I’ve been chasing down a mild misfire on my 1988 V12 with 50K+ miles. I know it’s a common complaint here, but bear with me.
It’s not a total misfire, it’s the kind where the engine vibrates ever so slightly. The RPM, fuel pressure and vacuum (17 inHg) all hold perfectly steady at idle. It happens in Park at 900rpm or in Drive at 750rpm. Happens when cold or warm.
I’ve spent a lot of time ruling out ignition, fueling, and timing.
My guess is now valves because a few months ago, I did a compression test (engine was cold though). All cylinders were 180-195 dry. Wet compression test, all cylinders jumped to 225-300 except 3A and 6B remained at 180. They did not budge.
This leads me to believe either the exhaust or intake valves on those cylinders are not closing perfectly. The car did run rich for years, I believe, and original owner I believe didn’t put a lot of miles on it. So there could be a lot of carbon build up. How common are burnt exhaust valves on the V12? And is there anything I can do for now? Unfortunately, the car is not highway ready for an “italian tuneup”.
Also, if spark plugs were not seated properly due to bad threads, would that explain my wet compression test results? Or would I not even get 180?