Low, rough idle on 4.2l
 

Ok so all of a sudden my idle is around 500rpm and its running a little rough with a noticeable vibration throughout the car. Sometimes after driving a while it will idle at around 750 but eventually drop back down. I have had the timing belt and both cam chain tensioners replaced withing the last 2-3 months but this became most noticable after the drivers side tensioner pads were replaced. The day of the replacement I was low on gas and the car had a blinking CEL and ran extremely rough, it threw codes for misfiring on all cylinders. The cel went away and didnt come back and the vibration almost disappeared but not completely.

Any ideas whats could be causing this?

BUMP hoping for an idea. Ive been running audi fuel additive for the last few days and it seems to be helping, a bit but it still drops on and off.

i will wanna take a look at my fuel filter and may be the injectors which may be clogged as a result of running too empty. pay attention to your vacuum lines as well

I thought about vacuum first because I recently installed a manifold spacer, but vac leaks usually surge this is very steady just low and rough. I Just came back from a decent drive with more of the Audi fuel additive in there, I held gears for a few seconds at a time to open up the injectors and its been idling smooth and fine for hours. Im still gonna replace the filter when I change the oil next week.

manifold spacer?
 

Tell me more about this....

OK, so I have decided to bring this back and see if anyone might have any ideas. I determined some months ago that the idle drops when the a/c is not running. If the blower is off or set to a temp well above the ambient temp the idle is around 500, if I lower the requested temp till where the a/c might kick in the car idles at 750. I had a broken vac line a while back that I repaired and it made no difference.

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