P0300 - misfire only at idle, none while driving

[Papikev252]

Been chasing this P0300 for a while now. 98 K3500 7.4L

Hooked up a scanner to view misfire data and it’s all over the place with 2 & 3 and 6 & 7 being the worst. I’ve replaced every ignition component, spark plugs gapped at .45, new Bosch fuel injectors. New crank position sensor and did a crank relearn. I did the upper intake manifold when I did the fuel injectors and also tested with a noid light to make sure they were all getting voltage. CMP is at 0, I’ve done a compression test and all were within 150-160 psi. Fuel Pressure is 60 with KOEO, and 55 when running. I’ve also replace the O2s with Bosch. It’s only at idle when the misfires occur, when I’m driving there are none whatsoever. I’ve smoke tested everything for leaks and don’t see anything leaking out. I’m at a loss, I have a scanner so maybe some insight of what to look for? I see sometimes the ECM may be an issue maybe was going to go that route.

 

[2jduenas]

Been chasing this P0300 for a while now. 98 K3500 7.4L

Hooked up a scanner to view misfire data and it’s all over the place with 2 & 3 and 6 & 7 being the worst. I’ve replaced every ignition component, spark plugs gapped at .45, new Bosch fuel injectors. New crank position sensor and did a crank relearn. I did the upper intake manifold when I did the fuel injectors and also tested with a noid light to make sure they were all getting voltage. CMP is at 0, I’ve done a compression test and all were within 150-160 psi. Fuel Pressure is 60 with KOEO, and 55 when running. I’ve also replace the O2s with Bosch. It’s only at idle when the misfires occur, when I’m driving there are none whatsoever. I’ve smoke tested everything for leaks and don’t see anything leaking out. I’m at a loss, I have a scanner so maybe some insight of what to look for? I see sometimes the ECM may be an issue maybe was going to go that route.

I’m going through same thing and wondering if you ever figured it out??? Would be great if you did. I would appreciate anything advice on this. Thanks

 

[Papikev252]

I’m going through same thing and wondering if you ever figured it out??? Would be great if you did. I would appreciate anything advice on this. Thanks

Gonna try the intake gaskets this weekend. All my live misfire data points to that possibility.

 

[2jduenas]

Thank you, let us know how that works out, happy wrenching and good luck

 

[Papikev252]

Thank you, let us know how that works out, happy wrenching and good luck

Sorry I never got back. My problem was my manifolds were warped and caused a big vacuum leak. I used a smoke tester. Hope you find your solution!

 

[Road Trip]

Sorry I never got back. My problem was my manifolds were warped and caused and big vacuum leak. I used a smoke tester. Hope you find your solution!

Sincere thanks for closing the loop with the solution for a misfire
that was resisting repair despite focused effort.

Theory is good, but real world proof showing that the theory is
valid makes a thread like this so much more valuable to others
who are researching similar concerns.

Well done. No doubt you've just added a little to your karma
bank account by your actions.
:0)

 

[2jduenas]

Sorry I never got back. My problem was my manifolds were warped and caused a big vacuum leak. I used a smoke tester. Hope you find your solution!

Thanks for the update, glad you got it figured out!