mcoughlin

I have a 96 Cherokee Country with 4.0 HO and AW4 and the check engine light is on. Initially, it had three codes for the downstream O2 sensor so I replaced that and reset the light. It was running great on my drive home from school last weekend and stopped for gas. When I restarted it, the light was on. Drove it home and checked it at the shop I work at. I don't remember the actually code names and numbers but they were:

1. System too lean Bank 1
&
2. System too rich Bank 1

WTF! The guys at the shop had never heard of that before. It also idles like a big block with a lot of overlap at about 500 RPM's and the smell of sulfer is disgusting. It is in fact rich because it will roll smoke like a diesel truck.

Anyone got any ideas? Possibly the MAP sensor effecting it? It has a short ram intake and headers but has had both for well over a year now with no problems until now so i doubt those are effecting it. Any help is appreciated because this whole 10 mpg thing is getting old, especially when driving 180 miles to school and using an entire tank...

tjwalker

Always best to post the EXACT codes as with OBD, the devil definitely is in the details. For a 96, while you can use the odometer trick to get a 2 digit code, it is always best to use a code reader or a scan tool to get the more comprehensive PXXXX code. Way more helpful.

If this were mine, I would proceed in this order...................

1. For starters, I would have replaced the upstream oxygen sensor at the same time you replaced your downstream. If one oxygen sensor is failing, the other is likely not far behind. And it is the "upstream" one that sends input that help calculate your mixture. I'd replace it even if it is not the root cause of these codes. NTK is my favorite brand of oxygen sensor for the 91-01. Also while we are on the subject of oxygen sensors, take a quick check of the mini 15 amp fuses inside of the power distribution center (black fuse box) under the hood. There is one mini 15 amp fuse in there (around slot 17 I think) that controls the heater circuit of your oxygen sensors. Be sure it isn't blown.

2. Be sure to inspect your exhaust system for leaks! A cracked exhaust manifold can screw with your engines ability to create the correct mixture.

3. Verify fuel pressure. Must be done with a gauge. (rent one from a parts store if you don't have one) For a 96, you should have 49 psi, plus or minus 5 psi.

4. The MAP sensor also contributes to fuel/air ratio. They can be tested with a meter and a manual with the procedure. Also be darn sure that the vacuum line leading to the map sensor is pulling good vacuum! Vacuum lines can rot and fail over time, especially where they connect.

5. Test or swap out your "throttle position sensor", followed by your "coolant temperature sensor".

Good luck and keep us updated!

mcoughlin

I tried to replace the sensor but i head some creaking noises from the sensor and didn't want to break it off or break the bung in the parking lot of school 180 miles from home. I've also checked all the fuses numerous times since mine has always seemed to be an electrical nightmare.

As far as exhaust leaks, they aren't there. It is quiet as hell with no leaks at the flange and the header is only a year old. I see what you are getting at though. I race motocross and a lot of guys have troubles with the slip fits on headers sucking air in creating popping on deccel which is normally because of a lean pilot jet....

I am starting to definatly think it is the MAP sensor since the mounts where is screws to the TB are broken and just hangs from the rubber elbow. This may be giving it false readings.

How would the coolant temp sensor have anything to do with it? My temp gauge works fine as well as the electric fan kicks in at like 215 degrees (I think, it is around there atleast). I get the throttle positioning sensor part, but why would this cause it to only idle weird when its warm? Cold, it has a nice smooth, slightly raised idle. It is REALLY bad when you come to a stop light.

mcoughlin

I noticed the other thread that is active about the same 500 RPM idle and rough running about they were talking about the steam cleaning. I have a Mazda RX-7 and I've seen some rotary guys do it to help keep carbon from building up on apex and side seals. Could this possible help as well? It does have 240,000 miles....