Ok ladies and gentlemen, I have been having a strange, intermittent misfire on #7 cylinder. Its a 5.3 engine with 241 heads and a 210/220 cam. The thing ran fine for years until about three days ago. It started misfiring suddenly on the #7 cylinder under low rpm, high load conditions. When the torque converter locks it will drive fine for a minute or two and then start to miss. It will do it bad enough to cause the torque converter to unlock. When the converter unlocks it will miss for a few seconds then stop. If I continue to drive the converter will eventually lock again and the whole process will start over. When it first started it would miss bad enough to cause the check engine light to flash. Originally, the scanner was showing misfires on #7 and #5. I think that the misses on #5 were just ghost misses due to 7 being so bad. So I changed the coil on 7, and all 8 spark plugs. Also swapped a plug wire from the other side to 7. The problem persisted.

So I pulled off the fuel rail and found the #7 injector to be slightly corroded, but not bad. But I changed it out with a new one anyway. I also cleaned the fuel rail. The problem got better, but still persisted. However, the misses on #5 ceased. So I then changed the plug on the #7 injector to a new one. Of course, the problem is still there.

Its better now, but its still there. The miss now is not bad enough to cause the check engine light to flash, but it is still bad enough to unlock the converter. It is still bad enough to feel a loss of power however. My mechanic friends and I are at a loss. We are wondering now if perhaps the injector driver for that injector is getting weak in the computer. It seems unlikely, but I know it is possible. Has anyone experienced something like this?

To recap, stuff that has been changed or checked is:
plugs, plug wires, injector, fuel rail, coil, valve springs.
We have not checked compression due to the fact that it runs perfectly fine when it is not missing. We will try that possibly tomorrow.
Thanks for any help. Sorry for the long read.

 

try changing your coil pack to that cylinder to a different one? see if it moves cylinders. i chased a misfire like that for a while and found that i needed an injector. the one cough... cough... autozone sells me has 2 nozzles and a different part number and though they say its the same lb/per hour it still had a stumble. went the the dealer bought one and what do you know it went away.

 

Well, I put a brand new coil on that cylinder. I did buy the new injector from autozone. It appears to be exactly the same as my stock one. But if I can't figure it out I'll try a dealer one.

 

have you thought about maybe swapping an injector around and seeing if the misfire moves?

 

Yea like said above, just move injectors and coils to see if the problem moves to another cylinder. Don't keep buying new parts if it's not needed.

 

I haven't swapped injectors around, but I have moved coils and that makes no difference. Also moving plug wires does nothing either.

 

oh i actually jumped out of bed as this hit me while i was falling asleep. try checking for a vacuum leak around the intake manifold. i have heard these intakes can start leaking by the o-ring and causing misfire issues.

 

Well we did check for leaks as best as we could. Sprayed carb cleaner around it and didn't see anything. But I may pull the intake off and check it.

 

Do you still have catalytic converters?

Years ago, I had an intermittent #3 misfire. It wasn't anything noticed during driving, but the PCM picked it up. After exhausting all of the fuel system & ignition system ideas, I consulted a friend of mine. I learned that a partially clogged catalytic converter will "pick on" the weakest cylinder when it's a problem in it's early stages. So I did a backpressure test. Sure enough, my left side cat had 1.5 psi backpressure at idle, and 3 psi backpressure at 3000 rpm. I ordered a new y-pipe from the dealer, and it fixed the problem.

 

I have seen this as well before myself.