might be the cause?

My thanks for your help. Jeff.
 
Hi

I have had the same problem when cold, even after changing Plugs, air filter etc. It could be the spark coil(s) being cracked. But the car has only 25k on the clock, and when changing spark plugs did not see any cracks. My car also stalls when cold , and only "smooths" out after about 1min fast idle.

Possible causes ? :

1 Air Leak in manifold - check all clamps and no leaks from MAF to Throttle Body. I did this - made no difference.
2 MAF ! - I cleaned it out, looked like new - no difference. (Faults would also show on MOT emission , but this was perfect).
3 Try disconnecting/reconnecting the battery. This may reset ECU - usually only temporary solves the problem.

I would also like to know as it is a real pain when cold !



 
5 years old might signify one of the coil packs starting to crack and fail - does it happen more in damp weather ?, and does the issue go away once you drive off ?

Coil pack failures normally result int he engine check light illuminating on the dash though.
 
Have not noticed that the problem varies acoording to the weather. In terms of it going away once I drive off, I tend to let the car recover before setting off. The misfiring/rough idle usually settles down within a few minutes then runs fine after.

Thanks for your help guys - you've given me some good clues, and I'll keep you posted when I get to the bottom of it. Jeff.

 
I agree, check the coil packs. Suspect that age is more of a factor than miles for these. Had one replaced on my 986 that was noticed to be cracked at a service - the others were fine. I never had a misfire so can't say for sure, but the veiw was that if it had been left, then it would have.
 
Coils would also be my first suggestion. I have just replaced all the coils and plugs on my '99 3.2 and it made a huge difference to the idle and also responsivenes of the engine in general. The check light was coming on intermittently and Coils 1 & 3 faulty were diagnosed and the light went off. When I changed those 2 I noticed the rest were like blobs of rust but were not triggering the light. If you have the cash spare I would advise changing them.

If the problem only happens on cold start then goes away it might be the cold start system firing in too much fuel - possibly water temp or air inlet temp sensors. These won't necessarily show up as a fault if they are within the correct range but giving out an incorrect value e.g. the car thinks it is minus five and prepares for a cold start but it is actually plus ten.

Hope this helps
 
ORIGINAL: dougster


If the problem only happens on cold start then goes away it might be the cold start system firing in too much fuel - possibly water temp or air inlet temp sensors. These won't necessarily show up as a fault if they are within the correct range but giving out an incorrect value e.g. the car thinks it is minus five and prepares for a cold start but it is actually plus ten.

Hope this helps
Click to expand...

Are these easily accessible to change? Mine only misfires on cold start up for a minute or so. Lots more exhaust smoke than normal too and worse when very cold i.e. when it was indeed -5 a week or so ago. As soon as I'm moving the misfiring has gone. Has never had new coils though.
 
If you can get under the car (and you know what end of the spanner to hold) it is very simple and you should be able to do it in less than an hour.

2 allen bolts and a multiplug per coil and they just slip out.
 
ORIGINAL: dougster

If you can get under the car (and you know what end of the spanner to hold) it is very simple and you should be able to do it in less than an hour.

2 allen bolts and a multiplug per coil and they just slip out.
Click to expand...

Sorry Dougster, I meant the access to the temperature sensors you mentioned.

I've decided to do the coils first (and spark plugs while I'm at it) because the car is 8 years old, so they're definitely on borrowed time. Hopefully they alone will fix the issue but I'm not banking on it at this stage so knowing where to find the temperature sensors would be useful. Cheers.
 
Sorry mate, I never read your post properly.

The air temperature sensor is on the top of the right hand air distributor (the plastic bit of the inlet manifold).

The coolant temp sensor is underneath the car near cylinder no 4 (front left).

Check out http://www.cannell.co.uk/Manuals.htm for more info.

If you are not sure what is wrong, changing sensors in the hope it fixes the problem can get expensive.

Try and find a good local specialist that can interrogate the ecu with the car running and interperate the output. Alternativley let the problem develop until the ecu sees a fault and lights the lamp.
 
A quick update to say that after changing all 6 coils (and plugs) my starting issues have been resolved. All the coils on the left bank were cracked while the ones on the right looked pretty good. The new coils (which I got a good discount on from my local Porsche centre) have a 997 part number that supercedes the previous ones for a 986.
 
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