Miss fire at # 5 cylinder only under load

[[email protected]]

Any ideas? tried new coil new injectors switched ignition wires checked and changed crank pickup.
All checked out but miss still there only under load
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[honkon]

Compression test?

 

[Schurkey]

WHAT MODEL YEAR?

WHAT ENGINE?

tried new coil new injectors switched ignition wires checked and changed crank pickup.
All checked out but miss still there only under load

Faulty spark plug?

Faulty distributor cap?

Vacuum leak into intake manifold, perhaps from a vacuum port on the #5 runner?

 

[[email protected]]

1997 chevy Tahoe 5.7 checked all of these new plugs and checked resistence before installing
Checked cap no problems. and vacum is correct .
I am really stumped

 

[L31MaxExpress]

1997 chevy Tahoe 5.7 checked all of these new plugs and checked resistence before installing
Checked cap no problems. and vacum is correct .
I am really stumped

Check out the GM TSB for sticking valve guides. I had a similar issue with one of these engines only while towing. Ended up having 2 exhaust valves that had the valve to guide clearance so tight they would stick when the exhaust valves got hotter than normal under load.

 

[[email protected]]

Brand new heads also . I had cracks in both. Not because of any overheating or anything.
They were a set of remanufactured heads but must have got passed inspection .So I bought a new set from GM.

 

[stutaeng]

I would do a compression test next. Might reveal a blown head gasket, but doesn't sound like it. Did it just start doing this?

Could it be an injector causing misfire?

 

[stutaeng]

I would do a compression test next. Might reveal a blown head gasket, but doesn't sound like it. Did it just start doing this?

Could it be an injector causing misfire?

 

[[email protected]]

Compresstion is perfect, Did new injectors also and check out
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[Eveready]

There was a thread here sometime back about a distributor getting out of kilter under load. I think it was a bad bearing or something and the truck would run rough when under load but at no other time. I wonder if you don't have early symptoms of that issue. I have no idea how you test for it but surely someone will have an idea. Good luck!