My car's been running rough. I finally found a big clue.

My number 5 cylinder fails to fire sometimes. Using a timing light, I was able to see that there is no spark whenever the car starts hesitating. I checked all the other cylinders and they all seem fine. I tried swapping the wires and the spark plugs and the problem didn't change. The cap and rotor appear to be in good shape.

If the problem was random among all the cylinders I might think it was the ignition coil, pickup coil, or the ICM... but just one cylinder? I mean it seems like it fire ~75% of the time....

So now I'm scratching my head. What could cause just once cylinder not to fire sometimes???

 

I take it the plugs are good too? Gaped good and not black. Should be a nice tan color.

When does it do that..at idle, under acceleration??

 

Check the distributor for looseness, sometimes that happens after alot of mileage. Remove cap & try to wiggle the rotor, it'll wobble alittle if its loose.
While you have the cap off look for arc trails or carbon tracks. Might be a defective cap & rotor.

 

My bet is on cap, its the only thing other than plugs and wires that can be specific to just 1 cylinder.

 

I guess I'll never know for sure... I ended up replacing the whole distributor, including the cap. My car runs okay now.

 

The only thing that can really go bad in the dizzy itself is the pickup coil, and its connections can corrode, but since the dizzy fires all 6 cylinders without using cylinder specific ones, I can assure with pretty good certainty it was the cap, but probably didn't hurt to replace the dizzy considering the age of it.

 

The bearings can go bad causing a wobble, not common, but its a place to look at on a high mileage engine.