backfiring 4.0L

japerry · Aug 16, 2007

Hey guys,

I've got a friend who has a nice Cherokee, but his engine keeps backfiring when he pushes it hard.

He's replaced the fuel pump and filter -- its stock EFI, and has a snorkel on it. He also replaced all the spark plugs.

Could the backfiring be caused from computer malfunction? He's had some electrical issues in the past but not sure what else it could be.

1 grumpy · Aug 16, 2007

see if the cat is plugged

japerry · Aug 16, 2007

heh he pulled his cat out a while back

1 grumpy · Aug 16, 2007

check the timing

blackcj71 · Aug 17, 2007

wires.

bobbed_84_toy · Aug 17, 2007

I broke a valve spring once n it backfired...alot. Could be he put the wires on wrong. Timing could be off. Timing chain and/or distributer could be sloppy which would cause the timing to jump around, blah blah blah. Swap in a V8

pokey · Aug 17, 2007

is it "backfiring" thru the carb (or t-body) or is is "backfiring" thru the exhaust ? General rule of thumb(old school) is if its thru the carb its an ignition or timing issue. If its thru the exhaust is a fuel issue

1 grumpy · Aug 17, 2007

what year is this jeep anyway?

japerry · Aug 17, 2007

1 grumpy said:
what year is this jeep anyway?

Its an '89 I believe or 90 -- its EFI though, no carb and I don't think TBI -- just stock engine.

I'll ask him these questions and see what he's done. I'm tempted to suggest he find a guy who knows a lot about these Engines to take a look, it might be cheaper than buying a bunch of different parts, and replacing things that didn't really need replacing.

The truck is in Bellingham so any local suggestions would be cool too.

junkyard jim · Aug 18, 2007

Sounds like a bent pushrod or maybe flattened cam. Cracked plug ceramic will do it too.

japerry · Dec 7, 2007

so we started it back up, and we have a little more details:

it runs GREAT for the first 20-30 minutes--then if you start pushing down the gas, it backfires and you loose all power. Once you backfire it, (flames come out of the exhaust AND you feel popping through the air intake) any time you press down the gas (even lightly), it'll backfire again followed by stalling out, or coming close to stalling.

I've also tried blending fuel, going from 87 to 92, and using 'expensive fuel' as well as the cheap stuff. No change. And its always within 20-30minutes of startup.

Oh and its an '88 with stock Straight-6, 4.0. Any other ideas?

1982cj7 · Dec 8, 2007

Check the crank sensor, resistance should be about 220 ohms. also check it cranking with your meter in AC voltage, it should be around .30 volts AC.

KarlVP · Dec 9, 2007

Open up the ECU behind the brake booster. Check the contacts for varnishing. If it is full of crap, clean it out with some brake parts cleaner, and then pack it full of fresh di-electric grease.

88 Renix XJ's are funny. Most of the problems I had with my 88, were due to bad connections in the ECU.