1988 MJ 4.0 Woes

The truck starts and runs very poorly, kind of like how an engine runs when its way out of time. It’ll attempt to keep running, but will eventually stall out. Giving it gas causes it to hesitate for a moment before the RPMS increase, at which point it’ll frequently have a misfire. Sometimes the truck will start up and run like it’s supposed to, but only for a few brief moments, before it goes back into what I’ve described above. I’ve done a fair amount of troubleshooting on the truck, and have replaced a few items, such as the crank position sensor, spark plugs, spark plug wires, and the intake/exhaust gasket (which seemed to be leaking and sucking in outside air….I thought a new gasket would be the end-all cure, but it was not). I’ve tested other various sensors, such as the TPS, IAC, O2, MAP, and a few others. All of them appear to be in good working order. I also had all of the injectors cleaned and flow tested by a professional, but that did not cure the symptoms. I also rerouted a bunch of the vacuum lines and made sure there are no leaks.


I’ve got a few other ideas as to why it is running so poorly. I was thinking that maybe the coil has a poor ground and is producing a weak spark or maybe the distributor was removed and wasn’t positioned correctly upon reinstallation. It just seems weird to me that the truck will sometimes run decent, but most of the time runs like a POS. I welcome any other hints, tips, or suggestions that you may all have. I understand that my description may be a little vague, so if you have any further questions, I’ll be happy to answer them to be the best of my ability.


Thanks everyone for reading!

"Sometimes run decent", lets focus on that first, when does it run decent, and how decent does it run????

What year and engine, manual or auto?

What are specifics of the TPS test results?

Have you replaced the cap and rotor? Checked & verified the plug wire locations for any crossed wires?

It's hard to pinpoint exactly when it runs decent and when it doesn't. It just seems to kind of come and go, whenever it pleases. When it does run decent, it's running on all six cylinders with no misfires and it idles where it should. I don't want to say it runs great because when you give it some gas, it'll hesitate for a moment before building RPMs and when you let off the gas, it'll sometimes backfire.

It's a 1988 MJ with the 4.0 Inline 6 and a 5 speed manual.

I couldn't quote you the TPS numbers off of the top of my head, but I went by this webpage: http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm and all of the sensors checked out fine.

The rotor and cap have both been replaced and the plug wires have been checked, double checked, and triple checked to make sure they are on the correct cylinders. When the truck runs decent, this further varifies that the wires are good.

Now, I worked on it a little more today and i definitely think the gas in the tank is a bit on the stale side. I actually have the bed of the truck removed so taking the gas tank out and getting rid of the fuel shouldn't be a problem at all.

One thing I forgot to mention in my original post is that a compression test was done earlier and the results are within spec. I have one cylinder that is significantly lower than the rest (and it's audible in the exhaust), but it's still within the deviation of the max cylinder.

How old are the plug wires, could one or more of them be breaking down and grounding out periodically?

Sounds like some bad ground wires and or a bad TPS to me. The tests on the site you listed for the TPS are not comprehensive and not conclusive! I spent 2 years beating my head against the wall :banghead::banghead::banghead::banghead::banghead:and finally got mine working with a new TPS and reworked grounds all over the jeep. My old TPS past several of the posted TPS tests, but it was still most of the problem. Check out my story, thread for all the details:

http://www.naxja.org/forum/showthread.php?t=905849&highlight=RenX+files

I have listed some comprehensive testing and data for the 87-90 TPS sensor there. Also be sure and read about the grounding issues with these year jeeps, 87-90 models and all the problems and grief they can cause. I was convinced I had good enogh grounds for months, but I was wrong on that too. Of course the upside of my hard headness is that I have a brand new, $6,000, 87 Jeep now, LOL.

TerraWombat said:

It's hard to pinpoint exactly when it runs decent and when it doesn't. It just seems to kind of come and go, whenever it pleases. When it does run decent, it's running on all six cylinders with no misfires and it idles where it should. I don't want to say it runs great because when you give it some gas, it'll hesitate for a moment before building RPMs and when you let off the gas, it'll sometimes backfire.

It's a 1988 MJ with the 4.0 Inline 6 and a 5 speed manual.

I couldn't quote you the TPS numbers off of the top of my head, but I went by this webpage: http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm and all of the sensors checked out fine.

The rotor and cap have both been replaced and the plug wires have been checked, double checked, and triple checked to make sure they are on the correct cylinders. When the truck runs decent, this further varifies that the wires are good.

Now, I worked on it a little more today and i definitely think the gas in the tank is a bit on the stale side. I actually have the bed of the truck removed so taking the gas tank out and getting rid of the fuel shouldn't be a problem at all.

One thing I forgot to mention in my original post is that a compression test was done earlier and the results are within spec. I have one cylinder that is significantly lower than the rest (and it's audible in the exhaust), but it's still within the deviation of the max cylinder.

Click to expand...

Thanks for the link and the tips. I may have figured out the problem. I disposed of the somewhat stale gasoline and tested the fuel pump to make sure it was working...and it was. I reassemled the gas tank and placed it back onto the truck as best I could (with the bed of the truck off, the tank just kinda hangs unsupported). I poured in five gallons of fresh gas and got to cranking. After a little bit, the truck fired up and gave me the same crappy symptoms as before, with a whole lot more backfiring! I decided to hook up the fuel pressure tester to the rail for the heck of it to see if it was a fueling problem. I noticed that the needle would hold around 15-20 psi and then suddenly drop off to almost nothing, which is when the truck really started having problems and sounding like poo. Then the needle would jump back up and the truck would run in its "decent" mode. Confused, I moved the gas tank around and the truck went back into "decent" mode and stayed there...for a while. I revved the engine and it sounded pretty good...better than it ever has and it didn't even backfire!

I think my problem is a bad ground at the fuel pump assembly. When I took it out of the tank, I noticed that the pump is grounded through the fuel tank, which I can only assume is grounded through the metal straps that hold it, which is grounded to the chassis. I'm assuming the large amount of rust on the straps and chassis are preventing the fuel pump from getting a proper ground and getting the necessary voltage it needs. I'm going to mess with it some more tomorrow and see if I can get a direct ground from the pump to a good spot on the chassis.