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L986

Ignition timing oscillating

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Hi,

I'm trying to have a smoother idle, now seems quite rough when warm (cold is OK). Otherwise car runs good.

Checking with Durametric, I found that the ignition timing on my manual MY98 C2 oscillates between 4,5º and 9º instead of being steady at 5,3º (that I understand is how it should be).

I have no OBD codes. Injectors are recently clean and tested, fuel filter is new, AFM is new, pressure regulator is OK. Data from oxygen sensors seems OK.

Any clue about what can cause this oscillation or what tests can I do to find the culprit will be very appreciated.

Thanks,

Luis.

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If you have a rough idle chances are it is a dirty throttle body and/or a dirty (or bad) idle control valve.

Both should be cleaned and inspected.

Details have been covered here many times before.

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Hi Loren,

Thanks for your response. I forgot to say the throttle body is already cleaned and I've tried with two different idle control valves. Same results.

The ignition time must be steady at 5.3º? If so, I have something not working properly, but I have not clue about what it could be...

Thanks for your help,

Luis.

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The DME controls timing as well as the sequential injection so I really don't see how it could malfunction and not affect all ranges of engine speed.

Have you checked for fault codes? What is your MAF reading at idle and again at 2500 RPM with no load?

I suspect something is telling the DME there is a problem and/or you have an intake leak.

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The ignition timing is also used to control idle speed as it reacts faster/more accurate then trying to move a idle speed motor or throttle plate. The timing will always fluctuate to some degree at idle for these reasons.

Even though you have no fault codes, I would carefully inspect for a vacuum leak and I believe the best way is using a smoke machine.

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Thanks a lot for your answers.

Loren: There are no fault codes. I'll check MAF readings and I'll post them here.

wvicary: You're right, as for the Motronic documentation I have, the ignition is used for fast fine adjusting the idle speed, idle valve is used for "gross tune" (it is a lower speed response regulation).

However, I'll be surprised if there is a vacuum leak, because last time I looked at them, TRA1 was -0.2 and TRA2 was 0,17. I understand that in case of a vacuum leak TRA values would be at least much closer to the richer adaptation threshold (0.37), and go beyond that if the leak is important (but in that case it would be fault codes present).

Anyway, I'll try to check for vacuum leaks.

Any idea would be very appreciated.

Thanks,

Luis.

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