Strange Tach behaviour and engine issues after distributor service '85 HE v12

[Aristides (Aristides Balanos)]

@Rasmus

now there is some hissing almost all the time near the B bank due to the leak.

@Rasmus

As stated earlier the engine also seems to have a slight misfire or other issue which makes the engine ‘shake’ or stutter,

It’ quite possible that those two facts are related Rasmus.
If for example you have a leak in the intake of one cylinder, that cylinder would run very lean, probably misfiring, and the other 5 of the bank rich as the ECU will detect the lean mixture and add more fuel.

I would first sort out the leaks before anything else.

Check this recent thread, lots of useful info.

Chasing Down Mild Idle Stumble

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 co…