Problem running smoothly. timing? valve? i'm stumped

Here's what i'm up against. This is on a 1994, 4.0, AX-15 vehicle with 203k mi on it. This weekend, I decided to work on the Jeep a little. It was supposed to be a simple tune up, some new plugs, cap, rotor, coil, and an optima battery. My Jeep just turned over the 200k mark and I felt the need to mess with these parts which had about 30+k on them, and really weren't causing me any trouble except some lengthened starts. Nothing to be concerned about I just decided I had the $ so why not get the thing in top shape. So my buddy and I changed all the mentioned parts out, it seemed to run fine. Later that night i took it for about a 5 mile drive. It was doing fine and I decided to test out the performance of my new plugs and all a little, so I was taching it up to about 4-4700 rpm on shifts on straight stretches. Nothing I've never done before. As I was nearing the end of my drive i started to notice the exhaust note sounding a little different. When i pulled in at my destination, I opened my door with the engine still runnign, and it sounded it was running a little rough. I did some rev's and rolled up the rpms to try and listen to see if anything was wrong, and it didn't sound too bad after a bit, so I shut it off. When I went to drive it home, it barely had enough power to pull out. It had pleanty of power on the way there, but I was running like I had 2 cylinders out and it sounded like an old tractor. I limped it home and have been trying to diagnose the problem ever since. I changed out the cap and rotor for a different brand, I have changed the wires, I have even changed the distributor to a new one. I have the stock coil back in it, and the old plugs. When i pulled the new plugs (which were bosch platinums, same as i had been running) all of them looked great except #6, which was discolored.

So tonight, I messed around with the timing while changing the distributor. everything I do makes it run much worse. Yes, the firing order is correct, unless anyone knows it to be diferent than 153624 on the 94's. I took the wire off the plug on the #6 and grounded it and the engine ran the same as with the wire on, even at different rpms. So...short of taking the valve cover off, is there any way to tell if I have a valve that isn't seating correctly? a bent push rod? I know this could be a possibility, and i also know that my tuneup could have had nothing to do with this problem! Any sensors going bad that would make this sort of thing happen? My bag of ideas is completely empty at this point...i've described this as well as i can, let me know what you think. thanks in advance

don't everybody hit me with suggestions all at once. i can only do one thing at a time. little update here-
i don't the platinum plugs will have anything to do with it (guys on jeepin.com sugested that) because i am running the old plugs now, which are also platinums and i ran them for 30k mi with no issue.

I did a compression check tonight and got 130-135 each time. I did have it running normally for about 2 minutes tonight. My buddy and i were running it, rolling up the rpms and such wiggling different plugs on sensors and trying to see if anything made a change. we sprayed some carb cleaner in the tb while it was running, and put some fic in the tank and it was definately running fine for just a few minutes. we revved it up a couple times and after a few revs it started sounding like a missing cylinder again. So i would venture to guess it may be a fuel delivery problem? clogged/bad injector? I filled it with 93 tonight and dumped in the rest of a bottle of fic. i'll run 1/2 of that tank thru and see if it makes any change i guess. still open to suggestions on things to try or check. thanks

check fuel pressure. sounds like whate my MJ just got through with. still not running good.

can i just use a tire gauge on the schrader valve on the fuel rail? what kind of pressure level am i looking for? thanks for the suggestion, that's definately something i will check.

No, you cant use a tire gauge on the schrader valve... I asked the same question . I got a fuel pressure tester at AutoZone yesterday for $36.99. And I am not exactly sure what the fuel pressure should be on a '94. I do know it should be above 30 psi though . Just check it and post up results and someone else will definately chime in exact #s.

Check the cylinder compression. If it is low you might have a valve problem. 120-150 lbs is the norm. If the compression is fine and the spark plug is firing then the fuel injector is malfunctioning. A fuel injector malfunction can be caused by water in the fuel, a fuel pump going bad or the fuel regulator setting the pressure to low. Oh, did i mention a fuel filter!

muddygp said:

I did a compression check tonight and got 130-135 each time.

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compression is fine. i will run the fuel pressure to see what i have, then start playing around with all the items suggested. my money's on a clogged injector, but we will see what happens. thanks for the help!

So Here's a new one...I let it run for about 15 minutes today while I ran inside. It heats up a little cuz i have my electric fan on a manual switch, so it was up at about 225 on the gauge. I came out to drive it home and it was running fine. I drove it for about 2 minutes, until it cooled down to normal running temp (210 on the gauge) and it was running crappy again, but not as bad. I got home and let it warm up again and rev it and it sounds great. Any ideas why this could be? Clogged injector working itself out, high heat burning off junk perhaps? I'm interested to get some input on this, as I really have no idea why this is happening.

So tonight, I messed around with the timing while changing the distributor. everything I do makes it run much worse. Yes, the firing order is correct, unless anyone knows it to be diferent than 153624 on the 94's.

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Exactly "how" did you change the timing?

"how" i changed the timing was by pulling out the distributor and moving the rotor shaft and lining up the oil pump drive slot and dropping the distributor back in...didn't do anything but make it run like crap so i wouldn't say it is adjusting the timing at all, rather, just screwing it up. it is right back where i found it now.

when it acts up again, pull plug wires one at a time and maybe you can narrow down which cylinder(s) are missing if so.

btw, if you narrow it down to 1 cylinder and rule out an injector or plug. might want to pull the valve cover and take a peek at the valves. make sure they are moving. check pushrods and rockers.

muddygp said:

I got home and let it warm up again and rev it and it sounds great. Any ideas why this could be?

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Is it possible that your temperature sensor is going bad? I'm talking about the one on the front of the head that tells the ECU what the temp is.

Perhaps your ECU thinks the motor is still cold, so it sends an inappropriate signal down to the fuel injectors ... a mixture too rich can make the motor run rough when it is hot.

I replaced the temp sensor tonight and put in some champion copper plugs. No difference, still all the same symptoms. I really would like to get to the bottom of this...

Fuel pressure should be around 39 psi with the vacuum line to the regulator disconnected at idle. Around 31 psi with the vac line connected.

You say the #6 plug looked discolored, but you have good compression on all cylinders, right? If it's not a plugged injector it might be a bad electrical connection on that injector circuit. Check the connector and ohm the wires with a meter.

Kyung

tried to do a pressure check today but didn't get to. I will mess around with injectors tomorrow. Thanks for the pressure values, hopefully I can look into that stuff this weekend.

My '97 with the 4.0L and the AW-4 had a similar problem when I did a tune up. To be completely honost with you, I still don't know what the problem was, but for a few months it ran like shit. Now it runs fine. It would have trouble idling, and would stall at stoplights. When I would drive down the highway it would miss but the feeling was like it hit a rock.

Anyway, you said that when you grounded the #6 plug wire it had no effect on the engine? If so then I would say that the #6 plug, wire, or terminal may have a problem.

I also know that a malfunctioning crankshaft position sensor can have similar effects. The ones I have had go bad work poorly while cold, but better when they warm up.

You might want to check and see if there are any loose electrical connections around where you were working. Sometimes they can wiggle and cause random misses.

I too suggest replacing spark plug wires, this sounds like a cross-fire. The whole ignition system has been replaced beside the distributor, might as well give it try.

With the new wires, butter every spark plug boot and the coil wire with dielectric grease to be sure theres no spark voltage leaking to ground.

I had a new distributor in it, made no difference at all...I have never had any problems with it so I took the new one back becuase it didnt help and it was expensive. Wires are new Bosch 7mm wires, which were what i had in it before. I'm going to pull injectors tonight to see what they say.

If i pull my cps and check reistance, anyone know what ohms i'm looking for to see if it's any good? Can i even check it that way? Thanks.

When the crankshaft is at TDC the oil pump slot should be a little past the 11 o'clock position and that will put the rotor at the 4 o'clock position. Is that about where you have it set?