This is general information about what happened with my 97TJ 4.0L 5spd. About a week ago I noticed that under acceleration (between idle and wide open throttle position on the TPS Throttle Position Sensor) my jeep was stalling and power totally dropping out except at idle or wide open throttle. As long as you kept it floored (don't recommend in the rain with lockers) you could limp it home. I was getting a check engine code of 21 (bad O2 sensor) and 55 (end of codes).
Now the chipmunk effect (I thought). My dog had been sniffing around the back finder wells of my jeep really after something. With the stalling I thought, stalling + critter = chewed wires. I dug around the drivers side first and there it was, enough moss and sticks to build a bonfire! I pulled out enough to fill a laundry basket! Cleaned that all out, washed it well and let dry (I do recommend BBQ tongs so you don't have a critter running down your arm!).
I traced the wires back to the fuel pump and unfortunately they were no where near the "worlds largest chipmunk nest". I cleaned off all the wires and connectors and felt for any nibble marks etc. Nothing. I put my meter on the wires to check for short to ground and nothing there either. All perfect. So much for the chipmunk theory! Nuts!
Then I thought it was a bad fuel pump. I put my fuel pressure gage on the test port of the fuel rail and because I have a long enough hose (fill in your own joke here) I routed it up and onto my cowl vent area so I could watch the pressure as I drove. Perfect 49psi even when the engine stalled and sputtered. It had to be somewhere else and electrical.
With the fuel pump eliminated from the formula, I went back to the strange action of the throttle position sensor. I thought that maybe it was going out intermittently, not enough for a code, but just enough to mess up the fuel mixture to trip the O2 code. Makes sense?
So I removed the two torx screws holding in the TPS and took a closer look. With my trusty Fluke, I checked it in all positions and had a perfect sweep of resistance all the way from closed to wide open. I cleaned it out with WD-40 and an air hose and double checked. No problems. Double triple checked, still perfect. I put everything back together and took another test drive, still stalling out...
At that point I thought, "...well better clean every connection and apply WD-40 to everything". Several hours later after cleaning everything I could find, battery post, grounds on block, all connections to computer, injectors, CrankSP sensor, CamSP sensor, both O2 sensors, the works. Test drive and it ran better! I thought good 'ole WD-40 once again saves the day. I gently placed my can in it's sacred location with pride.
Next day...
Sputter Sputter..... Bang Backfire.... limp limp x 100 from what it was before! I thought holy heck batman now what in the world is going on? As much as I liked keeping it to the floor, this was getting old and a bit dangerous! Back to the garage at home for more brainstorming.
The kicker was the action of the TPS. I kept thinking it had to be in there somewhere. Took it all apart again, cleaned, metered perfect and reinstall. Still chitty chitty bang bang. Darn.
At this point I figured I could do two things, start replacing everything until it stops OR suck it up and crawl down to the local shop and put it on the test machine. I called a few places and found one that I trust that would check it out for $80. I know what your thinking, DON'T DO IT MAN! IT'S A SCAM! Don't suck up to the modern electronic overlords and beg for help! I admit it did hurt, but I did it anyway. Why? Cause I wanted it fixed right, and do not want a failure out on the side of Mt. Rainier.
I took it to the shop and about an hour later I got the call. Shorted out O2 sensor upstream. Because of the way it shorted out internally it put power into the ground lead causing it to send 12 volts to the TPS on both the hot and ground wires! Because of the short they said it could have damaged the computer causing the 12 volt short. I couldn't believe them at first because that seemed just to (seemingly) simple of a problem. So I reluctantly let go of my $80 and went home to fix as reported all the while hoping it was only the O2 and not the computer as well.
For grins, I just did a simple unplug of the upstream O2 sensor, disconnected the battery for a quick reset and took it for a quick test drive. It ran so much better! No sputter or back firing. TPS worked perfectly, though performance was a bit doggy and I did get a solid Check Engine Light. Off to NAPA I went for an O2 sensor. As it turned out the Bosch sensors had a $10 gas card rebate on them. I had a local discount card that knocked off another $10 getting me my sensor for $47 bucks! Not bad!
Got home and installed, plugged everything in and test drove. Ran perfectly! I have to say I'm glad I paid the $80 for the analysis and I was still able to wrench it the rest of the way to a fix. So I now owe two apologies. First to the chipmunk who I assumed had chewed up my wires causing issues (can't blame 'em for making a nest in a Jeep can you?) and another for the computer tech who I swore would be an idiot and have no idea of how to tell me what was wrong. Sorry Chip and thanks Randy for a great job!
Tqm
97TJ