'96 XJ Help - Error Code

My son purchased a 96 xj, 4wd (country trim), 4.0, with @ 220k on it, about a month ago. So far we have had the transmission changed (not unexpected), and we have changed the cam & crank sensors (with Mopar parts), changed all the spark plugs (the old ones were almost completely gone) changed the ignition wires, distributor cab & rotor. This was all mostly maintenance since we had no idea what had been done to it historically.

The Jeep had previously just randomly stopped running but changing the cam/crank position sensors seems to have corrected that. Right now it is throwing a P0300 & 301 error code. We really cant feel any misfire and it seems to run fine. A tad rough at idle but otherwise good. The error will stay off for about a day if cleared, but will come back.

We have done an intake air leak test with carb spray (not sure if this is the best way to test) with no change in idle at all. Last evening we did compression tests on the cylinders. Results:

1 - 150 wet 185
2 - 175
3 - 150
4 - 180
5 - 175
6 - 150

We only did a wet test on #1 because we simply ran out of light. I'm definitely no expert but is the difference in the psi for #1 great enough to cause a misfire? Also, if it is, then why doesn't it throw a P0303 & P0306 since those have the same value? Ahhh ... questions from my non-mechanic brain. If the above values shouldn't cause the error then I am leaning towards a possible fouled fuel injector. We really have no idea how long this jeep was sitting and since it is now being driven I am wondering if some crud has gotten into the injector on #1. I hope to find some 703s this weekend and rebuild them but who knows if we'll find any available.

Am I missing anything here? What steps would you more experienced folks take next. I plan on doing a fuel pressure test this evening and maybe switching the injector from #1 to another cylinder and see what error code pops up.

Unfortunately my son is in kind of a time crunch since he has to get this to pass an emissions check here in GA so he can get a new tag by September 1.

I think you're on the right track with looking at the
number 1 injector. Also check the plug-in for a good
connection.

I recently had a misfire code on an Explorer, and it
was caused by a sticking injector. Had about 150K on it.

Thanks.
I have been doing some looking at other sites and someone else's post said the normal compression psi for a 4.0L XJ is 120 to 150? Wonder how am I getting a couple 175s and 180s? Is this normal? I'll run the checks again this evening since I was just turning the key last night and my son actually read the numbers.

To be a bit clearer, we removed all the plugs and the ASD relay before taking the readings. I read somewhere that you should also hold the throttle wide open? (we didn't do that).

Forgot to mention this was done on a hot engine that had been allowed to cool for about an hour or so.

I'd start with the simple stuff first.

Double check all the new spark plug cable ends, it is real easy to to miss or dislodge the cable ends in the boots and/or not get them fully seated. I do it by feel, I lube the part of the plug or the distributor cap the boot seats on (not the metal part) with a silicon spray or even WD-40 with a Q Tip. Makes the boot go on easy and you can feel (hear) that finally click as the cable end seats correctly onto the spark plug end or the distributor cap tower contact.

Pop off the distributor cap and gently push the rotor shaft to the side. The shaft can get a lot of play before it actually completely fails and the rotor flies into pieces from contacting the cap electrodes. The gap between the rotor tip and the distributor cap electrode can be variable.This can cause misfire or *crossfire*.

One of the hard things about diagnosing a misfire is you never really know if it is fuel or spark.

Quick spark checks are to look at the spark, open up an old plug to around 0.060 hold it firmly on a good ground and watch it fire. Sharp crisp blue white spark you can actually hear go crack is a good spark. A thick pair of dry gloves and not leaning your crotch against the fender are advised.

You can actually use a timing light to test spark on individual spark plugs. Use it on the suspect cylinder in this case number 1 (code P030 *1*) and hold the strobe light on something with a dark background (PDC fuse box cover), you can see a misfire.

Take the number one plug back out of there and look at the end, check for fouling, oil fouling etc.

Double check your initial setup. Put your compression tester into the number one cylinder, with the distributor cap off turn the motor by hand until the rotor gets near the number on position and for the last quarter turn of the harmonic balancer align the tick mark on the harmonic balancer with the ''O'' mark on your timing tab (hard to see under the water pump, remove the aux fan, use a mirror). The needle will jump on your compression gauge when you get close to the ''0'' mark and the rotor will end up slightly after the number one pole on your distributor cap. Might be a waste of time, but at least you know the last guy wrenching on your Jeep didn't screw up the initial setup some way. Look at the cam sensor magnet to make sure it isn't loose and check that the half moon metal piece is opposite the number one side of the distributor whne th emotor is set up to TDC ''1'' (intial setup).

I've heard a misfiring injector can set a P0300 code. Maybe even a plugged injector. Maybe even bad fuel. No first hand knowledge of the validity, but if other stuff doesn't pan out, something to look at. I've seen bad fule misfires often enough for it to remain a possibility in my mind.

Your compression is IMO good enough, I've seen them running with a hundred PSI compression, not running well, but no misfires.

Something that will cause a miss fire is a head gasket leak allowing some coolant into a cylinder, the coolant and eventual steam can short the plug and cause a misfire. A quick check is to pull of the radiator cap and start the motor cold and let it run until it comes up to operating temperature. Some of the coolant will spew but it will eventually stabilize at a high level and stay there as long as you don't hit the throttle and just let it idle. With the aux fan out you can put a wad of rags under the filler neck on the radiator and catch most of the overflow while the motor is warming up. Any bubbles coming to the top of the filler neck are suspect, rhythmic bubble surges coming to the top are a bigger deal. A few bubbles may come to the top without a head gasket issue, so no need to panic. You can also clean up a plug end shinny new, put it in (in this case number "1'') run the motor up to operating temperature and let it idle for a awhile, then pull the plug, should be some carbon on there, if there isn't you have a problem. A little head gasket leak may create enough steam to clean a plug up too clean, it actually can blow the carbon off the end of the plug.

Try the key method for retrieve the MIL codes and see what you get.

Not something I would look at first, but something I always check when I get stuck is the voltage at the orange wire at either the CPS or the Cam Position Sensor (in the distributor). You should check the voltage with the connectors connected, which can be a little tricky.

Grounds are always suspect. Ohm check your block to battery negative.

I'm out of wind. If you need any more ideas I can continue later.

Well, we pulled the #1 fuel injector and swapped it with #4. It ran for almost 2 days without tripping the error code. Today the service engine light popped back on with the same as before, P0300 & P0301. So got to try some other things. Thanks 8Mud for your response. I'll work through your list.

Any and all further suggestions would be greatly appreciated

The mileage is getting up there. Sounds like swapping #1 w/ #4 injector made a change. P301 is a cyl. 1 misfire(we all can figure that out, it's obd2). Maybe #4 inj. no good either. I would tell you to put new injectors in it but sounds like they may be getting fouled because of mileage. IMO.

You might try checking the continuity of the number
1 injector wiring to the ECM. Pay special attention to
the ECM plug-in. It might be something as simple as
corrosion at the connector plug, or one of the wires
partially pulled out so as to give intermittant connection.

I'd first try disconnecting the connectors and spray with
an electrical cleaner or WD-40, and re-connecting. Then
see if there are any changes...

Sorry if I missed it. Is the #1 spark plug gap bridged w/ crud?. An A/P store probably sells a pigtail connector for the injector. Connector is a good point.

OK. Time has been scarce the last couple days. Anyway, as I mentioned previously I switched the fuel injector from #1 cylinder to the #4 cylinder and #4 to #1. 2 days later the check engine light popped on and an ODB test showed a P0300 and P0301, the same as before. Tonight I did a vacuum test off the intake manifold. It was a steady 17 to 17.5 psi. I also did a fuel pressure test at the rail. It was fluctuating between 50 & 54 psi. All injector connections were inspected, looked good and clean, then reconnected. I pulled the distributor cab and checked for play in the shaft. It had very little movement when I moved it side to side.

So I still can't figure it out yet, but hopefully I am eliminating possibilities. I also still have a few things to check from the previous, much appreciated comments.

Again suggestions are more than welcome.

Transpoquick said:

Sorry if I missed it. Is the #1 spark plug gap bridged w/ crud?. An A/P store probably sells a pigtail connector for the injector. Connector is a good point.

Click to expand...

They are new spark plugs and were clean as of last week. I'll check again this week since I want to do a repeat of the compression tests.

Transpoquick said:

The mileage is getting up there. Sounds like swapping #1 w/ #4 injector made a change. P301 is a cyl. 1 misfire(we all can figure that out, it's obd2). Maybe #4 inj. no good either. I would tell you to put new injectors in it but sounds like they may be getting fouled because of mileage. IMO.

Click to expand...

re: New injectors. I found 4 703's this weekend at a local salvage yard. Need to go find another donor vehicle this weekend. So once I have those and have them cleaned up with new o-rings I'll be swapping all of the injectors.

Yeah, you should drive it for a bit. Pull all the plugs out, number then w/ a black sharpie on the insulator. It would be great if you could post a picture of them all. I would of liked to see the set you pulled out when you first changed them.
Try to get injectors from a lower mile truck if you can, pull the plugs on the donor too. Don't use them if the plug is bad.
Hey, at least you not having overheating problems. Bwah.

Btw, you may want to replace the asd relay that was removed for the compression test too. According to the drawing in Haynes manual 50010, 12-22 it supplies power to all the injectors.
Normally the computer uses a knock sensor in the block to detect a misfire. I don't see one in that same drawing though.

I redid the dry compression test this evening. I pulled all the plugs, braced th throttle wide open and turned it over 5 times on each cylinder. Results:

1 - 165
2 - 170
3 - 155
4 - 165
5 - 165
6 - 155

Here are the pictures of the plugs currently installed, #1 is to the bottom


Also here is a pic of the original plugs I replace. Pretty much gone .. LOL


I also redid a quick check of vacuum leaks using carb cleaner. No change in idle at all. I looked at all the grounds and they look pretty good to me though I was running out of daylight.

What about the coil pack? Can this be checked or is it simply better to change it out with a new one? Also is there a relatively easy way to check for worn valve guides?

Transpoquick said:

Btw, you may want to replace the asd relay that was removed for the compression test too. .

Click to expand...

I'll see about picking up one tomorrow. Thanks

Imo, if the original plugs are numbered like the old ones, 1 and 4 are lean. Maybe those injector's are clogged or not opening enough, that will cause a misfire too.
New plugs look good though, the old ones look like they are fully intact, minus the wear obviously.

Well I'm running out of things that I think I can check myself. I have gone through every one's suggestions with the exception of 8Mud's "Put your compression tester into the number one cylinder, with the distributor cap off turn the motor by hand until the rotor gets near the number on position and for the last quarter turn of the harmonic balancer align the tick mark on the harmonic balancer with the ''O'' mark on your timing tab (hard to see under the water pump".

The last thing I did on Saturday was pull the injectors and thoroughly cleaned them and the injector connections. Replaced the o-rings as well. I also replaced the coil, re-seated all the spark plug wires. I had the positive batt cable off for several hours when doing all this so the ECM reset. Check engine light went out as it should. My son drove it yesterday and today for about 60-70 miles and the Check Engine light came back on. I guarantee it is a P0300 and P0301 code.

I am officially stumped !!

Well I'll be da*#ed! It didn't throw a P0300 or P0301 this time. It threw a P0420 this time. I'm not sure if that is progress or not since I haven't searched for a P0420 yet.

book that came w/ My actron code scanner says: Catalyst efficiency below threshold, bank 1.
Sounds like your cat. converter is bad. Always something.

Just throwing this out there but have you tried seafoam'ing the engine through the intake manifold? I've had random issues similar to yours in the past and 7 out of 10 times this has worked. Again, just a thought. Good luck.