88 XJ Stumble...

[John(XJ)Jeep]

I've got a problem with my Cherokee that's got me a little stumped maybe it's the stress of it or whatever but for the life of me I just can't figure out how to fix this problem.

1988 XJ 4.0L 5 Spd.

Under normal driving my Jeep will stutter or stumble around the 2-2.5K RPM, it tends to happen when I don't move the throttle and starts approaching the point where the acceleration is leveling off. It prevents me from having a steady cruise. I've also noticed that it will chase my throttle input when the engine is cold killing my mileage.

It doesn't happen below that RPM in 4th or 5th but it happens more in the 2nd and 3rd gear. I've changed and checked everything from grounds to sensors to vacuum lines. I've re-torqued my manifold bolts and I know I have a cracked exhaust manifold already but this problem started about a week and a half ago, it's been cracked for years. I'm hoping that I've only missed something that is obvious but I just can't think of anything else that is wrong with it. All voltages are around 4.9V hovering between 4.8x to 4.9x volts. I'm gonna pull the sparks plugs and the cap and rotor and take a look at those again, see if I can see a difference in anything again.

If you want to see a small vid of my A/F gauge showing what it's doing the link is this. http://www.youtube.com/watch?v=rdSPF3a50Ng

When it's not doing it the lights stay at green (it's a little rich still) and when it starts stuttering it jumps into yellow (technically where it's supposed to be really) and red erratically and feels like the fuel is being cut.

 

[cruiser54]

Have you tested your TPS through it's operating range with an analog voltmeter?

 

[John(XJ)Jeep]

Ya, I was doing that today. I went to the scrap yard and managed to score a relatively new TPS. I checked the sweep thoroughly, it didn't jump or move rough up and down... didn't float around when held steady either. It went from 0.8v up to 4.65v pretty good actually. It's a new design TPS so it can only be probed from the harness side so I'm pretty sure the contacts in the plug are alright too.

 

[cruiser54]

How did you "check" your grounds? If you didn't remove, scrape, clean and polish them to bare metal, they aren't done.

After we're sure that's done, let's clean out the C101 connector up on the driver side firewall behind the brake booster, with the bolt holding it together. All sorts of communication between engine sensors, actuators, and the ECM need to travel throught this crappy connector. Unbolt the two halves. Pick out the road tar looking crap from the entire assambly. Spray all of it out repeatedly wiping out the now dissolved tar like crap with a soft cloth. When satisfied it's clean, apply dielectric grease to one half and bolt it back together.

Get some good contact cleaner as you'll need it to perform the C101 cleaning and I'd be cleaning all the connectors to every sensor/actuator I could find in the engine bay.

 

[John(XJ)Jeep]

I checked and replaced all grounds already. I wire brushed and wire wheeled what was good and replaced the rest. All reference voltages read 4.95 or better TPS ground came in at 4.9v on the dot.

The C-101 was fixed and cleaned during my A/F gauge install because it was flopping around after the mounts broke and was chasing down a rich condition with the O2 sensor.
All other sensors were already replaced along with the ignition coil, spark plugs, wires, that little box where all the connectors go to by the battery, relays, fuses.

I'm sorry if I'm being a big pain but this problem has really got me and it's pissing me off.
Chasing this actually fixed a few other problems that I had but this one.. I don't know.

 

[cruiser54]

I checked and replaced all grounds already. I wire brushed and wire wheeled what was good and replaced the rest. All reference voltages read 4.95 or better TPS ground came in at 4.9v on the dot.

The C-101 was fixed and cleaned during my A/F gauge install because it was flopping around after the mounts broke and was chasing down a rich condition with the O2 sensor.
All other sensors were already replaced along with the ignition coil, spark plugs, wires, that little box where all the connectors go to by the battery, relays, fuses.

I'm sorry if I'm being a big pain but this problem has really got me and it's pissing me off.
Chasing this actually fixed a few other problems that I had but this one.. I don't know.

First of all, you're not being a PITA. It's refreshing to deal with someone who appreciates some help and is willing to put in the effort necessary to chase down these problems without expecting a silver bullet cure to appear on their computer screen. As you can see, these problems aren't fixed on your keyboard. Glad to see that some other problems were solved on your journey.

Brain fart on my part... Search "distributor indexing." Properly indexing the distributor was a procedure AMC came up with on your vintage Jeep to address a surge at cruise. Have you checked the stator/sync generator inside the dizzy? I think I would do that before indexing, just in case the stator is bad.

 

[kcjeep6]

First of all, you're not being a PITA. It's refreshing to deal with someone who appreciates some help and is willing to put in the effort necessary to chase down these problems without expecting a silver bullet cure to appear on their computer screen. As you can see, these problems aren't fixed on your keyboard. Glad to see that some other problems were solved on your journey.

Brain fart on my part... Search "distributor indexing." Properly indexing the distributor was a procedure AMC came up with on your vintage Jeep to address a surge at cruise. Have you checked the stator/sync generator inside the dizzy? I think I would do that before indexing, just in case the stator is bad.

Also check the distributor for worn out thrust washer. Grab the rotor and turn it back and forth, it should move very little. I there is a lot of up and down motion as you turn the thrust washer is worn.

 

[John(XJ)Jeep]

I was just in there today and noticed very odd wear patterns on the cap and rotor from the last time I took it off. It is a little wet in there, I'm gonna have to change that seal and if I'm gonna change that seal might as well do a rebuild of the distributor.

I will look up the indexing too because a while back when I changed my rear seal I decided to do my front seal and my timing chain as well I wasn't sure if it skipped or not because the indexing marks were off each other by one tooth, I don't know if the cam was advanced or not or it was the other way and skipped a tooth. It had 1/4 inch deflection (an 1/8 either way) so I figured to change the gears and the chain and put all markings back to I guess "factory" really.

kcjeep6 - I did do that test and I believe you are right it felt a little too excessive play to just leave that option out, it is old heh heh.

 

[cruiser54]

I believe you're on the right track.

 

[John(XJ)Jeep]

I just finished re-indexing the cam and the distributor a day ago. I had a hell of a time trying to figure out what the matter was with the markings on the timing gears. For some reason the manual and the new gears I got a while back don't match what is suggested in the Haynes and won't align properly. I got it to work reliably but I lost quite a bit of bottom end and my top end is better than it was but not like it used to be. I'm not going to be too worried about it ATM I will be buying a Clifford header next week and will address these problems all at once. Hopefully my dads truck will be fixed by then so I can get to work heh heh.

The stuttering did disappear for now, sort of traded it in for stalling at idle issues. I modified my distributor for infinite adjustment for fine tuning. Seemed to make a difference, it will respond to adjustments from stalling to backfiring ;p but all in good, I'm starting to believe that manifolds are the root of my despair with the bad idle and the way rich condition it still has.