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1991 Ford 5.0 EFI. Weak spark on 2 cylinders checked at the distributor cap.
After a distributor replacement due to no start (faulty pickup coil), vehicle has a significant misfire. I checked for spark using an incandescent test light connected to ground as Paul Danner does in one of his videos: I am checking for spark at the towers of the distributor cap by holding the test light probe next to the tower while removing the plug wire and examining spark strength. 6 of 8 cylinders have good strong spark. 2 cylinders have weak to no spark. Distributor cap was changed out for another with the same result: same two cylinders have weak spark. I can’t see any part of the ignition system that would cause weak spark on one cylinder besides the distributor cap, wires, or plugs. With all three of those eliminated as causes, what could be causing this? Thanks for your thoughts.
I suppose the only other part that could cause a problem on a particular cylinder is the reluctor in the distributor. I could see the reluctor being perhaps irregularly shaped in such a way as the ignition module is not receiving a proper signal to fire two cylinders. Any clarification on how this would happen is appreciated!
If it is a triggering issue, then you should be able to measure the spark from the coil and get 6 really strong strikes and two weak/no spark events. Which would be easier to see with a scope, but to the eye it would probably just look like intermittent spark drop outs. If the coil is consistently delivering adequate spark but it doesnt make it out of the cap, I would investigate the rotor phase and condition considering it has a new cap.
I will check spark at the coil tower, good idea. I do have a scope and can scope the waveform of the pickup coil output if necessary. Unfortunately I’m very rushed in doing this repair so that step may have to be skipped.
The car has this problem after distributor placement, so rotor is also brand new. Two brand new distributor caps have the same result. I am thinking you are right, we would see the same weak/no spark result on two cylinders at the coil. I’ll do this quick check to confirm. I think we will just be putting another replacement distributor in it.
Any ideas what could cause such a triggering issue?
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