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!

Kalali

Speaking from personal experience, in the Jeep 4.0L single cylinder misfires fall into two categories; some are real simple to diagnose and fix and others are simply impossible to diagnose properly. The simple category is when you find a bad plug, plug wire, low cylinder compression, bad injector or pigtail/wiring. In general something specific to that cylinder. As soon as you rule these out, you enter the realm of infinite possibilities left with the option of either test and/or replace various sensors.
Sorry can't offer much help but I've also been trying to fix a cylinder 4 misfire, albeit with different symptoms, for the past several months with no luck.
Good luck and keep us posted.
Just as an aside, keep a close eye on the coolant level as the MY 2000 XJs are plagued with a poorly cast head - 0331 casting and the slightest overheating can be a death sentence.

prell

What codes pop up? are they a cylinder misfire and injector code? I would ohm check the #5 injector for starters. Does this problem only happen when the engine is at operating temperature? If you were to start it and immediately drive down the road would it have issues?

lafguy8778

PO305 Cylinder 5 misfire detected

The code will pop up within a few hundred feet of driving. It makes no difference whether the car is cold or not. I have not done any voltage tests yet, but will when I get a chance.

It is also now getting some EVAP codes popping up, but I noticed a few hoses back by the canister that were pretty worn. Replacing those should get rid of those codes and since they just recently showed up, I don't think they are related to the current problem with the misfire and the 3000 rpm rev limit. Any additional thoughts are appreciated.

prell

Well, I had the same issues, I was getting misfires on different cylinders and Evap codes 1494 and 0441. I didn't think they were related and still don't but it is odd how they come up together. My misfire issue was due to a heat soak injector problem. I only experienced my misfires when really hot, like driving a hundred miles in 90 degree temps, stop, then restart, it would idle rough. This is a pretty common issue with the 2000-01. I eventually wrapped all my injectors with heat shield and made a heat mat for the top of my intake, seemed to solve the misfire issue thus far. I put on about 1000 miles and no CEL. I do however still get the evap codes. I replaced all the elbow fittings on my canister and on my intake, replaced gas cap, and still get a evap CEL. I need to smoke test it next cause I am running out of ideas. I would definitly look at that injector as a start, you could pull it and swap it with another cylinder and see if the code jumps to that cylinder. Make sure you ohm test it first and go from there but I have had bad injectors on my 300zx before and they spec'd out ok.

Allusion

would you be able to post a pic of your heat sheild? im throwing a cel for a missfire in cylinders 1 and 6 and i cant figure out what is causeing it.

prell

yeah, I can try to take a pic of it tomorrow. Basically I went to Pepboys, purchased an underhood heat shield, a large roll of it, it is about 1/4" thick and 4 foot by 6 foot. Cost was $40, I know it is expensive but I tried everything so I was willing to give this a shot. First I cut large pieces of heat shield/fabric and placed it on the top of the intake cutting slits in it to wrap around each injector and I pulled it up next to the valve cover, then I cut another piece for each injector and wrapped it around each one but did not zip tie it or anything. My main goal was to shield the heat coming off the intake. Keep in mind it is not one big perfect fitting piece but several pieces fit in around the rail and injectors. Anything to cut down on the heat soak I thought would be a good approach. I know you can buy heat shield fabric on ebay too if you cannot find it locally.

Veachon

I have had the exact same ongoing problem. Mine is a 2001 went ahead and replaced the cylinder head and head gasket. Still misfires on cold starts and hot starts doesn't matter. After idleing and above 1,000 rpms its fine just runs like crap right after the start up. I too have had the flashing cel intermitently and could not rev past 3k. I can shut it off and restart during a misfire episode and my jeep will idle smooth with no misfiring like nothing has happened. I'm stumped as well and hope you find a solution for yours.

BimmerJeeper

Do you think it's the ECU? It doesn't seem like a physical thing since it can resolve itself with a restart.

salad

Have you done a compression test?

I'd just go ahead and replace all spark plugs. Should be NGK ZFR5N. Hit the electrical contacts with some electrically conductive grease like Gardner Bender Ox-Gard.

Tried swapping fuel injectors around?

It's also possible that the Crank Position Sensor could be involved. The ECU relies on this to fire spark and fuel timing. I *think* on distributorless 4.0s the spark timing is also had from the Cam Position Sensor where the distributor is normally sits. Misfire detection is done via the CPS. The ECU uses it to read the current position and match it up to the firing order. If cylinder 5's turn comes up and there isn't a corresponding "push" in the flywheel then it's considered a misfire - sorry I don't remember the actual threshold of fails before it throws a code, but it takes running a while to show, longer if marginal. CPS is known to go out on these things due to its location, I'd say even more likely on the XJs with cats in the exhaust manifold giving off tons of heat.

Veachon

It could be ecu/pcm related but I'm not sure how to go about testing that as I don't have a scope or any other proper equipment.

tjwalker

Agree with salad on how to proceed. Troubleshooting is a process of elimination. Simple things first and that is fresh plugs, swapping the #5 injector with another and if neither of those things help, then running a compression test.

A spark plug can have a cracked electrode that you can't even see. Plugs are cheap; would be a shame to chase your tail for a simple plug issue. At the very least, swap the plug from cylinder #5 with a different cylinder to see if the misfire follows to the new plug location.

The compression spec for the 4.0 is 120-150 psi, with no more than a 30 psi variation between cylinders. If you don't have a compression gauge, many parts stores will rent you this tool very inexpensively. Never ignore the possibility of an internal problem!

Computers can go bad, but it happens less than you think and often they are blamed for symptoms they are not responsible for. Replacing the computer should be a "diagnosis of exclusion" meaning you have ruled every other possible thing out first. My gut feel is that this is not a computer problem.

mikeoc1

I had the exact symptoms as OP. I had good fuel pressure. I replaced the coil pack. no fix. replaced the #5 injector (new) and have not had a problem since. I have driven it about 500 miles since then.

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..

x91evo

isnt there a spot on the harness behind the valve cover on the drivers side,
that gets torn up somehow?
I remember someone mentioning that somewhere before but couldnt be sure.