Originally Posted by x91evo

bumping up an old thead..
I have all these same exact problems,
CPS seemed to be the problem. Replaced it with the same problems.

Replaced plugs, Getting code for bad ignition coil.
But dont want to start replacing parts for no reason..

Originally Posted by tjwalker

Post the EXACT code number. With OBD, the devil is in the details.

Always chase the code; OBD isn't perfect, but it is pretty darn good at pointing you in the right direction.

Originally Posted by lafguy8778

OK, I am not the owner of the vehicle I will be discussing, but I am the one working on it. The vehicle in question is a 2000 Cheorkee with 4.0L engine and automatic transmission, fuel injection, and electronic ignition. I have spent a fair amount of time working on this truck and searching the forums for answers, but just can't seem to get the problem

resolved.



A little background information: The vehicle developed a problem of starting rough (took a few tries to get it to start) and after a period of time the engine light started flashing. My friend (prior to ever talking to me) took it to a trusted shop and they read the code as a Cylinder 5 misfire. Somehow after spending some time diagnosing the problem, the shop determined that the fuel pump and ignition coils needed replaced. With that information, my friend asked for my help in doing the work. This model of Cherokee has the fuel pump that is accessed from the top of the fuel tank and has an enclosed pump. My friend purchased ONLY the fuel pump (not the whole unit) and the auto parts store (NAPA) actually gave him the wrong pump kit. I was able to make it work, but had to re-use the screen, which I cleaned out really well. We re-installed the pump in the tank, hooked everything back up and then made sure the pump started, which it did. We then replaced the ignition coil pack. The coil pack (there actually three) is one unit that has the plug caps built in and bolts directly to the plugs. While we had the coil off, I took plug #5 out to inspect it and it looked fine other than a little bit of oil on the threads, but not enough to get worked up about. There wasn't much wear on the plugs, so we didn't replace them. After replacing the coil, I cleared the codes and we started the vehicle. Started rough, but usually takes a second after replacing a pump, but eventually started fine. Revved high and no code(s). Turned it off, tried to start it again, and fired the first time. He needed to leave, so we didn't have time for a test drive, but everything seemed OK. About two miles down the road, the engine light flashed again and at some point in the trip he tried to rev past 3000 rpms and it wouldn't (this was not a problem prior to the work we did that day). He checked for codes when he got to his meeting and it was reading a cylinder 5 misfire. After his meeting he brought it back to show me the revving problem. He truned it off in the garage and I had him re-start it and it revved past 3000 just fine. We took it out for a drive and when he tried to go past 3000, it wouldn't and it seemed as though it was hitting a rev limiter. I hooked up my fuel pressure tester and had him start the vehicle and initially I didn't get any pressure when he was turning it over, but eventually the pressure rose and the vehicle started. Once running it held 48psi no problem and when we revved it, no drop and again, it went past 3000 rpm. I must mention, when we got back from our drive, we put it in park and revved the engine and it wouldn't rev past 3000rpm. Not sure what that is about. I was never able to replicate the 3000 rpm situation while the pressure tester was attached, so I can't verify if the pressure drops when it gets to that point, although I don't think it does. It seems more like an actual limit issue than a pressure issue considering it hits at the exact same point every time. I was at a loss for what to do, so I wanted to do some forum research and get back to him. The vehicle was driveable.

After much research, it seemed as though the consensus was that it was a TPS (throttle position sensor) or pressure regulator issue. Most leaned towards the TPS being the culprit considering the list of problems. Since the pressure regulator (and filter, they are one unit) is with the fuel pump (really bad design if you ask me!) and the TPS is cheaper and easier to get to, I suggested he replace that first. He did and it started up right away and no code(s) popped up. He took it out for a drive and shortly after he began the engine light flashed and he couldn't go past 3000rpm. He read the code and it was a cylinder #5 misfire. Once again, I told him I would do further research the issue before we even consider dropping the tank. After additional research, someone suggested unhooking the upstream O2 sensor and taking it for a drive as the symptoms could also be related to a faulty O2 sensor. He did that yesterday and initially had no problems. He took it out, got it to rev all the way to 4000 rpm and then after that the engine light flashed and he once again couldn't go past 3000rpm. He read the code... and you guessed it... cylinder #5 misfire. I find it odd that it did run fine for a bit. Any thoughts on that?

I am at a loss right now. I don't want to have to drop the tank and replace the regulator without good reason to believe it is the problem. I also don't want him to continue to throw parts at the problem until everything is replaced. I have decided before doing anything else, I would post this to the forum and see what others say. I have heard it could be MAP sensor, the crankshaft position sensor, one (or both) of the CAT's clogged, the pressure regulator, and on and on.

I am sorry for the length of this post, but I wanted to be sure to explain everything that has been done and what problems have been noticed. I often find that too little information is often given and then the next 20+ posts are spent gathering more information that should have been given right away.

I welcome any suggestions for how to fix or better diagnose the problem. I would ask that you also provide a reason why you think your suggestion may be the problem so I can evaluate where you are coming from. I have quite a bit of knowledge working on cars and am not afraid to do most anything, so hit me with your best shot! Thank you in advance for you help and suggestions!