RussB

I've been trying to locate the source of an intermittent WOT full load stumble for weeks. The engine runs fine under light to medium loads.

The steel breather pipe (approx 3/4 inch o.d.) that conducts vapors from the bottom leftside of the crankcase to the cam cover was intermittently touching cylinder 5's spark plug wire insulation.

I wasn't especially concerned, as I reasoned, "both ends of the pipe are insulated from ground by rubber couplings". Then on Saturday, for grins, I insulated the pipe with bicycle innertube and dressed the spark plug lead to keep it away from the pipe. Voila! No more stumble.

A friend pointed out, "The inside surfaces of the rubber couplings are coated with (conductive) oil." . . . so the pipe was closer to ground potential than I thought. D'oh! Apparently the resistance through the pipe and rubber couplings' insides is fairly high: under normal conditions the spark plug gap offers a lower resistance to ground. Under full load conditions, though, combustion pressure rises and the resistance to ground across the spark plug gap becomes higher than the resistance to ground through the breather pipe, so that's the path taken by the spark current rather than through the spark plug.

I've added 1/4 inch i.d. split loom insulation to #5's spark plug wire in the vicinity of the breather pipe, and added more electrical insulation to the breather pipe. I'm gonna fabricate a simple standoff on an 8 mm bolt with 3/4 inch Adel clamp to position the breather tube so that it's well away from any spark plug wires.

I wonder if others here have experienced this problem?

Car: 1988 5000CSTQ

CARSNPLANES

Russ,
I can't attest to the same experience with the breather pipe and evidently the experience was real with you, however, repectfully disagree with the notion that just because we know that the combustion pressures are high under full load conditions that anything within a small distance from any wire is going to jump a spark. I'm tending to think that you may have a bad wire or a gap that is already too wide (worn plug) or some other high resistance problem. Most wires if not all, are designed to hold the spark in with it's insulation, even when sitting right on a metal piece. Of course there are better wires that offer more insulation than others and they are meant for the high energy type systems. In the loom, wires area actually sitting next to one one another and if everything is right, gap, no resistive corrosion, weak or bad insulated wires, there should be no concern of spark jumping or, "cross cyl firing". I don't know how old your wires are or the gap of or age of your plugs, but i'll bet you could possibly be experiencing a resistive jump or leak. Yes, the spark is trying to find a way out,and yes, the closest thing was your pipe. But if everything was right, that shouldn't happen. Insulation does break down after a while especially if it's tryng to send a spark down a wire to a plug that is too wide or tryng to pass through a very corroded plub connector, or just a defective wire.
I'll bet if you changed your wires and rechecked your plug gap, that the miss, even if wires were resting on the pipe, would go away.

boxerfan

If the wires are old just fire up the car at night or in a dark garage and watch for the light show.

Anyway, it's a good idea to keet the wires away from possible alternate paths to ground. Good work.