Jeep stumbles and backfires.

**BIG_J** · 12-17-2008, 02:47 PM

Hey guys, Im trying to see what you might think the problem is. My jeep runs fine for the most part. Only every now and then, it is almost like it is flooded, and only does this from a dead stop. Got to take off, and its stumbles and if you dont let off the gas, it will backfire. I have not been able to see if its backfiring through the intake or into the exhaust. once you left off the gas, and get back into it, it is fine. Any idea's on what might be causing this?

**tjwalker** · 12-17-2008, 02:48 PM

Backfiring can be an indication of a "lean" fuel condition. Test fuel pressure at the schrader valve on the fuel rail for starters. Compare your readings (both running/not running to factory spec).

**BIG_J** · 12-17-2008, 02:51 PM

I will start there. what make it even worse, I Must use this at night when i deliver pizza's. My wife is prego, and cant drive the jeep. to hard for her to get into.

**Ed Rico** · 12-17-2008, 03:04 PM

Even if your check engine light isn't lit, I would go ahead and see if there's any codes stored. My '97XJ acted like this when the O2 sensor heater circuit fuse was blown (especially on restart after sitting for short duration) and it never set the check engine light, although I did have a high-voltage code for the rear O2 sensor. The problem completely resolved when I fixed a loose ground connection for the O2 sensor heater circuit, so I assumed that the lack of O2 heater allowed the 4.0L to come out of closed loop operation due to the front sensor not being up to operating temp.

Might be a long shot, but it's easy enough to check.

-Ed Rico

**BIG_J** · 12-17-2008, 03:17 PM

Ill check that too.

**BIG_J** · 12-19-2008, 06:10 PM

Well i got to looking under the hood today. I was working, but we were dead. I looked at the coil, and the housing was cracked open. sent one of my guys to get a new one, installed it, and so far no problems. I will be driving it more tonight, so we will see.

**BIG_J** · 12-19-2008, 07:46 PM

OK well i just got back. I also stopped by vatozone and used their OBD2 Scanner. No codes. I bought the pressure tester and checked the fuel pressure, with the key to run, I have 42psi, when i turn the key off, it was at 40psi. I'm guessing the pump isn't the problem, but i am going to go to a shop i deal with and hook up their computer. It tells you EVERYTHING. I just hope it will tell me why the hell my jeep is acting like it is. If anyone has any more ideas, please chime in!! Thanks!!!

**tjwalker** · 12-20-2008, 06:41 AM

Taking it to a shop probably not a bad idea, especially since your fuel pressure checks out and there are no codes. Throwing any more parts at it probably isn't a great strategy. Good luck and post back what the shop finds out.

**BIG_J** · 12-20-2008, 03:49 PM

Ok, i went over to the shop. Their code reader is the bomb, and it found only 1 code. p0320. We checked around the dizzy, pulled the cap off, and everything looked new still. I only have about 2k on spark plugs, wire, cap, and rotor. Only about 30 miles on the coil. They got busy, and we had to stop playing with the jeep, so maybe tomorrow or Monday Ill go back up there and see what we can find out. If anyone knows what could cause's the P0320, please let me know. The shop I am going to is just friends, so its not coasting me anything to check stuff out, but i still have to do the work myself. Not a bad deal.

**BIG_J** · 12-20-2008, 03:50 PM

oh yea, and we found that 1 of the o2 heater's was not ready. when everything else is. Maybe a bad o2?

**tjwalker** · 12-20-2008, 04:01 PM

PO320: No crank reference signal at PCM

In my experience, that can mean either the crankshaft postion sensor OR the camshaft position sensor. Your symptoms match up more with the camshaft position sensor rather than the crank sensor IMHO.

The camshaft position sensor is located inside of the distributor. The computer uses the camshaft position signal to determine the position of the No. 1 cylinder piston during power stroke. The computer uses this information in conjunction with the crankshaft position sensor to determine spark timing and proper injector firing.

You can test both the crankshaft position sensor and the camshaft position sensor. Meter and a manual needed here. If it were mine, I'd start by testing the cam sensor...Testing is the best strategy here; don't throw parts at it.

Good Luck and post back what you find!

**BIG_J** · 12-20-2008, 04:33 PM

Well looks like ill start there tomorrow in the cold. I am about to go out for the night. Ill be taking the wife's car though.

**BIG_J** · 12-21-2008, 12:55 PM

Well i just got back from a trip in the jeep. On my way back, it started acting up, but this time it set the CEL off. I stopped by auto zone, because i passed right by it on my way home, and found codes, p0132 o2 sensor 1 bank 1, and again p0320. I am gonna get a new o2 sensor, and try and find out how to check the cam PS, and crank PS.

**Ed Rico** · 12-21-2008, 01:08 PM

Quote:

Originally Posted by BIG_J

Well i just got back from a trip in the jeep. On my way back, it started acting up, but this time it set the CEL off. I stopped by auto zone, because i passed right by it on my way home, and found codes, p0132 o2 sensor 1 bank 1, and again p0320. I am gonna get a new o2 sensor, and try and find out how to check the cam PS, and crank PS.

The P0132 high voltage code can be set when the front O2 sensor heater isn't working, the sensor is essentially open electrically when cold and the PCM substitutes a base voltage that doesn't vary instead of the voltage normally generated by the sensor due to the difference in O2 content between the exhaust stream and the outside air. This can be an internal problem with the heater or it can be due to a wiring issue with the harness to the front O2 sensor. I'd suggest you take a close look at the wiring to the front sensor, if it's a wiring issue a new sensor won't cure the problem and you might have a cheap and easy fix.

-Ed Rico

**BIG_J** · 12-21-2008, 01:26 PM

well i know for sure the wires are good, already checked that. and it wold explain why the heater was not ready when i hooked it up to the shop's computer.