Poor Performance, Misfire...

Hello all,

Background:

88
4.0L 275K Miles, Rebuilt at 250K
New Plugs, Wires, Distributor, Cap, Rotor.

We are fighting a nasty problem with our Cherokee misfiring. Sometimes it seems like it is running smoothly, but the power is really poor. It's almost as though the spark is generally weak and we aren't getting a good start on the combustion.

It will frequently begin missing steadily on one cylinder. It sometimes misses on multiple cylinders.

When the problem first began, I noticed that it would misfire when it was started warm (engine had only been off for a short time). It would run really bad, but then almost instantly clear up when accelerating and then run fine until it was shut down again. This was about 3 months ago. Now things have gotten steadily worse. It is now nearly undriveable.

Any ideas most appreciated.

Thanks,

WillyS(LC)

90 XJ Limited 4.0 4x4, I have the same problem and I'm thining along the lines of an injector 1st or pressure regulator. I can't keep pressure in the rail. We really need some help from you fuel guys.
Craig

Our problem is definetly Ignition. I know because the Cat Converter gets really hot from the unburnt fuel.

WillyS

Just a shot

Check your coil.
I had the same problem, and crap mileage to boot.
I was getting error codes for irregular voltage from the coil and was misfiring as well.
Problem solved with new coil.

The mounting on the Renix years' coils can cause that (just remove and re-mount clean) - Also, you mention new plugs - have you pulled the plugs to determine which is your issue?

Has this been happening since the rebuild? (did you reinstall your block or a reman? and is the cam new?) Often these will necessitate the reindexing of the distributor due to distributor-deck-height issues with the later blocks and the cam-grind changes to off-set the modified deck-height.

Simple check is to loosen the dist's hold down bolt enough so you can give it a little twist - without upsetting anything -- then check how the engine operates on both extremes of the rotation afforded to you (it's only a few degrees, but the change was very small; why they thought that it wouldn't cause any issues.

There's a TSB on how to properly index the dist if needed...

Also - I know you indicate new plugs -- oncew fouled they are hard to clean while in the engine & they will tend to get worse - pulling, inspecting, and cleaning can help the problem and the diagnosis.

The plugs have less than 2 operating hours on them. i'm sure they aren't the problem the block is the same one. the cam is new and it is an after market cam. thanks for all of the tips. i will check on some things in the morning. To add to what my dad said. It will run good until it gets warmed up and then will continue to run good until you turn it off and restart. after it has been shut down and then restarted it will misfire. Again thanks all for the help.

Jerm S. (son of willy)

Jerm S. said:

The plugs have less than 2 operating hours on them. i'm sure they aren't the problem the block is the same one. the cam is new and it is an after market cam. thanks for all of the tips. i will check on some things in the morning. To add to what my dad said. It will run good until it gets warmed up and then will continue to run good until you turn it off and restart. after it has been shut down and then restarted it will misfire. Again thanks all for the help.

Jerm S. (son of willy)

Click to expand...

It's a long shot, but I had a problem almost exactly like this on a 93, which turned out to be a wiring harness fault. Given that this is an 88, with the potential glitches of the C-101 connector added in, it's worth a look. You should at least check and clean every connector and every ground related to the engine that you can find. What happened in my case was that a single splice in the wiring harness to the fuel injectors had just enough resistance to heat up and cut one injector out, but only after it warmed up. There was still enough voltage to fool an injector tester, too, but not quite enough to fire the injector itself. Like yours, it ran perfectly until it either heated up after an hour or more, or after it had been shut down hot. I went through a lot of stuff before I tracked this down, but finally went through the entire system with an ohmmeter, both hot and cold. After weeks of struggle and many bucks' worth of tuneup parts, the repair required a couple of feet of wire and took 10 minutes.

I hit on the diagnosis, by the way, when I ran the Jeep with a vacuum gauge (long hose, gauge taped to the side-view mirror). When it started misfiring, the vacuum gauge did not register anything, which is "negative diagnostic" for engine internals or ignition problems.

Harmonic Balancer

Matthew Currie said:

It's a long shot, but I had a problem almost exactly like this on a 93, which turned out to be a wiring harness fault. Given that this is an 88, with the potential glitches of the C-101 connector added in, it's worth a look. You should at least check and clean every connector and every ground related to the engine that you can find.

Click to expand...

Actually, my bet is on something electrical like wiring harnesses, grounding or whatever. I really don't think any components are actually involved. I think it is a matter of corroded wires or something like that. It is going to take tremendous patience to figure out. I actually think it might be the ignition switch itself. If you look at the electrical schematics, the switch is not entrirely out of the question. When you think about the hours and hours that the switch stays in the on position, couldn't it wear out?

WillySLC

The ignition swith is a definite possibility, judging from past threads here.

Have you tried a vacuum gauge yet?

I know you probably hate to toss $$-$ at this right after rebuild, but consider tossing a fresh 02 and a CPS, and 2nd the fresh plugs (or at least pull one at a time and inspect).

The 02 and fresh plugs I installed Saturday fixed my 89 4.0, but my stumble/misfire/whateveritwas more intermittent than what you're describing...

Good luck

Willy, there's a camshaft position sensor (CMP) inside the distributor. It's a hall effect pick-up coil , that determines the #1 cyl power stroke, and determines timing in conjunction with the CPS. If the pick-up coil fails completely, the engine will continue to run (poorly), but will not start after it's shut off.

If it starts and runs fine when cold, and then acts up when it gets warm and when it's re-started, I would suspect the CMP. The Chilton's manual has a testing procedure for it. Basically you're looking for a 0v to 5v signal change when the distributor turns 1 revolution.

In your post you said it has a new distributor. If it was changed to fix this problem, and didn't, then disregard my post. If the new distributor worked fine for a while, maybe a wire is broken going to the sensor or in the pick-up coil itself, and is giving an erratic timing signal.....

engine hesitates and sputters when restarted hot

Hi-

My 96 4.0 w/ 190k hesistates and sputters for a short period when it is restarted hot as well. Then it runs fine. Was there and resolution to this thread?

If it sputters first, then clears up, I'd check first for a fuel injector that isn't closing properly. If it dribbles fuel, it will miss until the fuel burns off, then run fine. To test this properly requires a fuel injector tester and a pressure gauge. The tester fires the injector, and the gauge records whether the pressure keeps dropping after it stops. This presupposes that your fuel pump check valve holds pressure, at least for a little while.

If you have a miss, and it is possible to pull over with the engine misbehaving, try pulling the connector to each injector one at a time. When you pull a good one, the engine will slow a little, then the computer will compensate and idle will come back to normal. If there's no change, or something else happens, you know you've got a bad one. Although the 91-up system fires the injectors individually in sequence, it is not actually critical, so you can also swap adjacent connectors to determine whether the fault is in the injector itself or the harness. The REnix system, by the way, fires all 6 at once, so they really are interchangeable.

There seem to be at least 3 different styles of injector connectors. If you have the kind of connectors that have a split metal ring on them, you can remove the rings temporarily, otherwise the plugs will be very hard to pull, especially when everything is hot. Practice pulling the connectors before you do this, so you know how they go. They can be pretty unfriendly.

I keep harping on this, but will mention it again: once upon a time, mechanics would learn all sorts of things with a vacuum gauge, and it's still a good thing to try. Generally speaking, ignition faults, along with internal engine problems, will show on a vacuum gauge, but glitches in the fuel injection will not, and this is a good way to determine which system you should be concentrating on.