Re: Misfiring just above idle (archive)

Motronics with O2 sensor control gives a very precise fuel mixture at idle. Any additional false air from a vacuum leak or as when you remove the oil cap will affect the mixture by leaning it out sufficiently to cause either a stall or rough idle. I would assume that your mixture is way to rich at idle. First check the O2 sensor voltage signal with a digital VOM. If it's reading .7 or above you've got a rich condition that the Motronics can't adjust. If you're not getting any voltage then the o2 sensor is kaput. Before you replace it make sure you didn't destroy it with non-o2 sensor safe silicone.

I would also verify proper TPS operation. Both at idle and at WOT. A rich condition could be caused by a faulty TPS switch. You perform these tests with the engine off. The 5-series Bentley manual is a great resource for trouble shooting the fuel injection on our e23's.

And last but not least check the proper functioning of the temp sensor. When the engine is hot you should be getting much lower resistances than when it's cold. Also there are engines out there that have a special dealer fix on the temp sensor wire consisting of a inline resistor. Make sure you don't have one installed on yours and that it hasn't broken off.

demet

Good thought about the vacuum leak. You are right about removal of the oil cap causing the engine to stall; I remember now that it used to but now it doesn't. Now if you take the cap off, it idles OK. There is little or no blow-by from the open oil cap when the engine is running. How can such a giant vacuum leak not bother the engine?

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