1994 Volvo 960 has an intermittent engine miss?

[the tenor man]

Hello all and thanks for looking.

I have a 1994 Volvo 960 Estate that has an intermittent miss in the engine...sometimes it runs and idles as smooth as silk...but it's the other times....

So far, I have changed out the spark plugs and repaired the positive battery cable which was corroded, but didn't solve the problem. I have also cleaned the throttle body and checked the vacuum lines for breaks, but have found none so far.

I do have the same heater vacuum motor problem as many do in that the front vents shut off during hard acceleration...I'm sure that's causing a lack of vacuum but I'm not sure if that affects the engine miss problem.

It always starts hot and cold and has never died on the road or in the driveway. Sometimes it will start and run on all six cyliners and sometimes just five. Sometimes it will start and run on five and then immediately begin to run on six and vice versa.

I did notice in a parts diagram that here are two power modules, but I don't know if one of those may be causing the problem. I don't know how to check the ignitors other than swapping a known good one and seeing if I can find the weak one...

Your help, thoughts and prayers would be appreciated...

The Tenor Man

[jimmy57]

No OBD-II diagnostics on that one to tell you which cylinder is misfiring so the investigation gets a bit harder.
If the miss will occur for a long enough period you can figure out which cylinder by unplugging injectors until you find the one that does not add an additional miss when unplugged. The change that coil.
The two power stages each control three coils (igniters) but those have been pretty trouble free and when they fail they usually are done for with no on again/off again stuff.
The power stage connections could be corroded.
I would pull the coils out and inspect the coils and also look at the wire leads to each coil to be sure the wires are not chafed or the end connectors insulation is deteriorated. The connectors can make poor contact at coil when the plastic over terminals falls apart and the terminals do not get pushed onto the coil connections by plugging in the plastic connector. The wires cna be pinched by installing the plastic coil cover.
If all this looks good then remove each coil and inspect for cracks and trails that could be where that coil has arced.
I have used a spray bottle with water to find bad coils by squirting them with water one by one.

[the tenor man]

Many thanks....

I'll check the coil connections because one of the wires did get pinched from someone who serviced the car before I bought it.

I'll also check the connectors...

Thanks again!

[the tenor man]

Problem solved...It turned out to be some broken insulation on the wiring to the #6 coil (ignitor). I simply wrapped it with some electrical tape and the miss is gone...smooth as silk again.

Many thanks!

The Tenor Man