what can cause exhaust backfire?

91 jeep 4.0 HO manual tranny.
loss of power and a loud exhaust backfire when engine is warm. thanks
nathan

Most backfires are from unburned fuel, leaky injector maybe ?

A crack in the exaust manifold or header gasket will also cause it.

Does this begin as soon as the engine reaches operating temperature, and is it consistent?

If so, I'll guess O2 sensor. When the engine is warming up the O2 sensor is ignored and the ECU operates according to a hard-coded, pre-mapped fuel curve. When the engine temperature reaches somewhere around 170 degrees, the ECU switches to "closed loop mode" and starts adjusting the fuel mix according to various data input, including the O2 sensor. If the O2 sensor is weak, it doesn't detect unburned hydrocarbons as effectively as it should, so it allows the engine to run rich.

How old is your O2 sensor? They should be replaced every 75,000 to 80,000 miles.

It only seems to happen after it has been running for a while (30 min +) then does it every 10 mins until I stop and let it cool down. I just picked up the MJ and don't know if the O2 sensor was changed. I will get one today. Thanks
Nathan

Might be easy. Unburned fuel is what's causing the backfire. Any incomplete burn or misfire in the cylinder will mean that unburned exhaust gas can ignite in your exhaust system. That's why it doesn't happen until the car (and the exhaust system) is warmed up! A worn rotor, bad spark plug and/or bad spark plug wire can all cause the same symptom. Start cheap & easy --for only about $40-50. Buy a new distributor cap, rotor, plugs & wires and replace 'em. That fixes a lot of these issues (and rough idling, poor engine power, fuel mileage, etc...) . If that doesn't solve the problem, at least you've eliminated one common potential source. Good luck.