93 grand cherokee fuel problems

[kc9djp]

Greetings All,

My 93 grand cherokee inline 6 4X4 has a unique problem. The fuel pump works as do the relays. It will start a couple of times with no problems then it will fail to start. I have completed all the troubleshooting recommendations that the Haines suggests, but I am still scratching my head.

[WJCraig]

Sounds like you either have a relay problem, a spark problem or a sensor problem. Is your engine light on? Have you scanned the computer for any codes?

[kc9djp]

Engine light is not on. I had the luxery of having new relays on hand and all worked as they should. Even the low voltage pin displayed accordingly. Will the scan be sensitive enough to indicate a relay prob or will it hit on the lack of fuel. The fuel pump is new and works fine when the relays are by-passed.

[sadass]

PCM. that is where I would put my money. So what was the culprit???? did ya fix it yet???

[kc9djp]

I am leaning towards the PCM, but I want to rule out the crank sensor first before I drop some coin on the PCM unit. Currently, it is refusing to fire, so another symptom to diagnose.

[sadass]

It is somewhat of a common problem on the early ZJ for 3 capacitors on the PCM to leak. this will give you your exact symptoms. It is repairable for only a few bucks and a trip to radio shack.
Also when the crank sensor goes, it will not start at all.

Do a search for ZJ PCM leaking capacitor and you should find several write ups on the repair.
I will try to find a link to one of the better ones now!

[sadass]

Ok One other thing to add aside from the PCM. I had a fuel pump fail in my 93 too,as well as a PCM. When the pump went, I was able to get it to start pumping just by kicking the under side of the tank. Like yours , It worked fine then randomly it would not start. So to rule out it not being a fuel pump before you look at your PCM> Just try to lightly hit the center of the gas tank on the under side to see if that fuel pump kicks in while someone else is in the cab trying to start the Jeep.

Let me know if you can not find the PCM leaky capacitor fix on line.

[kc9djp]

I'll check for the leaky capacitor and get back with you. Thanks for your replies.

[Rustys95ZJ]

Cleaning the PCM connections did the trick for me only after I had wasted hundreds of dollars replacing the fuel pump and and other components. Cleaning those connectors brought about such a wonderful transformation I just couldn't believe it. I wish I had done that before I begain throwing money at it.

Almost everything about the car is controlled by the PCM. I have a factory service manual that gives a pinout diagram for my PCM. Truly. Most every function from engine to brakes is controlled by the PCM--and that includes, of course, multiple systems involved in starting and running your Jeep. A loose connection to the PCM can cause the symptoms you have explained.

I have a 95ZJ but I presume yours is similar. One bolt holds a multi-cavity connector to the PCM. Disconnect and clean the 60 or so terminals in there (female barrels of the connector and the mating male pins on the PCM).

Get some electronics cleaner in a spray can. One I use (because it's available on the shelf at auto parts stores around me in Kentucky) is made by CRC Industries called QD Electronic Cleaner. (QD is quick-drying). It's a mixture of hexa-benza-thanes some of which I'd never heard of. I love the stuff. I've never used anything so flammable in my life. I'm definitely careful not to generate a spark while using it and if I think vapors have drifted into a closed compartment, I give plenty of time for natural ventilation to exhaust the area.

My PCM connectors looked fine even under a 10x magnifier. There were visible tiny globs of silicone grease in the cavities but I think we would expect to see that. All of the female connectors looked perfectly shiny and clean. Nevertheless, I had tried everything else. I considered replacing the PCM. I got hold of myself and talked some logic in my little brain. "Let's first be sure the connections are clean and sound," I said to myself. "You know," I said to myself, "it's usually not the PCM at fault. Most of the time, you just waste money replacing it."

Of course, we disconnect the battery first and foremost.

To do the job quickly, I'd spray the female connectors and then blast them clean with an air hose. On mine, the female connectors are open like tiny barrels and air will blow debris out the other end. This method won't polish the barrel interior but it will, perhaps, dislodge tiny grit particles encompassed within silicone grease globules and move them out of the barrel. If your Jeep is fixed after this, you'll know that's all you needed to do.

I chose a different tactic because I absolutely wanted to eliminate the multi-cavity connector as a source of problems. The best method I could think of was to get a brush the size of each female connector so that I could scour the interior of each of the 60 connectors using the cleaning spray. Well, I'm good at internet searches but I couldn't find such a brush. I expected Molex to have something but they told me, "no."

So, my alternate was to get a stiff plastic bristled brush at the auto parts store--like the ones used to detail cars. I'd cut a few bristles and bunch them together so they'd fill a cavity. I'd hold them together with a pair of forceps. I'd spray a female connector in a cavity and then I'd "ream" the connector with that collection of 3 or 4 bristles held together with forceps. For each cavity, I changed the bristles.

We do not want to deform or enlarge the female barrels. We must choose a number of bristles that fit snug but not so tight as to cause those problems.

Yes! A tedious chore! 60 times I had to do that. (I situated my little garage TV so I could see it during this 1 1/2 hour ordeal.)

I did not repack the cavities with silicone grease. It's been running so well, I really don't want to muck up the fun by repacking the cavities if it might bring back problems. However, I know that the lack of grease may cause other problems over time.

Then, I used a plastic bristle brush to scour all of the PCM male pins in unison after spraying with the cleaning solution. I wouldn't use a metal bristle brush. That might short pins that could disturb chip program instructions.

Amazing! I swear! That was 30,000 miles ago. The old ZJ now has 150,000 miles and runs as smooth as factory new. It starts, runs, accelerates, decelerates--all just perfectly.

That little cleaning procedure kept me from replacing the PCM ($200 U.S.) and replacing the injectors ($450 U.S.). So, everytime I cruise through a Jeep forum looking around, I feel compelled to tell my story. I'm sure there just has to be some people out there just like me who would greatly benefit from 1.5 hours well spent. The electronics cleaning solution is less than $10 (U.S.) per can.

By the way, if anyone finds brushes that fit those connectors, please post here? I can use those types of brushes in every other electronic device I tackle for repair.

[kc9djp]

OK, it has been a couple of months but the problem has been fixed. I found that an aftermarket security system had been installed in which the hood latch sensor was shorting out. Who knows how long the short had been happening. The short that apparently was not discovered by the original owner affected the PCM. The aftermarket security system was completely removed and a rebuilt PCM was purchased. A DRB scan tool was able to talk to the vehicle and actuate the fuel pump and sensors through the new PCM. I then discovered a faulty neutral position safety switch and have fixed that. The Jeep is working well. Many thanks to those who replied.