gm 6.2l cylinder 8 misfiring only at idle

[Serge19]

How did you check the valves? Did you measure rocker arm travel compared to a good cylinder?

Just visual check, they were moving fine like others.

 

[clinebarger]

Yes, there is vacuum leak at valve covers, I’m planning to change the gasket for it, not sure how valve cover vacuum leak can make that difference. Please advice

The only vacuum leaks the valve covers will cause is PCV related which will lean all 8 cylinders....Not just one.

 

[clinebarger]

Do you have a way to perform a Leak-Down test?

 

[Serge19]

Do you have a way to perform a Leak-Down test?

Yes, smoke test

 

[Serge19]

Do you have a way to perform a Leak-Down test?

What you think, causing this issue ?

 

[mattd]

have you done a dynamic compression test? Your best bet is to apply compressed air into the cylinder with both valves closed and see if you have any leakage.

 

[Serge19]

have you done a dynamic compression test? Your best bet is to apply compressed air into the cylinder with both valves closed and see if you have any leakage.

But my question is, if there is misfire because of compression issue then why it’s acting on cold start or idle speed.

 

[mattd]

Static compression can be normal but you can still have a dynamic issue. Idle misfire is likely from a valve train problem, whether a valve not fully opening (cam problem), excessive carbon buildup, etc.

That is why I asked you if you actually measured the rocker arm travel instead of just watching it.

 

[Serge19]

Static compression can be normal but you can still have a dynamic issue. Idle misfire is likely from a valve train problem, whether a valve not fully opening (cam problem), excessive carbon buildup, etc.

That is why I asked you if you actually measured the rocker arm travel instead of just watching it.

Thanks bud, I’m not that much technical to measure all this, I’ll take it to my mechanic and let him do it for me. Thanks

 

[wag123]

Static compression can be normal but you can still have a dynamic issue. Idle misfire is likely from a valve train problem, whether a valve not fully opening (cam problem), excessive carbon buildup, etc.

That is why I asked you if you actually measured the rocker arm travel instead of just watching it.

mattd has a valid point. I remember when the Chevy 305 V/8s had a widespread problem with cams going flat (yes, I'm old). One of the first signs of this problem was an idle miss.