Cherokee Laredo difficulty starting
Hi, everyone. I can see most of you have some constructive comment to make
when people post their problems, I respect people who value other
I own a ´98 Jeep Cherokee Laredo, 6 cyl, 4.0, manual. It worked just fine until one day, it just would not start. I brought a “generalist” mechanic, who after some testing, discovered the gas pump was not getting the ground signal from the PCM. He fixed it by giving the gas pump a direct ground connection, which helped me get it running.
Since it was a quick fix to help me get it moving, I took it to a Jeep dealer, hoping I would get a tech-wise repair (they have the software and tools). They told me what I knew, PCM is not passing the ground signal to gas pump. The mechanic told me he connected the ground terminal of the pump to the ground of the ASD relay in the fuse box (he said they work the same way). The truck started fine when I tried it, so I thought it was a good repair. The mechanic told me that I should start getting ready to replace the PCM because that is something that can’t be fixed.
BUT, the car gave trouble anytime it was not started for a weekend, and twice when I was at the beach on different ocassions, it wouldn´t start. The Jeep mechanic told me to check the ASD relay, and to have some extra relays ready in case they were giving trouble. The truth is that I changed it, and, though not easily, the engine started. Last time, when I opened the fuse box to check the relay because the engine wasn’t starting again, I found the plastic of the ASD relay had deformed by heat. Someone told me that happens there is a electrical “drop”, and the relay overheats.
In short, the engine starts fine if is used on a daily basis, it won’t start after a whole weekend unused, or at the beach (sea-level). Another generalist mechanic told me it could be the crankshaft sensor. After a weekend without use, he went to my garage to check the gas pressure, and the gas pump did charge to appropriate gas pressure, though the engine still did not start. I had to keep trying the ignition and pressing gas pedal all the way down. Finally it started.
If it is the PCM for sure, I would go ahead and get it, but if it is something else, I would like to discard other possibilities, because it is a very expensive part to replace if it is not the solution to the problem. If you have ideas about what other things to discard, or a way to make certain a new PCM is required, I would be listening.
Thanks for your time and know-how. I appreciate your help.
And most of us respect someone who introduces themselves properly in the appropriate section prior to asking for help.
It sounds as though the people who are working on your Jeep are A) not
working in a logical sequence, B) unafraid to cobble something together no
matter what damage may occur to your vehicle and C) unconcerned with your
For just a moment let's forget about everything else except the safety standpoint. You said "Last time, when I opened the fuse box to check the relay because the engine wasn’t starting again, I found the plastic of the ASD relay had deformed by heat." Then you went on to say "Someone told me that happens there is a electrical “drop”, and the relay overheats." Well that person was right. I don't know the specific differences between your Jeep and a US market one but I can assure you that is not a good thing. Just off the top of my head I feel there is high resistance in the circuit based on the melted relay. That same resistance could very well damage the ground circuit inside the PCM.
Before replacing the PCM you need to determine the cause of the high resistance and repair it or a new PCM will go bad in short order. That in itself may fix you up. I also have to ask if they've scanned the PCM for codes or just jumping to conclusions on the no-start?
You can see I've stayed focused on the safety aspect here. The possible existing problem along with the shadetree repairs make for a very dangerous situation. An overloaded circuit of the type you obviously have could cause a severe fire while you're driving. If you're unable to affect the needed diagnosis and repair I hope you can find someone who can. Good luck.
Right after the first time the gas pump failed, I took it to a Jeep shop, they connected the scanner (I saw them doing it, plugging the scanner under the dash), and they told me what I said before, the PCM is not passing the ground to the gas pump, and they said the PCM had to be changed. Then I took it to a Jeep dealer, and I told them about the problem the way I described here (until that moment, the "fix" made by the very first mechanic was in place-direct ground cable for the pump, bypassing the PCM). This second "authorized Jeep" opinion gave the same diagnosis: ground terminal is not working from the PCM to the gas pump relay in the fuse box. That is when they connected the ground terminal from the gas pump to the ground of the Auto Shutdown relay. (they said it was a better "fix"). Their advice: get a new PCM.
Thinking it over, they seem to think the problem is within the PCM, it stopped working properly. They didn´t make any comment as to whether some other factor could have damaged the PCM terminal. I think you are right when you say they should check the fuel system to discard any other malfunction that could also damage a new PCM. Is there a way that can be checked, how could I get to use codes to tell those guys what to do, considering they haven´t been able to think that by themselves? By the way, I checked the ASD fuse again, and it wasn't melted. Ooh, I remember, the radio is like losing volume and apparently losing power from the battery, could that be related to PCM, or is it just a radio problem?
I would say the radio and fuel pump problem are unrelated, but you never
As for checking out the fuel pump circuit, it will just take a little time for a decent tech with a DVOM to check the wiring, pump, etc for dynamic voltage drops and resistance. Jeep should have specs for it in their literature then it's a simple matter of finding the wires and doing it. All the codes will do is tell you there's a problem with the system, not exactly where to look.
Let me throw another guess out there. It's my understanding that Venezuela has an extremely high level of alcohol in their gasoline. If that is the case, I have seen excess alcohol damage fuel pumps. Why? Because the fuel cools and lubricates the pump and alcohol doesn't lubricate as well as gas. Also it can cause internal deterioration of the lines making the pump work harder to push the fuel. Again, that is all conjecture but it might lead to a pump and a PCM.
I'll have the fuel system checked in terms of voltage and physical installation, which will require to remove the gas tank. I'll take advantage of that work and have the tank cleaned (there was a gas strike here, and we started getting uncontrolled quality gas imports from we don't know what tankers). I'll let you know what I find out.
Before, I failed to follow-up on the fact that the engine started OK on second and subsequent starts during the rest of the day. Any idea as to why the engine wouldn't start fine the first time in a day, but just fine on subsequent tries? Thanks