4.0 sporadic idle and no power.

DimebagDave · 06-18-2009, 12:21 AM

I recently received a 1999 Jeep Wrangler 4.0 from my father, which is currently at about 81,200 Miles. Run's great, used for plowing, and it's a pretty strong little thing.

But recently it has been giving me this MAJOR issue that I can't seem to solve.

When I start the car, it will turn over no problem, and slowly drop down to normal idle. Than sometimes it will start idling rough, or even as low as 200RPM to where I need to feather it to keep it alive. Than I pull out of my drive way to leave, and when I'm applying constant pressure, about 1,400 RPM it will start bucking, and while I'm applying the same pressure, it will start dropping, than catching up, than dropping and catching up. When I come to stops, (only 3 times now it has completely died) it will start dropping below 500 RPM.

We also noticed the Oil Pressure Gauge says it's at 20 Sometimes, than 40 sometimes, than 20 again (normal idle for all) meanwhile the dipstick reading say's it is perfect.

We did a OBD and the computer came back clear for everything. No codes at all...Makes no sense, obviously there is a problem. So we started to do what would be logical, and try our sensors. New TPS, New CPS, New MAP, New IAC, New Rotor and Cap, New Fuel Pump/Filter, New CCV Valves, Cleaned out the Throttle Body. I don't know what else to do, so please if you have any ideas please help me out

**TJftw** · 06-18-2009, 12:24 AM

i have something similar but it only starts when the motor is about half warm. it's in the shop now so ill post when they tell me whats up

DimebagDave · 06-18-2009, 12:29 AM

Thanks man. I took it to the shop too after we tried unplugging the TPS. CEL came on, than we plugged it back in and the CEL stayed on, so we thought it was the TPS for sure. Ran okay for about 20 miles, and she's doing it again. My mechanic said he thinks it might be the main computer. But I'm not willing to go through guess' and not fixing the problem right away. I spent over $200.00 already and still nothing, glad I do it myself and get 50% off parts.

**TJftw** · 06-18-2009, 12:40 AM

i gave up trying to figure out what it was. im good at mechanical work but not trouble shooting. hopefully ill have an answer for you tomorrow

DimebagDave · 06-18-2009, 12:42 AM

Awesome, that would be a real time saver. I already went through about 200 post's, and finally decided to join and ask myself. Did you try changing your TPS or anything or just straight to your mechanic with it?

LJameson · 06-18-2009, 12:46 AM

Take the battery out and have it load tested. That should've been the first thing you do, and clean the terminals.

**TJftw** · 06-18-2009, 12:49 AM

i took a look at it but not really. ill give you a more detailed description of what's happening:

Ill start my jeep just fine, its running at the correct idle. Ill start driving it and once it warmed up a little it will start to jerk lose power and then regain it lurching forward, sounds like it's harsh on the U-joints and other drive train components. all this time my foot has stayed steadily on the accelerator. It will do this for up to a minute or 2 so i normally pull over and wait it out. while im sitting on the side of the road in neutral i can hear the engine revving its self and sputtering. The rest of the trip will be un eventful. I think it may have something to do with the fuel breathing system but for once i just took the lazy way out and took it to a shop. didnt feel like lowering the tank

DimebagDave · 06-18-2009, 12:52 AM

Yea I hated dropping the tank. That is exactly what mine does, and yes, pulling over and waiting the 2 minutes is worth it other than driving like that. I also hear something clicking, it sounds like a butterfly valve flapping open when it's jerking. Does your make that noise too?

**TJftw** · 06-18-2009, 12:52 AM

it's funny because i have a '99 too

**TJftw** · 06-18-2009, 12:53 AM

i don't hear a clicking no

DimebagDave · 06-18-2009, 12:53 AM

I read a lot on this, and apparently the 4.0 engines do this a lot, especially the 99's. But all ended up being something different.

Got any pictures of yours? I'll take some of mine when I get my camera back tomorrow.

**TJftw** · 06-18-2009, 12:56 AM

here are some pictures, the second page has some of the cooler ones :P
http://www.facebook.com/album.php?aid=248343&id=535800316&l=27748bd5c2

DimebagDave · 06-18-2009, 01:11 AM

What a beauty. I just sold my 88 Ramcharger. She was a beast.

**TJftw** · 06-18-2009, 01:42 AM

nice those things are beasts. in the mean time fill out your profile, it will help people get a better idea what you're running.

**Jonny Jeep** · 06-18-2009, 02:13 AM

Gentlemen, I believe the upstream oxygen sensor may be the cause of both of your Jeeps problems. Have a read of the following thread and try disconnecting the sensor connector to see how it drives.

http://www.jeepforum.com/forum/f9/97-tj-sputter-stall-o2-sensor-612880/