'93 XJ giving me fits! HELP!

Hello everyone,

So here is the long and short. I have had my jeep for many years. She is a 1993 Jeep Cherokee XJ Country. 4.0 HO with Hesco aluminum head. Engine was rebuilt about 5 years ago. Many add-ons such as borla headers and exhaust, open air intake, throttle body spacer, coil pack and wires.

The poor girl sat for about three years. My oldest son wanted to fix her up to drive to school so we started working on her a bit. A few weeks ago we took her out and romped on the gas a bit. Stripped our brass distributor gear out and she was dead in the water for a few weeks. Got that fixed, now we are having an issue that is driving us to our wits end. She fires right up and runs smoothly. In park/neutral I can run the engine to 3-4k RPMs without any hiccup or any felt/heard issues with the engine. However, once we take the ol' jeep out and she warms up she begins backfiring under load. The backfire and loss of power happens throughout the RPM range, but seems most noticeable at higher RPMs and while accelerating. I believe the backfire is out end and not through the intake as it is felt though the seat of the pants.

We have replaced the CP sensor, Engine Temp sending unit, TBP sensor and coolant temp sensor. The CP sensor seems to have improved idle, but did not get rid of the backfire and sputtering once the engine heats up. We also have replaced the fuel filter.

We need some help. Don't know what is causing the backfire, loss of power, or sputtering... these are the best ways I could describe them. I am out of ideas and do not want to resort to either junking the whole thing out of frustration or spending hundreds/thousands at a repair shop.

Please help if anyone has this experience or knows what would be causing this issue.

:banghead:

My call is the oil pump. Esp. if it's a high volume.

1.. The distributor gear drives it... too much resistance from the pump will strip the gear.

2.. high oil pressure over-pumping the hydraulic lifters, causing the backfire.

You can test run a thinner weight oil to verify.

Might be weak valve springs causing the backfire as well.

Please help me then, because maybe my understanding is flawed... IF the high volume oil pump is the cause, wouldn't pressure be reduced as the oil heats up. So the problems you stated would be present when the engine is cold, but would go away as it warmed correct?

I'm diagnosing over the internet, based on your descriptions of symptoms.. I can be completely wrong, but everything you describe leads me to a mechanical problem, not electronic.

high volume pumps are not recommended for the 4.0

Please help me then, because maybe my understanding is flawed... IF the high volume oil pump is the cause, wouldn't pressure be reduced as the oil heats up. So the problems you stated would be present when the engine is cold, but would go away as it warmed correct?

Click to expand...

not necessarily with a high volume pump. also, the relief valve could be stuck, with grindings off the destroyed dizzy gear.