Small Backfire and Low Compression on Middle Cylinders

[Acgallen]

I recently did a full head gasket replacement to fix it blowing white smoke out of the exhaust, which worked. Now after putting everything back together ive noticed a few small problems. I bought the truck knowing it ran but i never layed on the gas because of the bad head gasket, so not sure if the backfire was from the fix or it was from before. I only notice the backfire when i really give it gas, and its a low popping noise with the occasional loud one. I did a compression test on all the cylinders and found 4 were at 160-165 and the middle 2 were at 140-145. The middle drivers side spark plug was completely black and not sure what that's from either, as all the others are brand new clean plugs. Any help is appreciated, i can try to give more info but i only know so much.

 

[rusty ol ranger]

140-160psi spread isnt bad. 20% is generally the rule of thumb.

But the backfireing is concerning...did you take a good look at the valves when you had the head off? Did you torque everything correctly?

 

[Acgallen]

They looked fine when i had it out, made sure everything went back the same. Torqued everything to manufacturing specs and the right pattern, thats why im scratching my head on this one.

 

[rusty ol ranger]

Is it backfiring out the intake or exhaust?

If the exhaust is popping you may have a rich condition. If youre getting to much fuel the popping noise is raw fuel exploding in the hot manifold.

If its coming out the intake you might jumped time.

 

[Acgallen]

Not quite sure, how would be the best way to test that?

 

[rusty ol ranger]

If that one plug is black that means that cylinder is rich as hell. Either a stuck injector dumping fuel or its not getting good (or any) spark.

 

[Acgallen]

Ill definitely check that out and see if that fixes the issue

 

[Acgallen]

Cleaned out all fuel injectors, didnt fix the backfire but it runs a little better