shaky and coughing engine at idle

[911cedric]

Hello,

i found out working on 993s, that the 96' and 97' with varioram use to have shaky idle. I mean that idle is quiet fine, but the engine shakes on a pattern of few seconds every 15sec; exhaust coughs pretty badly at the same time.
Performances and mpg are unaffected.

i found out that this problem is pretty common on late 993s.
Without being a real threat, it's kind of annoying to feel the engine coughing and shaking when you are at idle.

I scanned many cars with this problem without pulling relevant error codes.
What drew my attention, was the varioram air injection system, and those 4 O2 sensors in the exhaust system. As the 95 never have this problem, i assumed that it was related to the additional systems they fitted on the late models.

It seems to me that without being sure about it, O2 sensors appear to be the problem.
The 95 as only one central o2 sensor when the 96 97 98 have 4, which make them more vulnerable and sensitive to emission and sensors related problems.
The problem is that when checked with a PST2, the O2 sensors values seem normal and do not trigger error codes. Replacing the 4 O2 sensors is not a cheap fix: almost $600 the 4 OEM sensors; so i'd better be sure about it.
It might also be caused by tiny vacuum leaks.

I might be interested by your experiences, if you had the same kind of problem

cheers

[911cedric]

i have to add, that those symptoms appear when engine is hot only

[911cedric]

still no comments??

i have found btw that the cars idle better with AC on; however, it's still not perfect.
it increases by 100rpm the idle speed.

i have checked myself a 96' carrera 4s, tightened the admission hoses, cleaned the ISV,
however, the problem remains.

I don't have any error code and no vacuum leaks, but i think that the o2 sensors might be lazy. It always starts to idle rough when engine is hot, while the o2 sensors start to regulate injection.

I can't see another potential cause

[Britwrench]

If the O2 sensors are reading correctly and the adaption numbers are within normal specs, then there is something else causing it.