View Full Version : Rich/ lean rough idel

April 11th, 2008, 00:02
1998 4.0 AW4

I just cant seem to get my engine to run smoothly. I have had problems since the day i bought it. I have got it better, it all started with a damaged upstream 02 sensor wire. Fixed that, replaced broken cat with a new one. But it still dosnt run smooth. When its ideling it has a slight rough idel,and occasional when i smoothly accelerate it likes to just slightly hesitate.

i hooked up a scan tool and the thing that seemed weired was the up stream o2 sensor kept switching from rich to lean readings, back and forth constantly, what could be causing this, could this be the root of my problems?

Iv had the jeep for amost 3 years now and ive replaced i think about all the sensors in my quest to cure it, i thought buying a newer jeep would be better but my 89 ran 10 times better then this one.

April 11th, 2008, 04:05
I'd look at a vacuum leak. One tip off would be lighter colored plugs on the cylinder/cylinders with the leak.
Not a bad idea to check the torque on the intake/exhaust manifold bolts periodically. A lot of hot and cool down cycles is bound to have some affects.
You have to remember the O2 sensor is after the fact, it tells you what has already happened. Either too much or too little residual oxygen from the burn and it constantly adjusts after the fact, with the initial settings usually coming from another sensor (mostly the TPS and MAP with some input from the coolant and intake temperature sensors).
Vacuum leaks are often not uniform, you have a couple of cylinders firing lean and the rest normally, then the O2 sensor adjusts for the residual oxygen and no cylinders are firing ideal mixes.
Replacing the sensors can be helpful, unless you have excess resistance in the wiring or connectors, then the sensor values are likely skewed from the get go.
Do a compression test, uneven compression can cause some of the same symptoms as uneven intake flow (leaks).
Check your spark, weak spark can also cause rough idle, cutting out (hard miss) and high speed misses. You often loose some top end RPM's. Along with poor burns and fuel/air ratio metering problems.
Sometimes getting back to basics, before deciding the sensors are the problem, produces better results. In my experience the problem solution probability is often 33 % sensors, 33% wiring and 33% general automotive stuff, like vacuum, compression, worn internals and worn ignition parts. I usually troubleshoot from cheap to expensive and try not to cut corners. I've fixed way more problems with time, than I have with money.

April 11th, 2008, 04:25
Cracked exhaust manifold? You have any ticking?

When you say all sensors - what all have you replaced?

April 11th, 2008, 08:44
i do have a slight ticking, and the sensors would be, tps, map, cps, the computer,. I didnt buy them new i pulled all them off a running jeep at the place i got it cause they couldnt figure out what was wrong with it either so they just gave them to me. i did buy new o2 sensors though.
i think thats all of them, i might be forgeting somthing, i guess thed good thing is i have all spare sensors...

April 11th, 2008, 10:32
I damaged an O2 sensor once and after it got replaced the jeep ran horrible (rough idle and stumble). After about 8 months of searching I found that there was a blown 15A fuse under the hood. Once that was replaced it ran perfect.

April 11th, 2008, 10:37
i had the same blown fuse exept i drove like that for alomost 2 years!, after i replaced it it ran much much better but i still have the other little problems

April 11th, 2008, 11:33
i had the same blown fuse exept i drove like that for alomost 2 years!, after i replaced it it ran much much better but i still have the other little problems

What fuse are you talking about?

April 11th, 2008, 13:18
the upstream o2 sensor should be switching for rich to lean on a quite about every 5 to 10 seconds. the one downstream should also switch but at a much slower pace.

April 11th, 2008, 14:17
What fuse are you talking about?

i cant remember what slot but its a 15 amp under the hood, the upstream o2 sensor is ran through it

April 11th, 2008, 14:58
It's in the row closest to the relays and second from the left if your looking under the hood.