My car misfires under load. Usually in 3rd or fourth gear. Seems like it stumbles and if I give it too much gas there is a popping noise. I'm thinking its backfiring. I check the ignition system and coils, good. Checked all plugs and wires, thats good. Good fuel pressure. No vaccuum leaks that I could find. Not a clogged filter checked those. Any idea? I was thinking maybe a bad fuel injector but would that cause a popping noise like that?

 

Did you just "check" the plugs & wires or did you change them? How long since a tuneup?

Run some injector cleaner thru it, which might also clean out any carbon buildup that might be in the combustion chamber

 

Ya I checked them and changed them about 8 months ago. Should I run some sea foam in it I got some of that.

 

That stumbling/popping is a misfire. You may have a cracked spark plug. You should recheck your spark plugs and look for hairline cracks in the porclain or black marks where the porcelain goes into the plug. Or go buy another set of plugs. Did you use a TQ wrench for tightening them or just crank them in?

 

It is not a spark plug. Could it be the timing chain? Maybe it stretches a little and retard the timing of the valves which would explain the backfire.