I have an 89 4x4 auto with the 3.9, it has been back firing for as long as i can remember. only has 74,000 miles on it. but lately it is getting worse just blew the muffler off.

back fires when i suddenly left off the throttle.,
just did a tune up and gutted the cat, still backfireing though, and im only getting 11 mpg on a stock truck please help???

 

First I would say check the timing. It may be the timing chain. Or enternal engine manfuction. If it is backfireing I would not drive it, at all.

 

First where do i start with the checking the timing?? i know this has been backfireing for atleast 2 years. thanks for the info

 

start with the number 1 spark plug on top dead center and the make sure the distributor is pointing at top dead center. not sure what the timing should be, check the faq2 service manual and it should say and give extra instructions.