check engine light

good afternoon all, my 98 996 has the check engine light on. it drives fine, but the only symptom I can see is a slightly lumpy idle. I have it booked in for diagnostic but wondered if any of you guys may be able to give me a heads up? cheers chris

Hi Chris It could be lambda sensors, you have to change 2 at a time or all four worst case.

My 2003 C4S was just in at my local Indy last week with the exact same symptoms. Turned out to be lambda sensors that were badly carbonated and a dirty throttle body and vacuum hoses. All cleaned up, two new lambda sensors and now running like a dream (with no check engine light!).

Could be all sorts of things. A £15 OBD2 reader from Euro Car Parts will tell you what is wrong, even down to the correct cylinder, if it's not exhaust sensor related. Maybe O2 sensors as above, but equally could be a plug failing, coil failing or even a coil pack connector slightly loose, as one was on mine, where the rubber sleeve was on back to front, not allowing the locking tab to lock.

:yeah: With no fault code its pure guesswork im afraid .. a duff lambda sensor which the car has flagged up a fault code ( so knows it's an issue ) won't normally cause a running issue as it ignores this sensor . Don't even think of throwing a couple of lambda sensors at the car until you get the fault code read out young man ! Please also be aware .. some lambda sensor fault codes mean the sensor is fine but there is a separate issue such as maf , air leak etc .

:agree: I had a cel, with alambda senor code. It turned out to be an exhaust leak on a flexi pipe.

I got a CEL every so often and when I plugged my cheap OBD-II reading it was a Catalytic Converter Bank 1 efficiency below threshold. I cleared the code and it's not come back since. This was all around the same time my coils were damaged by a failed water pump which covered them in steaming hot coolant. Like others have said, without the actual error codes then it's all guess work.

Great stuff guys. I'll get a reader from eurocarparts tomorrow, have a plug in and see how we go. Many thanks for your help! I'll report back Chris

All. I got an ODB2 reader and it gives 300 to 303, cylinder misfire. I have cleared and idled the car on the drive (which is when the cel came on initially) and it hasnt re-appeared. I'll drive it later, but any ideas about what threw the codes up when the car was just idling? Cheers all!!

When did you last have the coils changed? Do you notice any spluttering at low revs? When my coils were on their way out idle was rough and it was a challenge to not stall the car in 1st unless I really pushed the revs over 3k!

The codes are general misfire and cylinders 1,2 and 3 misfiring .. On a later car this would be the high lift solenoid but as yours is a 98 and does not have this i would say the vario cam unit might have very worn slippers or perhaps just isn't working .

Thanks guys. Demort, is this something to worry about, or just run it and see if it pops up again? Cheers

Difficult to answer .. I've seen the wiring on the solenoids burn out and the car runs ok .. ish and owners leave it like this .. I've seen very worn slippers on the vario cam and again the cars were running ok ish .. Best guess you should be fine and as variocam replacement is not cheap and worn slippers can only be proved on a strip down then you need to be prepared to spend a few bob on it . Cam cover off , vario cam removed and inspected basically . There is a test that can be done using a tester but even with Piwis testers i find it a bit hit and miss with proving an inop vario cam . In short .. i would think so judgeing by what ive seen but as always .. caveat .. i can't say for certain . caveat in this case .. Before you make your final decision, you might want to consider the caveat in this agreement. Please also bear in mind .. this is an educated guess from me .. to prove a garage needs to diagnose it .

Thanks so much for your time. The lumpy idle would suggest exactly what youve suggested, so i'll run it and see how we go. Of course i'll update. Thanks again.

Just an update all. Once tjhe codes were cleared, car is now running great (still a slight lumpy idol, briefly until it settles). Engine light has stayed off. Thanks again for your help.