Okay, I am totally baffled, the truck in my signature has a miss on #3 cylinder. I checked codes, nothing in CM or KOEO. Checked compression, #2 95psi, #3 90psi, #4 95psi. Replaced wires, cap and rotor, moved plugs around but still no spark on #3. It will spark when engine is first started and is at high idle, then after about 15 seconds spark quits. Checked spark with timing light have spark on all other cylinders. I'm just about out of ideas.

 

It must be lower in the distributor, the part that fires the individual cylinders.

 

Do you have a misfire, and that's what prompted the investigation? 95 psi static compression is pretty dang low. I had a destroyed top ring and damaged piston develop 90psi with a little oil..


If you plug a spark plug in #3, ground it, and crank the engine is there a visible spark?

 

Just so I understand what you did... you put the timing light lead on each wire and the light flashes? Have you swapped plug wires from one cylinder to another and the miss didn't move to the other cylinder? Have you done as Goingboarding suggested and checked for spark at the plug? It could be that a gap opens up in either the plug or wire when the part gets hot.

88n94:
I'm curious what part would be "lower in the distributor" that fires the individual cylinders?

 

Most likely in the PIP or stator(which I don't claim to understand). With all the testing wag53 did I would say the next step would be to try a different distributor. I don't see any other solution but maybe there is one.

 

Okay, rechecked compression with another tester, tested all 8 got high of 125 and low of 120 guess 40 year old tester has finally died. Put plug on #3 wire and grounded it very weak (yellow) spark in air, pulled dist. cap and found approx. 1/16 play in shaft I think that might be my problem. Any recommendations for good dist.

 

I would think that a worn wobbling distributor shaft wouldn't just cause a problem in one clyinder. I'd think the problem would jump around to different cylinders. Are you getting the same weak spark at other cylinders? Only the coil generates spark energy so I don't know how it could only be cylinder #3 that has weak spark unless there is high resistance in that wire.

 

I wouldn't have thought so either but, the other 7 have a good hot spark, took it on the road and it shows the miss very obviously. also new plug wires as that is what I thought was the initial problem.

 

Have you swapped the wire around to make sure it isn't the wire that is the problem as has been suggested? What about swapping the plugs? I don't see how the other 7 would have strong spark but not the one, unless the problem is with rotor, cap, wire, or the plug itself. Have you verified wires are going to the right cylinders and are routed to avoid crossfire?

 

If the bad spark stays on one cylinder, then the problem is with what supplies spark to that cylinder.

Since you changed the plug and wire, the next logical place is the distributor cap. I have seen the wire insert in a cap so corroded it was completely gone. Could also be a crack in the cap or the contact point on the inside corroded, burnt, or broken.

 

He did change the cap, read under #1.

 

I must be having reading problems today. I read his post twice and didn't see the cap part. Read it just now and there it was.

 

Problem cured! Pulled distributor and this is what I saw.

The timing window (for lack of a better term) was rubbing on the pip. Went to pullapart and found another distributor body, cleaned and lubed it, put my original pip in and reassembled. All is well, Big thanks to 88n94 for pointing me in the right direction.