rdr8887

1994 XJ
1993 Motor 120k
4.0L
Automatic
4x4

So, I've been working on this jeep for about 2 years now. Bought it from a friend after he drove it overheated until it was tired of running. I replaced the motor. Twice actually. Stupid mistakes....

Anyways, I got the motor installed and running as good as I can. What's left is a rough idle condition with no codes. Under load in neutral it isn't noticeable at 3000 rpms. Motor sounds and feels strong.

I did a compression test last year, although, I can't find where I wrote them down. Something 150-160 in cylinders 1-5 and like 120 or 130 on cylinder 6. This indicates a wearing o ring, but, not a major concern of mine. with oil it shot up higher than the others.

I am not one to just shotgun parts at a car until it's fixed. I enjoy diagnosing to an extent. I'm just a novice. So, I'm hoping for some help (again).

I know the main components are Vaccuum, Fuel, and Spark. I have one or two of every bolt on part for this motor. I can interchange parts as needed to troubleshoot.

Spark

I'm going to start here. I know I replaced plugs and wires last year. Sincerely doubt they are the issue. Even changed out the wires with a new set (failing connectors). The cap and rotor are old, however, I have replaced with a known working one and the same issue. Same with ignition coil.

Timing could be off, but, I would assume rough idle would be noticed under load if timing was the issue. Thoughts? I have no clue how to check or change the timing. I know it's moving the rotor, but, I don't know how to get it right. I'll look it up if someone deems this necessary to check.

I'll post an update with results or check something else if you think of something I'm missing.

Fuel

I do not have a fuel pressure tester, but, will rent one tomorrow. Could be the issue. I have two additional sets of injectors, so, finding 6 good ones might be easy.

Other than unplugging each fuel injector at a time to listen for changes, don't really know what else to test here. I will check pressures tomorrow.

Vacuum

This could cause rough idle. However, don't know how to check the system. I've electrical taped many of the vacuum boots in the motor as they are dry rotted. I suspect this could also not be helping. Not sure if this would trigger a code.

toasterknight

Have you cleaned the throttle body and iac? If it runs fine at 3k its unlikely to be fuel or spark.

rdr8887

Throttle body looks as clean as it's going to get. I'm pretty sure I cleaned it prior to installing the intake manifold.

Removed the IAC valve. Looked okay but I cleaned again anyway. Pic is after cleaning.

Reinstalled, still rough. Almost seemed to run smoother sucking in air from open throttle body. Reinstalled air box and it was back to rough. Could be crazy though.

toasterknight

When you cleaned the iac did you take the housing off the throttle body and clean the inside of the housing?

rdr8887

I did not. Just removed the sensor. I think the housing will be as clean as it's going to get as well. I tried to clean up all parts prior to assembly of the motor.

EZEARL

Test it.


IAC ohms test:
Pin sequence;
A B C D
A to D/B to C = 40 to 80 ohms
A to B/C to D = open

toasterknight

That was gonna be my next reccomedation. I'm glad you did it because I can't remember the procedure.

EZEARL

No problem. I hadn't read anywhere about testing them so after cleaning my IAC and it still not working quite right I did some surfing around and found the test specs. Thing is my idle was never rough. It was low,I would have to use the throttle to start it,and I had no fast idle. His IAC may test good.

rdr8887

Per the below photo, this is what I'm getting. Differs from yours, however, wasn't sure which one was "A".

A/D - 53
A/B - 0.L
A/C - 0.L
B/C - 58
C/D - 0.L

EZEARL

Hey guy SORRY but in my post above I typed in my test wrong(corrected it though).


Your A to D and B to C are within spec. A to B and C to D are good to go. No need for A to C. Although I didn't have your problem it didn't hurt to test it and now you know it's within spec.

rdr8887

Yes. Thanks. I never mind troubleshooting. Now I know it's good.

CCKen

Verify your MAP sensor tubing is in good shape and is connected properly.


You can test the MAP sensor if you want.

rdr8887

I'll check that shortly. I'm still in coffee mode, so, forgive me if this is a stupid question:

The testing of the MAP sensor is telling me to look for an output voltage of 4-5 between terminals A & B. Is this referring to the connector? I assume the sensor won't have any output voltage since its disconnected at this point for the test. Basically what I'm doing here is checking to make sure the sensor is getting the required voltage.

What is a neutral hot idle condition and how do I recreate it?

rdr8887

Also, this general rough idle thing is weird to me. How many conditions would cause a rough idle, but, drive well? I can only verify this in neutral revving up the motor. Currently has some tranny issues and has failing brakes. It's not drivable to see what mpg looks like.

When I swapped the motor, I replaced every gasket, but, I didn't take the head off and do any valve work or replace the gasket. Wishing I did now, but, i didn't want to go too far in this financially as I wasn't sure what else I'd find. Coolant pressure holds during test.

This thing has a lot of problems from tranny, suspension, and brakes. I want the motor running like a top before I move on. Hoping the tranny is just low on fluid (noticed a puddle) as it worked well prior to the motor swap. It surges when shifted in gears.

If someone has a tip on the tranny, I'll take it. But, I don't want to stray too far away from the post topic.

rdr8887

Okay. Vacuum line appears to be in good shape. Solid connections at boot.

Voltage at connector is bringing 4.8 volts to sensor when key is in position 2.