My 1988 4.0 XJ has started bucking when driving faster than 50 mph. What happens is when the throttle is engaged, the car has no problem accelerating or coasting, but as soon as I let my foot of the gas, it bucks pretty aggressively just once and then it's back to coasting without any trouble. If I keep my foot just barely on the gas it bucks consistently until I let off, or push past that dead spot. The issue goes away when the TPS is unplugged, the car performs fine without it. Here's what I've done to troubleshoot:

Replaced the TPS (twice) and calibrated the new ones, replaced the IAC, Replaced the O2 sensor, new plugs, cap and wires, replaced motor mounts, refreshed all the ground wires, refreshed the electrical connections, relays and the C101, cleaned the throttle body and cleaned the EGR.

It seems like the issue is related to the throttle somehow but I am having trouble pinpointing it. I would just leave the TPS unplugged, but I've heard that this can cause long term issues. Any thoughts? Thanks in advance!

 

I dont know much about older XJs and how the "computer" works, but just off the top of my head it almost sounds like something from the throttle position is acting up just in that range of position so the "computer", or whatever controls your fuel into the engine is getting out of whack when its at that specific point of throttle application. But I could be way off and thinking much too deep into it....

 

How did you clean the C101 connector? Before I just deleted mine it was packed with old grease that became a tar like substance that would of been more work to remove fully than just removing the entire connector. What does your crank position sensor look like? It could be a wire on the connector is shorting itself or the connector is dirty sending false codes trying to kill the Jeep. Just some off the top of my head ideas based on what you changed. I've had the same issue before but too far apart to do anything about it honestly.

 

This was my initial thought as well, but I'm reluctant since those are a low fail part. That said, this makes sense to me and I haven't tried that yet, so maybe I will see if I can find a spare ECM next time I'm at the junk yard. Thanks a ton for the suggestion!

 

I haven't checked the CPS yet, that sounds like a great place to start! I want to see about troubleshooting it since I have replaced some sensors with aftermarket sensors and I'm trying to avoid those. If I see any noticeable damage or if it fails any tests, then I will certainly look into replacing that. I cleaned out the C101 with electrical cleaner and a pick. It seemed to work well and the resistance is in pretty good shape with the grounds. I would love to delete it at some point since I have heard that can solve a lot of problems.

 

UPDATE. I was able to make it to a pick n pull and get a new ECU for the jeep, but unfortunately the problem remains. At least I can rule that out now. I found that the previous owner hard wired the CPS into the ECU, which is strange and could be an issue later on, but I checked the wires, made sure they were taped, and checked the resistance and found that it was about 225 ohms which is in spec, so I think the CPS is alright for now. My other thought was that the EGR is bad, but the spindle is moving when the engine revs, so I'm not sure. Could a faulty EGR cause something like this? I think my next step is to triple check all my grounds and replace the ground strap to the firewall and check the O2 relay. Any other thoughts or advice to point me in the right direction? Thanks!

 

Have you done Tip 5?

 

Hey Cruiser,
You're a legend, thanks for chiming in! I've been going through your list and trying to tackle as much as I can. I have done Tip 5, but I am going back to triple check everything. The TPS to the battery ground is showing .7 ohms pretty consistently. I wiggle the wires and it fluctuates between .5 and 1.2. It jumped quite high once, but I have not been able to replicate that since, but I think my next step is to go back into the harnesses and re-check the crips / the ground wires that I soldered. I just bought a battery cable to replace the ground strap to the firewall and I am going to re-clean / sand the grounds at the dipstick to see if that helps at all. Are there other sensor grounds that you would recommend checking next to make sure the resistance is good? Could it possibly be an EGR issue like I was guessing? Thanks again for your incredibly helpful tips!

 

I don't like the results of Tip 5!!! That same ground services a few other sensors. Tip 6 outlines what to do.

 

I had an 88 some years ago that behaved like this. Turned out to be vacuum l leaks.A guy on the NAXJA led me to look at this. I first replaced all of the vacuum lines. The issue remained. I then proceeded to plug a line or 2 and drive it. When i plugged the line to the EGR the problem disappeared. Mine Turned out to be the small tube that runs from the exhaust to the EGR was plugged. You said you cleaned your EGR. So maybe you have an issue elsewhere. When I replaced all of the vacuum lines it helped but did not solve the issue. The line to the Vacuum disconnect on the front axle was particularly deteriorated. If you have a vacuum gauge that you could hook up and have visible to you while you drive you will be able to see if loss of vacuum is your issue.
Hapdad

 

UPDATE: Hey all. I upgraded some grounds on the Jeep and replaced the vacuum lines (part numbers 46003 and 46004. The other lines looked good. Thanks @Hapdad ) and am unfortunately still having issues with the jerking and bucking. For further context of the behavior that I'm seeing, the jerking happens when letting off the gas at about 60mph and when getting from gear 1 to 2 at about 15mph. The issue is worse when the engine is cold, and sometimes it lets out a puff of exhaust when bucking. I found that driving with the shifter in the bottom position at '1-2' makes the bucking worse and occasionally the car makes a clunk sound if the RPMs get too high. A little concerning...

@cruiser54 I was able to upgrade the ground strap from the engine to the chassis and now I am regularly getting between .2 and .3 ohms of resistance from the TPS ground to the negative battery terminal. I have already re-soldered those ground splices before the C101. I am seeing similar results from the MAP, so I didn't mess with their splices like you mention in tip 6. The CPS is hard wired into the ECU and there are some extra wires that look modified around the fuse box, so clearly someone was tinkering in there before me. Additionally, it looks like there is an unshielded wire wrapped in conductive tape with the other wires from the CTS going into the ECU which seems super janky. Picture included:

https://drive.google.com/file/d/1Sxb...ew?usp=sharing

Short of tearing apart all the wiring harnesses and tracing individual wires all through the engine bay, I am at a loss for what to do next. Does anyone have any other next steps that they would recommend to help troubleshoot this issue? I was planning on looking into fuel pressure issues and checking vacuum while driving next. I also ordered a mt2500 to hopefully get any more insight from the engine data!

 

Check fuel pressure and make sure your EGR isn't hanging open.

 

Hey all, unfortunately no change yet, but I'm persistent.

The fuel pressure is about 30 while idling and pops up to about 39 when the fuel pressure regulator vacuum is unplugged. The Fuel pressure holds after the car is turned off. It even went up after about 10 mins.
The EGR is freely moving once the car is at temp and throttle is applied. Additionally, the engine stutters and eventually dies when vacuum is applied to the EGR. Feels alright, anything else I can test to make sure it's not stuck open?

Thanks again for your help. Any other next steps I should look into? I feel a bit skeptical about the coolant temp sensor and the CPS since those looked modified. I think I will test those next. Any other pointers would be greatly appreciated!

 

Oops wrong account.

 

I found a few minor exhaust leaks, could that be contributing? Additionally, I received my MT2500 and all the sensors seem to be set up correctly and sending the ECU correct info.

I need to replace my head gasket, so I am going to do a big overhaul in a few weeks and replace the water pump, transmission cooling lines, injectors, thermostat and all the gaskets. I will continue to post updates, but fingers crossed that she will be purring without any other trouble once everything's put back together.