Engine idles, but backfires

I can and will check timing. As far as where it backfires, sometimes out intake. Sometimes out exhaust. If you open the throttle rapidly, out the intake. If you open the throttle slowly and try to force the engine to accelerate, it runs very rough, almost like a misfire, then out the exhaust.
Thanks for your response.

Timing is about 6 to 8 before at idle and goes up to around 14 or so when accelerated to about 1800 to 2000 rpm. This doesnt sound so far advanced as to make it backfire, I am thinking a vacuum leak, so this is the direction I am going. Got anymore ideas?

I appreciate the input about the plug wires, but if they were wrong, it would run crappy or not at all. This thing idles beautiful. Just cant push the gas.

Thanks for all the great advice. I worked on it a few hours this evening and this is what I found.

First. Went through timing again, all correct, on TDC rotor is at #1 dead on.

Second. The engine is a 97 and the truck is a 98. I compared the distributor that came with the engine (used engine) to the 98 out of the vehicle. The cam pick up that rotates is not clocked the same. So I put the 97 distributor in the vehicle. But this made no difference in how it ran.

Third. Timing at idle is about 8 degrees before TDC. Sound pretty correct. This did not change with either distributor, stays the same.

Fourth. Found forward O2 sensor connector very dirty, cob webbs and all sorts of stuff. Cleaned this out and THE INTAKE BACKFIRE IS GONE. Punch the gas and no backfire.

Now. At about fifteen hundred RPM it gets very rough will popping out the exhaust. Sounds like very rich mixture? Why at fifteen hundred RPM and not before?

Have any advice on what would make it pop out the exhaust and run very rough above 1500 RPM?

New plugs, rotor and dist cap.

After some more troubleshooting today.

It does not backfire out the intake at all.
It runs great until about 1400 - 1500 RPM, then it begins to miss, keep going and you get unburnt fuel popping out the exhaust.

Cant tell if the miss is from ignition or too much fuel. I think ignition. I changed coil and plug wires to some known good from a running vehicle. No change. Changed all plugs, no change.

I have followed this post because my jeep is doing the same thing it ended up being a clogged cat converter FYI it cost me $560.00 for a new one all fixed I love this site.

I have a 1998 Jeep Cherokee with the 4.0L I6. It had a problem where intermittantly it surged and sometimes backfired on acceleration. The more the throttle was depressed the worse the symptom became.

When it first started doing this I discovered that the fuel pressure was sometimes as low as 20 PSI. I changed the fuel pump/filter assembly and the problem disappeared for about 2K miles.

It has now come back but only on moderate throttle. I have checked the fuel pressure at the fuel rail and it appears to always be approximately 50 PSI.

This time the symptom is slightly different. If full (or almost full) throttle is applied during acceleration it accelerates properly and will then continue to run fine under all conditions until the next time the engine idles for an extended period (like at a stop light) or ran at very low throttle for a while like when in 4WD on level but rough surfaces such as a field or if the engine has been turned off.

It appeared that maybe the IAC solenoid was sticking open. I cleaned the solenoid and the the IA port. Again the problem went away for a few days then returned. So I changed the IAC solenoid. This did not change the symptom.

So, as it stands, I have a new fuel pump and fuel filter, a new IAC solenoid and a new rear oxygen sensor (changed for another reason). I still have the problem.

I had the codes read and they didn't show anything.

Any thoughts?

I bought a code scanner and even though it had scanned clean right after I had a new fuel pump installed it now showed codes for both O2 sensors. I knew that the front sensor was old so I replaced it.

Immediately the Jeep ran better but still surged under the same conditions described above. Sometimes it showed a O2 sensor heater code and sometimes it didn't.

To make a long story short. There is a fuse that is only used for the O2 sensors. This fuse was blown. I replaced the fuse and NO MORE SURGE.

Maybe I didn't give enough information.

The fuse is for the O2 heater circuits. So the O2 heaters didn't work. Anytime the computer called for open loop operation and ignored O2 sensor input the Jeep ran fine. These times include cold start and WOT.

The problem presented itself when the computer called for closed loop and the O2 sensors were too cold. These times include idle and part throttle after warm up. If the engine load is great enough the exhaust stream temperature brings the O2 sensors to acceptable temperature and things work great.

In my case, I was in very rough country off road and a bush caught the leads for the rear O2 sensor and tore them out. While they were flopping around the heater wire shorted and blew the fuse. This started a long string of errors on my part trying to resolve this. I don't regret changing any of the components. The miles on the Jeep dictate that they be changed anyway. It was sure frustrating though!

Thanks for the ideas. They were good thoughts. Glad I didn't have to do that too!

Started with stalling cranking for long periods of time and wot to get started
once started would run rough somtimes fine.
Then stall when going to take off and not starting again unless wot hard to get started.
New distributer cap wires plugs, tps, etc nothing worked.
I then took the injectors out with rail cleaned best I could, found carbon build up where injectors enter the manifold cleaned & now will start no problem idle perfectly hit the gas revs fine.
But when driving after 2nd gear shift engine starts to bog down looses power really bad & backfires through the intake manifold, I can w.O.T it and it clears up until trying to maintain road speeds.
I checked the fuel pressure with a gauge reads 49 psi @ idel & stays almost the same during driving, when I shut the engine of with key of the pressure almost instantly starts to fall.
Drops to below 30psi in under 3 minutes, hanes manual says its the fuel pressure regulator, this model is equipped with a vacuume assited pressure regulator, could this be the reason for the backfire and loss of power very costly want to make sure this is what wrong before dumping another 100 into it
thanks jim