4.3L Backfiring under acceleration

ONLYGODKNOWSWHY

MEMBER
CHEVROLET TRUCK

88 C1500 halfton - 4.3L 5spd standard- currently 211K bought with 203k

Had the Timing Chain and both sprockets changed, plugs and wires changed, set the timing, runs great idling can rev it up as high as you want runs fine. Then you take it on the road pretty much from 2nd gear on it just backfires, sounds like it wants to choke out. If I drive it slow in 2nd or 3rd it doesnt seem to mind, but when I give it gas it just backfires like crazy.

A little background which may help determine, the wife wanted to drive back from camp, never driving standard on highway before, and while going to downshift from 5th put it into 2nd at 110Kms/hr (70-75m/hr im guessing). I dont have a tac but im sure it went from 1500rpms to about 8000-9000 for about 5 seconds, before she got out of panic mode and pushed the clutch in. Right after that it started backfiring.

4 Replies

TOBYU

MEMBER
89 POSTS

Unfortunately, it sounds like the high rpms caused the problem.
A piston probably banged a valve when over-revving.
It probably bent the vavle head stem and is not sealing now. It would be an intake valve if it is popping thru the intake plenum.
It could have put a hole in the pistion too. If so. You would have a LOT of blowby out of crankcase breathers and oil fill hole if you take it off while running.

It probably just has a bent valve.
It is possible it just bent or jammed a pushrod.
Find out whick cylinder in doing it.
Block the wheels good. Hold the brakes on and parking brake too. Load the engine down while giving gas to get it to do it constantly.
Power braking is more difficult with a stick shift.

When it is popping. Take one plug wire off at at time. When you find the one that is doing it. It will stop backfiring as completely or a lot less.

That will tell you which side. Turn it off and get the tool box out.

Pull that valve cover off and look at the rocker arms on the popping cylinder.
You can loosn the nuts and take those two push rods out to check for a bent one.
If all looks good there. You will have to pull the intake and at least that head to get at it.

You can also do a compression check on all cylinders to find the popping one.
Kill the spark. Hold throttle wide open.
Crank it to highest reading.5-7 hits.
You should have about 140-150 psi on all cylinders. If you have a bent valve. You will bet less than 40 on that one.

Good luck with it