TJ is spitting, sputtering, and backfiring....any thoughts please?

karstman · #1 10-21-2002, 06:44 PM

Subject sums it up.
I got the rear disc brake conversion complete and went for a 15 mile test drive. Jeep ran fine as usual and the disc conversion worked perfect

I returned home, cleaned the garage, and went to the grocery store to get some goodies to cook for dinner. On the ride home, the jeep started bucking and sputtering as I entered the development, going up a slight hill. This continued till I got to the house and shut it off. I checked all sensor connections and they seemed fine. I drove it around the development reproducing the symptoms. It idles fine at startup and revs fine in neutral. When going up a hill or putting a load on the motor, it sputters and backfires through the intake. Ahhhh....96,000 miles on it, I bet the TPS finally took a crap. I swapped the TPS out today and found that my diagnosis was completely WRONG. Damn thing is still sputtering under a load, especially after warmed up. I'm out of town for work for the next few days, but any thoughts would be appreciated. I'm leaning towards the fuel pump and plan to do a pressure test when I get back.

Any thoughts? Hints? Suggestions?

thanks

mark

Art Welch · #2 10-21-2002, 07:06 PM

Could be an O2 sensor. I had a similar prob, whenever the PCM went into closed loop mode the jeep would run like crap, particularly under load when it would nearly die. I'd check fuel pressure too as you suggest (I think it's supposed to be 48 +/- 2 PSI?) but my money is on the sensors.

karstman · #3 10-22-2002, 03:34 AM

Hmmm...I will have to double check when it first begins sputtering to try and isolate when it begins. I did not really load it hard until it was somewhat warmed up, but I did 2 loops around the block with a cold motor and it ran fine after installing the new TPS. 3rd run up the hill and it started backfiring through the intake and sputtering bad until I cut back on the throttle. Thanks for the suggestion. I'll be reading up on the old FSM in the hotel tonight

Oh - FWIW - It has new plugs, wires, cap, rotor, coil, and air filter.

thanks

Mark

Robert J. Yates · #4 10-22-2002, 09:11 AM

Are you sure that new tune-up stuff you put on is ok? How long has that stuff been on? Recently or has it been on there for some time? Sounds like a bad plug wire to me. Good luck - I hate tracking down that kind of crap but one of my tricks for anything going wrong is to go back to whatever I recently changed if the problem seems to crop up outta nowhere.

Jerry Bransford · #5 10-22-2002, 04:02 PM

If you have the right year TJ (some can do this, some can't), how about trying the old ignition key on-off-on three times within five-seconds trick to see if there are any diagnostic codes that will display in the odometer? Be patient, they sometimes take 5-10 seconds to display. A '55' code is normal, anything else in addtion to 55 could point to the problem.

Art Welch · #6 10-22-2002, 05:15 PM

If you find yourself spending much time on this at all I really recommend one of these:

http://store.summitracing.com/partdetail.asp?part=25149

It's $260 but can pay for itself really quickly given the sensor capture mode.

Robert J. Yates · #7 10-23-2002, 08:29 AM

Look here if you don't want to pay the $260

http://www.jeepbbs.net/forums/showth...&threadid=3065

Art Welch · #8 10-23-2002, 09:21 AM

The code reading is helpful for basic stuff but it gets to be really useful when you are able to drive around with it plugged in running monitor/capture mode to reproduce a problem that only occurs in specific situations. I'll admit that part of the coolness factor is because I am a geek

but I've had two cases in which there were no trouble codes but the jeep ran horribly some of the time. When you can see PCM parameters in real time it helps to narrow down the search.

Robert J. Yates · #9 10-23-2002, 10:35 AM

I was being sarcastic about the $260

But I guess if I want to geek out, I can always borrow yours

karstman · #10 10-23-2002, 03:59 PM

Thanks for the replies

Here are some more details after a quick session a few minutes ago.

Jerry - it's a 99 so the key trick won't work

Robert - I have about 6,500 miles since the tune-up with no problems except the one that just popped up. I did it in prep for a road trip to Colorado at the end of August. I'll double check the wires for arcing and everything else for good, dry connections.

Art - thanks for the link. I have a friend...who has a friend with an OBDII scanner. He may be getting a call.

I tested it this evening after it had been sitting for 2 days. When cold, it runs fine. I made many runs up the hill and I could not reproduce the miss. Once warmed up, I was able to get it to backfire through the intake one time. Then it almost seemed to run better.

I let it sit at idle for another 5 minutes to make sure it was at operating temp and took it for a short run. It ran fine for all of 1 minute. It then started bogging. I could not get it to backfire. At WOT, it would accelerate and pull fine. At part throttle, in 3rd gear, trying to maintain about 40 mph, it would fall flat on it's face once it hit an even speed. If I gave part throttle to accelerate, it would bog, then accelerate, then bog. If I gave it full throttle to accelerate, it would accelerate fine. When I say bog, it's as if the fuel and/or spark is cut 100%. It's not like a miss from one cylinder, it's more like cutting the ignition for a second or two then turning it back on.

If I pull the leads to the O2 Sensors, will it force the PCM into an open loop? I'm about to start swapping parts off Jill's 2000 TJ onto mine to test them out one-by one

She's gonna kill me

Again, thanks for the input and any other thoughts are VERY helpful. I am beginning to miss the HEI and Holley on my Chevelle a lot.

later,
mark

mbryson · #11 10-23-2002, 04:30 PM

Quote:

Originally posted by Robert J. Yates
Are you sure that new tune-up stuff you put on is ok? How long has that stuff been on? Recently or has it been on there for some time? Sounds like a bad plug wire to me. Good luck - I hate tracking down that kind of crap but one of my tricks for anything going wrong is to go back to whatever I recently changed if the problem seems to crop up outta nowhere.

I've been having the same problems as Mark and mine turned out to be plugs and wires. I think Yates is on to something here!

Mine were 'newer' plugs and wires so I ruled that out. Check the cheap and easy stuff first! I spent $350 before pulling my plugs and noticing one tip that was CRISPY!! and to others that I'd classify as just Crispy (that's 3 out of 6 for those counting--surprised the poor little thing ran at all--The ceramic 'tip' near the electrode was pretty much burned off on the bad one. New to me, but very true. I walked that one over to Brent Orton just to show him.....it was freaky)! New Champions {damn Autozone for not carrying decent plugs} and the thing ran fine for 2000 miles. It started sputtering yesterday and I just put some Autolites in (I've never liked Champions-esp. not in GM products) and everything seems like it's back to normal.

I replaced the wires, cap and rotor as they were not in 'killer shape', but on their way out the door so I kicked them out prematurely.

Art Welch · #12 10-23-2002, 08:11 PM

Quote:

Originally posted by karstman
I let it sit at idle for another 5 minutes to make sure it was at operating temp and took it for a short run. It ran fine for all of 1 minute. It then started bogging. I could not get it to backfire. At WOT, it would accelerate and pull fine. At part throttle, in 3rd gear, trying to maintain about 40 mph, it would fall flat on it's face once it hit an even speed. If I gave part throttle to accelerate, it would bog, then accelerate, then bog. If I gave it full throttle to accelerate, it would accelerate fine. When I say bog, it's as if the fuel and/or spark is cut 100%. It's not like a miss from one cylinder, it's more like cutting the ignition for a second or two then turning it back on.

If I pull the leads to the O2 Sensors, will it force the PCM into an open loop? I'm about to start swapping parts off Jill's 2000 TJ onto mine to test them out one-by one She's gonna kill me

At wide open throttle you're open loop and it totally ignores the O2 sensors. Under normal acceleration you're open loop but I think it still listens to the sensors (can't say for sure though).

When I was in smog hell early this year I experimented running without the 02 sensors for troubleshooting and if I remember all was good with downstream unplugged. With upstream unplugged the reported sensor voltages were *really* weird and the engine ran like crap. I don't remember more specifically than that though.

You're lucky to have a TJ to swap from, that should make it easy

I'd start with the upstream one and see what happens.

karstman · #13 10-24-2002, 05:33 PM

Did some more work this evening. Pulled the upstream O2S and checked resistance as per FSM. Both O2S passed fine, BUT, the wiring harness to the upstream sensor has several small abrasions where the wires had been chafing against each other. One wire appears to have been exposed for a while. The strands are clearly oxidized and dirty. I taped the exposed sections up and drove for 15 miles without a burp from the motor

Then I got one short burst of backfires/bogging.

Regardless, the sensor needs to be replaced. I'm not about to rely on e-tape within 1" of the exhaust system. I'll post back if something else turns up, as I hope to have a code reader hooked up tomorrow to check for any other issues. If I find them I'll post it here, if not.....it was the damn wires to the oxygen sensor. I have a feeling there is a bit of corrosion inside the one wire, leading to the intermittent problems.

THANKS for all the help

These things can and do drive me nuts

later,
mark