minikee

Hello everyone. I'm at the end of my rope with a rough idle issue with a 2000 Cherokee 4.0l. I'm new to Jeeps but not new to mechanics. I've been working on cars since I was a kid. Not an expert by any means but I know my way around.

Here are the symptoms:

When I first start the jeep, it has a mild misfire. You can feel it a little (not much) and you can hear and feel it at the exhaust. Put it in gear, it gets a little worse but still not bad. It runs very rich the whole time.

After it warms up (<2 min), the misfire is more pronounced at the tail pipe and also at the engine. Once it finishes warming up and is at running temp, the misfire is the worst, especially after shifting into drive. At this point, I can now hear a popping/knocking noise coming from the engine. At first I thought it was backfiring due to the misfire, but it almost sounds like fuel knock. Somtimes it sounds mechanical but it's not consistent. It's a lower knock, not lifters. I have a stethoscope and can hear no abnormal noises from the valve train. The top end of the engine sounds very good. The bottom end of the engine sounds tight as well. I can't figure out where this popping/knocking noise is coming from but it's random and inconsistent and only does it when the engine is hot. The misfire seems to be inconsistent as well, unlike say, a bad plug where the misfire is always in the same place. I have tested and replaced the TPS, IAC, clock spring, and coil. There are no codes present. I unplugged the MAP sensor and IAT sensor, both threw codes so I know the computer is communicating. I reset the codes. The jeep starts strong and quick so I'm ruling out the Crank sensor. Would bad/failing o2 sensor cause any of these symptoms? Live data shows both upstream o2 sensors somewhat within range, but the results from each one are not very consistent with each other.

When I bought this jeep, it ran fine. Needed a little TLC but didn't have the issue I just described. About 20 minutes into the drive it started acting up. I could hear relays clicking sporadically near the glove box, which correlated to the rough idle. After that drive, the relays stopped clicking and the misfire/rough idle is constant. So now I'm thinking wiring somewhere? Any common places for wiring/electrical issues I should be looking first?

CCKen

What does live data show for Short Term Fuel Trim (STFT)? And Long Term trim?


Is your 2000 Federal Emissions or California Emissions (exhaust)?


If STFT shows fuel being pulled off it only takes one leaky fuel injector to dump excess fuel in the exhaust and the PCM is responding by pulling fuel off, resulting in the other cylinders running too lean.

minikee

This is the only pic I have with me. I'll get some more comprehensive readings later and post them.

CCKen

Looks like you have a Federal missions exhaust system.


-16.4 is pulling a lot of fuel off.


1/1 O2S should be toggling every 2 seconds and the 1/2 O2S should be a little slower.


Check the STFT at idle. If it's high like the -16.4, in neutral, advance the throttle to around 2000-2500 RPM and see if the STFT goes to near zero. If so, I would suspect a leaky injector. If it stays at a high negative trim it's not an injector but something else, like a cracked exhaust header leaking oxygen in to the exhaust stream upstream of the upstream O2S.

mrbill

OK......this is going to sound crazy, but I had a similar experience. I was having some window tint work done to my 2000 Cherokee and the guys left the doors open which ran the battery down so low it would not start. They jumped it and it started, but ran so rough it could hardly move when trans was engaged. After talking to my mechanic he recommended that I disconnect the battery for 10 minutes or so and then reconnect and try to start again. It worked and the engine purred like a kitten. I later asked my mechanic for an explanation and he stated that this resets the vehicle onboard computer much like rebooting your desktop or laptop computer. Hope this helps.

minikee

Ok, got some more data from the scanner.




I'm still new to interpreting this data, thank you all for the input.

CCKen

If I see this correctly, you are at 2250 RPM and still show a -13.3 fuel trim. Correct?


If so, the fault lies somewhere else, like a cracked exhaust manifold - but this is not conclusive.

minikee

Yes, that's correct.

Just went through the entire undercarriage and engine bay looking for loose/chafed/damaged wires. Nothing found. Unplugged and replugged all connections, followed the harness and found nothing indicative of a short so commonly found on these cars.

I started the jeep and found yet again that it runs smooth and even until it warms up. On a cold start it still smells rich, and even after closed loop it runs pretty decent. The closer it gets to 200 degrees, the worse it runs, chugging like a diesel, still rich. Couldn't find any cracks or leaks in the exhaust manifold or intake. The engine bay looks pretty good for 155k, alot better than some of the jeeps I looked at. Could a bad o2 sensor be to blame? No codes yet...

CCKen

Might be an O2S but they usually show their **** at cold start up idle/drive away stumble, not when it warms up. Might check the O2S heaters fuse in the PDC to make sure. I'm posting a 2000/2001 PDC fuse layout but it's for a California XJ. Maybe the O2S heaters fuse is the same, regardless. The 2k / 2K1 may have the same fuse heaters relays (?).


Let the Heep set over night to let the coolant temp stabilize to OAT, then using your scanner check the ECT sensor temp and the IAT sensor temp to see if they differ a lot. Max is 3*F difference.


Checked mine the other day and saw the IAT was about 6* off from the ETC temp and was jumping around. Installed a new OEM IAT and if matched up with the ECT perfectly. OEM IATs are expensive dudes but BWD sensors suck and are a waste of money.


Rogue ECT or IAT sensors can cause a rich mixture, which may be your issue.


I bought a digital meat thermometer from Home Goods and used it to measure the coolant temp at the radiator (cap removed) to get a base line for checking the temps with my scanner.

BlueRidgeMark

Might that vary between brands? I just solved a similar problem by replacing the 02, and it was very definitely worse once it got well warmed up. I have no idea what was in there before. I replaced with a Mopar unit.

EZEARL

MOPAR brand. I was wondering why you had to pay so much. If I not mistaken MOPAR O2 sensors are NTK/NGK.

minikee

What I have just discovered and realized: this is a 2000 with 2000 exhaust manifold, with the 2 cats deleted and the ports merged into 1 downpipe. This downpipe travels through a single cat further down the line. There is a o2 sensor before and after the cat. I saw this yesterday but didn't realize that there was a difference between 1999/2000 and that this had been modified. That being said, I wonder what they did with the other set of o2 sensors? I don't see any unplugged harnesses hanging around. Could this be causing my problem? It's basically been converted to 1999 exhaust setup on the 2000 motor. I have no idea if the computer was swapped.

EZEARL

What makes you believe the exhaust system was altered? How many o2 sensors your vehicle has depends on the emissions package. California(and maybe a couple others?) had 4 the rest had two. After 2001 I think all had 4.

CCKen

I think you're making a church out of a ****house.


There should be a placard on the firewall that states what emissions rule the vehicle complies with. Look at it.


Here's a pic of the exhaust system for the 2000 XJ California and 2001 50 states.


.

.
Here's a page from the 2000 FSM showing the 2000 federal exhaust system and the California dual cat exhaust system. No majic involved.

minikee

According to the VIN, it's federal emissions. The picture above is the type of exhaust the jeep had originally. I can see where the upstream o2 sensors (directly under the intake) were removed and the holes plugged. Where the manifold to cat flanges are, the cats have been removed and a straight pipe has been welded on each one, merging into 1 pipe and proceeding down into a single cat next to the tranny. So it's set up like a 1999 exhaust but was originally a 2000. It looks like figure 33 above but has o2 sensor holes that have been plugged.

See example here:

https://www.cherokeeforum.com/f2/200...roblem-202384/

Rock Auto shows the difference in exhausts:

1999:



2000:




Mine is set up like the 1999, but was modified to be that way. I have the 2000 manifold, with the o2 sensor hole plugged.


For the 2000 setup, it would have 4 o2 sensors. But now it only has 2 because someone modified it. I'm wondering if this isn't what's causing problems. If they swapped the computer to accomodate the exhaust, does the 1999 computer have all the same variables for the rest of the engine as the 2000?