[CustomCab66]

Been driving my C-10 around town a lot. There is a nuisance random misfire at idle that i have been trying to clear up.

I determined that it is coming from cylinder #5 and it is hit and miss. meaning the cylinder is not dead all the time. The misfire is usually only noticeable under hard throttle. I really haven't worked hard at solving the issue because i figured when i did the top end a valve seal or valve seat would be the fix.

Once i completed the top end: New cam, New lifters, new push rods, new rocker arm trunions, new valve seals and springs, I figured I would take another look at the misfire. Referring back to my notes and this is where i'm at.

Checked the PCM ground wires for C1 (blue) pins 1 and 40 and C2 (red) pins 1 and 40 to the chassis they have 0.02 ohms.

With the computer plugged in I checked the following

Coil plugs.
Terminal A ignition 12vdc
Terminal B trigger from PCM
Terminal C Low circuit reference 12vdc with key on to terminal A
Terminal D chassis ground.



Checking all the coils on left bank with key on they all the pink wires (ignition power) have 12.48VDC

Checking the grounds on the left bank coil pack (wire D) to chassis on and all coil plugs have .4 ohms.

Checking voltage on the right bank coil packs terminal A to chassis ground with key on and they checked out at 12.44vdc

Checked the grounds on terminal D on the right bank and across the board they were .4

Checked low signal reference Terminal C to terminal A both banks and I have 12.41-12.42 VDC. Also checked the low reference wire back to the PCM and it had .4 ohms resistance so that is good, also not shorted to ground.

Checked the coil triggers from the PCM to coil for cylinder 5 and they were good @ .4ohms resistance and it was not shorted to ground.

Checked the PCM trigger for the injector 5 and it was good .4 ohms resistance. Not shorted to ground.

Again checked the ignition voltage at the coil and the injector with the key on. Now it was 12.6 because there was a fresh charge in the battery from running the engine.


Swapped the injectors from #5 and #1 the misfire remained in cylinder 5. The misfire is intermittent or random.

Swapped the coils between #5 and #1 and again the misfire did not follow the coil. Misfire is still intermittent or random. (completely dead cylinder)

Surely it couldn't be this simple right so i swapped the spark plug wires between #5 and #1 still the same.

All this is done with a brand new set of AC Delco Platinum plugs in it that I installed the day before. (just in case an insulator was cracked).

Completed a compression check on the left bank 1,3,5,7 and all the cylinders are within 5 psi of each other 165-170 lbs which rules out a piston ring issue.

I’m leaning toward a bad PCM at this point.

Might as well check the vaccum at the intake to see if there is a bad intake valve leak. I doubt it because the compression is good.

I went ahead and bought a used PCM that i found on facebook so that’ll be $70 all together plus shipping.

To Be Continued.