1997 Jeep GC Runs Lean / Rough Idle when Hot

[quote=”Mike2914″ post=124257]Hey All,

I know there are various topics already on rough idling vehicles and the like, but I decided to post this one because it seems unique. If I\’m wrong – my apologies for the added thread.

I have a 1997 Jeep Grand Cherokee Laredo (I-6) with ~230k miles on it. I got the car used ~100k miles and have never changed the transmission fluid on it due to the \’you might break it if you do\’ folks out there. I know it\’s not great, but once it was driven so long without a fluid change, I figured it was best not to chance it.

I live in Orlando so the weather here is normally warm – hot. Back in October it might have gotten ~50 or 60 F and when it got colder, I noticed the transmission started to slip. I\’m no expert, but this is what I am calling it a transmission slip: when accelerating, the car will go to a high rpm or almost rev high with now power and then \’lurch\’ into the next gear. I normally just let off the gas when I hear this happening and re-engage so it auto downshifts without all the extra gas/noise. Please correct me if I am misinterpreting this as transmission slipping and it\’s really something different.

So my transmission began to slip but it was only when the engine was cold. Once ti warmed up, it was fine. I figured the transmission fluid is old/bad and so it was only working properly once it got warmed up.

Over the Christmas holidays, I treated myself to some much deserved vacation and took 2 weeks off work. While off, I drove my wife\’s car because it\’s newer and nicer 🙂

Now is probably a good time to mention that I have had a coolant leak that I can\’t track down for over 6 months. I\’m not too concerned about it though because I just keep the reservoir topped off and everything seems to run OK.

So I start up the car this morning and everything is fine. A few miles from my house, I notice that the car is idling really rough and sounds like it\’s going to shut off. The Tac is bouncing up and down between ~200 to 1,000 rpm. If I accelerate, it purs just fine. I drive mostly highway to work so the rest of the trip it was fine, until I start slowing down below ~35 mph at which point (foot off the gas coasting) it starts to rough idle again. I also notice that my oil pressure (which is usually ~40psi) is \’stuck\’ at 30 psi. It used to increase when accelerating but now it just sits at 30 so I\’m not sure if the gauge is working properly or if this is a real problem.

So after work I start her up and she starts fine. She even drives/idles fine. Then I notice that once the engine temp reaches ~190 (210 is nominal) the rough idle begins. If I shift into neutral it seems to go away (at least it did at stop lights).

On the way home I got a CEL. When I got home I read it with my scantool and saw P0171 System Too Lean Bank 1. This would seem to indicate what I was experiencing at idle, however the freeze frame data shows this happened on the highway.

Freeze Frame Data
Calculated Load: 42%
Engine Coolant Temp: 204.8
Short Term Fuel Trim: 32.8%
Long Term Fuel Trim: 32.0%
Intake MAP: 10.15 psi
Engine RPM: 2304
Speed: 71.42mph

Any ideas what might be causing this? I had the pre-existing coolant leak and I\’ve read that if the thermostat/coolant temp sensor is broken it can think the engine is hotter than it is and the PCM will lean out the mixture but I don\’t know if that makes sense since the PCM detected the mixture was lean. The lower oil pressure may be related or not – it\’s definitely unusual for my car to run lower than 40psi. There was coolant in the reservoir at the 1/2 line when I got home so I don\’t think it was coolant but I\’m stumped. I also thought about the O2 sensors but not sure if they would have caused this or not. I\’m hoping you can look at the MAP and tell me if it was in a good range or not.

I also want to reiterate that as soon as I accelerate, she sounds great (which again signals to me she isn’t getting enough gas but I don’t know why).

Any help/suggestions would be much appreciated!

Mike[/quote]

Mike I was having this exact same issue for the past few days in my 96 cherokee 4.0L i6, purchased and ran my scan tool and got the same code for bank 1 sensor 1 to lean, it is a VERY common issue in these 4.0 jeep motors all the way up to 2004, mine ended up getting worse were it started as a sporadic idle, than got worse to were it was surging and “bucking” like it was out of fuel while driving, than to the point were once the engine ran longer than 10 minuets it wouldn’t go, just had no power. I say catch it before it gets worse, but a “test” you can do is to crawl under the jeep and by the oil pan on the driver side, unplug the harness going the the upstream O2 sensor. And see if that changes how it behaves after a short drive. If it doesn’t than plug it back in and pull out the IAC (idle air control valve) located on the throttle body closest sensor to the fuel rail (held in with 2 t20 torques screws) and give it a reall good cleaning being carful not to push on the rod as it can throw it out of setting for your jeep, but spray it with carb or break cleaner both will work. And screw it back in and thy should make it better. Althoubwith the O2 sensor code that your getting 99% gaurentee that’s going to be your issue. And again just unplug the O2 connector and take it for a spin. If it stops acting up than go away and replace the O2 sensor. Upstream sensore is located on the driver side. Looking up from the bottom on the exhaust pipe above the front driveshaft you’ll see it just after the flange off the header has 4 wires sticking out of it… Hope this helps ya.

Attached a photo of how dirty my IAC was, and also a video of how bad my jeep when’s in a matter of 4 days from the O2 sensor. (Hope the upload)