Rough Idle - No stall, No CEL

Thanks for having me yall. I'm a member at a competing forum and have been unable to diagnose this rough idle for a year or so. When I have issues, I typically find the best answers on this forum, so, I figured I'd join and see what happened. Thanks in advance for all help/criticism I get. This may be a long post.

I've been working on this jeep for a few years now. Bought it from a buddy as a project after he drove it overheating until it quit working. Replaced the motor with a JY motor from a 1993 Grand Cherokee. Used the head and long block. Swapped everything else out.

1994 XJ
1993 motor
175k miles
4wd

It's got a very slight miss at idle. Hard start when it sits. Maybe 3-4 seconds to start. Runs rough when it's cold and hasn't been starting for about a minute. Evens out then idle has a very slight miss or stumble. No check engine lights. At 2500 RPMs I can't notice the miss. Stumbles like its starved for something when throttle is smashed down. Then it catches up and purrs. Friend said it smells rich?

I've done a compression test about a year ago. I can reconfirm, but, it's been sitting this whole time and I doubt it changed much. With putting a little oil in 6 it raised up and met the others. Little wear on those rings, but, not sure it's enough to cause my issues?

1. 131
2. 126
3. 130
4. 131
5. 124
6. 114

Spark

I've replaced plugs (not champion), wires (twice), and cap (two used). I've replaced the ignition coil. Seems to be getting good spark, but, willing to recheck if deemed necessary for troubleshooting purposes.

Fuel

Put a pressure gauge on fuel rail and it primed up to about 40psi. Took a couple rotations of ignition. Drops pressure slowly. Starting the jeep yielded a 32psi at idle. Removing the vacuum line on Fuel Pressure Regulator increased fuel pressure to 41psi.

Checked each injector and all had 15 ohms resistance. No clue if they are clean or clogged. Going to check for a leaking injector today using the paper towel trick with the fuel rail out.

Replaced the fuel filter.

Timing

I assume timing is good. I don't know how to check and moved the distributer a few times till it ran well. First shot was off a tooth or two. Very slight miss so I assume timing is good. Thoughts?

Vacuum

Found a few cracked hoses and boots. Replaced what I found. No tester to check vacuum. Something I could rent from parts store? Worth doing?

I probably left some stuff off, but, hopefully this gives us a direction to begin diagnosing? I have 2-3 of every bolt on part for this long block. Should be able to troubleshoot effectively.

I didn't realize we were having a competition. Sounds like we're winning.

Mine has a stumble at idle. I don't bother.

Ha. The winning part is yet to be determined. Lol

I'm trying to turn this into a reliable vehicle capable of playing on the trails as well. Thought this was going to be a $1k project, but, I was wrong. I'm on the third motor....oops.

It's got a host of other issues like AC, brakes, transmission, regulators, radio, suspension, pretty much everything that can go bad did. Motor and transmission are by far the most important to me to have running like a top. Perfectionist I guess.

I'm in the process of tossing on a Smittybilt XRC bumper. Turns out I have to take a grinding wheel to the fenders since mine hang too low. Just turned an hour long project into a 4 hour job. With beer at 11:15 am. Ugh. Ima's body shop is open for business.

Okay, pulled out the fuel rail and attached the fuel lines to it. Laid down at shirt and primed the fuel pump a few times. Checked and everything seemed good. No squirts on the shirt. I assume they are not sticking now. I can hear them all operate with a screwdriver.

However, I did find a broken o ring on cylinder 5. Replaced it. Idle seems to be a little higher now. Was at 1000 or so when I first started it back up. Dropped to the 750 range. Before this I think I was idling at 600. Could be crazy though.

Just so I know, should additional information be posted as an edit, or, should it be a new post. Don't want to seem like I'm artificially bumping the thread. Just trying to add relevant info that could point to the cause....or rule one out.

Any thoughts on this?

Have you pulled off and cleaned the throttle body thoroughly with good tbody cleaner and qtips (especially the IAC pintle + channels)? I'm really only familiar with RENIX-era 4.0s but that can go a long way towards fixing idle problems.

Have you put in a new bosch or NTK 02 sensor/cleaned or replaced your MAT sensor?

Evens out then idle has a very slight miss or stumble. No check engine lights. At 2500 RPMs I can't notice the miss. Stumbles like its starved for something when throttle is smashed down. Then it catches up and purrs. Friend said it smells rich?

Click to expand...

These were exactly the symptoms I was fighting with mine for months. If you manually flip the throttle wide open under the hood, does it bog momentarily and then attempt to catch up? I troubleshooted that endlessly on the distributor side, but it ended up being that my 3&4 and 5&6 injector harness pigtails were switched. Might be a possibility since you swapped engines? I was able to tell by pulling back the rubber boot on the pigtails until you can see the wire colors, which at least on my 88 are supposed to be:
1-light blue
2-light green
3-tan
4-yellow
5-white
6-brown

I don't know if 1993 harness colors are the same, but since it fixed my issue that's the solution I'm pushing :lecture:

Yes, I've cleaned the throttle body and intake manifold prior to re-assembly of the motor. I'm pretty sure I did at least. Looks clean as a whistle looking inside of it from top.

I have not changed the 02 sensor or MAP? sensor. Haven't cleaned either. Is there a way to test these in lieu of swapping parts?

Your symptoms are exactly as mine are. Swapping the pigtails is defintiely a possibility. I really thought we were onto something here. I looked up the diagram for a 1994 xj and it labeled the fuel injectors by color. However, I looked at my wiring and all the wires going to the fuel injectors were the same. At least 1-3 were. Boots are already tore open with exposed wires.

Amidoingitwrong?

I was going to show a picture of the wiring diagram, however, I can't post attachments?

Ima Doit said:

Yes, I've cleaned the throttle body and intake manifold prior to re-assembly of the motor. I'm pretty sure I did at least. Looks clean as a whistle looking inside of it from top.

I have not changed the 02 sensor or MAP? sensor. Haven't cleaned either. Is there a way to test these in lieu of swapping parts?

Your symptoms are exactly as mine are. Swapping the pigtails is defintiely a possibility. I really thought we were onto something here. I looked up the diagram for a 1994 xj and it labeled the fuel injectors by color. However, I looked at my wiring and all the wires going to the fuel injectors were the same. At least 1-3 were. Boots are already tore open with exposed wires.

Amidoingitwrong?

I was going to show a picture of the wiring diagram, however, I can't post attachments?

Click to expand...

There are ways to test the O2 sensor with voltmeters, both in and out of the car but they're over my head (I'm still learning voltmeters). If you are better with them than I a google search will turn up specifics for your H.O. I went with the $50 option and replaced mine. I think your Jeep should be OBDI so you could find someone with a scanner and have them take a look at your live fuel mix to see if it looks normal. I have a cheapie bluetooth OBDII dongle + the Torque app on Android I use for my BMW; I don't know if anything exists like that for OBDI but it's super useful.

The MAT (Manifold Air Temp) sensor is different from the MAP sensor, and it sits behind your throttle body screwed into the intake manifold. It's also known as an Intake Air Charge sensor and sometimes other things. Along with the 02 sensor and Coolant Temp Sensor it helps determine your fuel mixture. It's supposed to look like this:


But if yours is original and untouched it probably looks like this:


Looks like you can get a new one for your year for $8 on amazon right now. You can also clean them up with throttle body cleaner but a can of that will probably cost you close to the whole new sensor.

Might as well check your last two injector pigtail colors; my first two were fine and all the rest were switched. If you want to post attachments you can upload your picture to http://imgur.com and imbed the image using bbcode tags

Well, the past two or three times I tried to start the motor, it's had a very hard time starting. Violent shaking and stumbling before it will fire up. Tonight's was the worse. I decided to give it some throttle to see if it would even out and idle, however, it starting knocking. Loud. I got it to idle smoothish, but, it's ticking/knocking very loud.

Don't know what's going on. Definitely internal. Couldn't pinpoint it with a 18" screwdriver, so, going to pull off the valve covers and see what happens. It's running real rough now. Maybe this was my issue all along...I just broke something loose.

This really started after I drove it last when I noticed the terrible shaking. I habe another thread on here. Apparently it's been in 4lo for a while as I hooked up the tranny wrong when the motor was re-installed. Don't know if the two issues are related.

I really don't want to yank this motor again...

you need to determine if its missing. should throw a code and light the CEL but might not. you can verify by pulling injector or plug wires one at a time and see if it changes

sounds like exhaust leak but if its missing and shaking then its more likely to be a lifter problem. could also pull the valve cover and feel each rocker to see if there is any slop on any of them (should not be)

Yea. I'm going to pull the valve cover this weekend and see. Start there. I'm pretty sure it's coming from internal. Would any of this explain the extremely rough starts? The only way to keep it running is by giving it throttle till its warmed up. Then it idles like in the video above.

Maybe I should just set it on fire. Might get more enjoyment out of it.

Could be your camshaft position sensor, or distributor. Check your distributor shaft for excessive up & down play and rotational play.

All previous suggestions are good. Also, invest the $20 in a vacuum gauge and learn how to use it.
http://www.gregsengine.com/using-a-vacuum-gauge.html
You'll be blown away by how much you can learn with a simple vacuum gauge. Read the article.

Also, first thing I'd be doing when troubleshooting your issue is running the motor and pulling power to one injector at a time to see if you notice any change in idle or sound. That knock/tick is pretty loud though, and my first suspicion would be lifters (unfortunately). Do the vacuum tests and if nothing, try adding some Lucas to thicken up the oil, which should help with the noise if it's a lifter issue. Also verify your oil pressure.

We have lots of XJ's in my family and causes of stumbling, miss, rough idle, and hesitation/bogging down have been bad 02, clogged cat, faulty injector wire, alternator that is going out. Alternator can be checked at most auto parts stores for free. For the O2 at least check the connector and clean it and make sure the wires haven't been damaged and you should replace it if you haven't ever. For the cat just unbolted it looked inside. For injectors unplug each one, one at a time and it should run worse, if it doesn't their is an issue with that cylinder and you can start checking everything simple like if the pig tail for that injector is getting power, see how the spark plug looks and the spark plug wire.

I definitely have a rough idle issue. I'm just wondering why it all of a sudden got much worse at startup and the ticking/knocking. I'm going to check the rockers tomorrow. Then go from there. Likely I'll be pulling the head to check the valves and lifters.

How would I go about verifying oil pressure?

In the video, it sounds to me like one cylinder isn't firing. That tick isn't fast enough for all 6 cylinders. Unplug fuel or spark from one cylinder at a time while running. If pulling one of them doesn't make any change in that ticking, that's your cylinder.

If the problem has changed or gotten worse very quickly, I'd suspect bad wiring first, especially with regards to a single cylinder. Also try pulling all your plugs after running for a minute and check if one is soaked in fuel or oil. Get a picture of all of your spark plugs (keep them in order) as well. That'll probably show you tonnes of info.
Yea, problems like this suck, but I've had more than my share of them. I ended up giving up on the 4.0L and dropping in a v8 because of them. The 4.0L is a ridiculously reliable motor that is very forgiving of being run out of oil, over heated, beaten to shit, but you're dealing with 20+ year old wiring on a harness design that wasn't very good to begin with. I would've gone with another 4.0L except that in my area, they're very hard to find in decent condition and anyone selling one is selling it for a reason (ie: everyone here is dodgy, and as I've been quoted before, we're all rapists in Canada). The v8 ended up being the safer bet given I could probably buy 5 4.0L long blocks up here before getting one that wasn't destroyed.

For this problem, come up with a plan and stick to it. You need to know if all your cylinders are firing. Verify it as I've said above and post results. Your posts tend to get scattered pretty quickly and a lot of good suggestions seem to be going un-heeded .

I definitely get what your saying. But I think this is two different issues. I've never had a cylinder misfire make a ticking sound. I've had lifters do that.

I've previously pulled all injectors and wires and never noticed anything I wasn't expecting. None resulted in a "no change scenario". All worsened the idle. This tick is an issue that just started three starts ago. The rough idle has been around for a year.

Wouldn't you think finding the source of the tick would be more important?

Possibly, but if the problem is a lifter, you should notice a change when you mess with each cylinder.
Maybe pull the valve cover and just watch to see if any of the valves don't seem to be opening as far as the rest.
To me, that compression test you showed is low. I know spec says 120 to 150 or something, but I've always had problems with any block below 130. I'd look into that as well. Check if the low compression is rings or valves first (eg: couple drops of oil in the cylinder should increase pressure if it's rings). If not, probably just a head issue, in which case it may be time to pull the head.

I know it's the rings. At least cylinder 6 is. I put oil in and it went to about 140psi if I remember right.

And it's less of a tick and more like a clank. Just started it again. Definitely seems to be coming from motor. Long screwdriver told me. But, I wish I had a mechanics stethoscope to play with. I'm going to try and pull of valve cover tomorrow to see what I see. Might repeat compression test as well.

Well...I think I found the problem. At least solved the ticking and clunking issue.

Ever seen a rod this warped? Lol. First for me. Dropped another rod in and bam, she's loving it. Anyone know how to re-size the image link?