Engine backfires at high rpm

[dean1roberts]

I have a 98 k1500 5.0 305 recently got it replaced the distributor cap and rotor along with the Ingnition control module
It has a misfire on cylinder 3 and backfires without load at 70% to full throttle under load around 50-60% so around 2500 to 3500 rpm. The rpm gauge flutters as it drives as well. Just need help with the diagnostic as you can’t get specifics anywhere else. Thank you

 

[dave s]

Start with the easy stuff. I would try and eliminate the misfire on cylinder 3 first. Did you put new plugs in? Check plug wires? You could run a compression test on that cylinder as well.

 

[dean1roberts]

Haven’t done the compression test but I have swapped out the wires and plugs probably do the compression test in the next couple days

 

[pressureangle]

Double check you got the plug wires on the correct cylinders.
I drove my old 318 Dodge pickup a thousand miles with 2 cylinders swapped, never ran bad enough to make me look until I got home...

 

[dean1roberts]

I’ll look over them again I was thinking it could be some sort of cross fire but I did check them not to long ago

 

[Schurkey]

Can't rule out restricted exhaust when the backfire is at heavy throttle.

Plugged converter, damaged exhaust pipe, worn cam lobe/lifter so the valve hardly opens, etc.

An exhaust valve that doesn't open isn't really going to show up on a cranking compression test.

 

[dean1roberts]

I will do one within the next two days and let y’all know what happens