What would you check next? Missfire on my 98

[Tom]

I'm having a problem with a cylinder 7 missfire. It only happens when I've been accelerating mildly for a long time, like an uphill freeway ramp to 80mph. Eventually #7 starts missfiring, which gets worse the longer you accelerate. Let off the gas, missfire is gone.

-I started by swapping the plug and wire from another cylinder.
-Tested compression.
-Then I changed the cap+rotor
-Then I replaced the fuel injector.
-Finally I plugged in an OBD2 and watched fuel trims while spraying around the intake to cylinder head surface area with carb cleaner.

I've eliminated everything that's cylinder 7 specific. Driving around with the OBD plugged in, I can see its only cylinder 7 having any miss fires. I'm at a loss of what to check

[AussieinNC]

"Tested compression"...what were the results and cyl to cyl variation?

Does the truck have cats?

Where is the vacuum port for the power brakes drawing vacuum from? Port runner 7?


The fuel trim on these can cause a cylinder to lean out.

Did you use a HEI rated spark check tool?

[biketopia]

The fuel injector spiders on those also fail and will cause a miss fire, especially under load. My dad's truck had a similar problem, had a shop near him replace it and then he was back on the road. Occasionally after that it would get a misfire code and run rough. Found the timing was not exactly right, re-set it with a scan tool per ALLData and it cleared all the issues up.

[Tom]

Originally Posted by biketopia

The fuel injector spiders on those also fail and will cause a miss fire, especially under load.

That's why I replaced the #7 injector, although to be fair the entire fuel injection [conversion with mini injectors] is only 1yr and about 20k miles old.

Originally Posted by AussieinNC

"Tested compression"...what were the results and cyl to cyl variation?

Does the truck have cats?

Where is the vacuum port for the power brakes drawing vacuum from? Port runner 7?


The fuel trim on these can cause a cylinder to lean out.

Did you use a HEI rated spark check tool?

1- It was 147psi. I test two other cylinders on that side and got 137 and 140 in those. So the misfiring cylinder was the strongest.
2- yes its bascially all stock besides a muffler
3- It draws from the fitting in the back middle of the intake manifold specifically for brakes...
4- No. I merely swapped the wire+plug from a non-mis firing cylinder, then did the new cap and rotor.
5- If it was fuel trim wouldn't the other cylinders have issues?

[AussieinNC]

My next move would be to replace the plug lead on #7 with a brand new one.

I would also be testing the spark with a tester designed for HEI ignition systems.

Finally, and maybe you should check this first...do you have the firing order correct,,,,,it is so easy to mix up #5 and #7 leads...

I have witnessed bad fuel trim readings on worn engines especially on these older systems. I have also witnessed a failing cat causing all sorts of backpressure issues under sustained loads like your example.

"biketopia" has a very valid point in the ignition timing must be set correctly....plus any vacuum leaks anywhere will play havoc with the injection system.

[leegreen]

I'll throw this out there. Can you actually feel/hear misfire?

Force the computer to relearn the crank position sensor correction factor.
If it is not accurate it can misidentify the cylinder or even the misfire condition. It has to know accurately when the power stroke from each piston is due to detect misfire. if you changed anything in the rotating assembly, sensor or computer itself the correction may be off.

Crankshaft Variation Relearn is a feature of some GM scan tools. I think the redneck way is to disconnect battery for a while, then drive around being sure to do some long decelerations from speed without brakes.

But I haven't done this myself, I know just enough to be dangerous, YMMV etc