cherokeexj21

Hey guys, Ive been having this weird issue with my 98 XJ 4.0. It won't stay idling unless I give it gas. So I check the fuel pressure and its fine I bought a speed idle control also cleaned the throttle body and it seemed to fix the issue but it started doing it again the next day. Battery is holding 12.4 to 12.5 volts when off and its charging at 13.4 to 13.6 volts, checked all the grounds and connections everything seems fine.
Now the weird part is that I went to see if there were any codes and noticed none of my monitors were set?. which was weird because I never disconnected the battery. Last night I did the idle relearn process for the idle control and it was ok also ok this morning. I drove it to work checked for any codes and nothing came up but my 02 sensor monitor and heater monitor were set.
When I got out of work today it started to do it again and now all my monitors were reset again and had put it D and R a few times then it was good. So Im thing the PCM keeps resetting itself so it doesn't hold the idle memory. ANY opinions would be appreciated.

SirCyber

Speaking from a computer tech standpoint, this sounds like a memory controller issue. Bad memory in a computer could cause this issue, but in a normal computer we would say it's the CMOS battery failing. As far as I know this would be the battery of the jeep. So maybe it's losing power to the pcm when ignition is disengaged. Input power or bad memory module would be my thoughts, but I haven't delved into the computer for Jeep's yet. Again, just from an IT background thought. Hope it helps.

4.3L XJ

Anything else acting up?

cherokeexj21

I was also thinking the battery but nothing shows me enough evidence for me to change it maybe Im just cheap but I guess I will buy one tomorrow

cherokeexj21

No actually once I can get it to stay one it drives perfect, thats another things thats throwing me off..

4.3L XJ

How about radio loosing presets, wonky gauges while this acts up?

cherokeexj21

No gauges are fine. And I let it sit last night and all day today battery still at 12.5 and radio presets aren't erased.

SirCyber

Still sounding like a memory module issue. I just don't know how to fix it on a car computer

cherokeexj21

Yea.. looking more like the PCM

4.3L XJ

OK, I agree

'90Cherokee

There's a constant power wire to keep the computer's memory, that would be something to check before buying a computer.

DaveXJ

Check your wiring harness coming out of the plastic housing by the fuel rail where it bends 90 degrees and attaches to the firewall. There is a lot of movement on that small area of slack as your engine moves! I had a broken wire inside that intermittently shorted open. Couldn't tell by looking at the wire as only the metal strands inside the plastic came apart. Engine idled sometimes fine and other times very erratic. The dealership replaced the TPS (throttle position sensor) 3 times under warranty and did nothing to correct the problem. I ended up replacing the entire wiring harness, cut about 1" off the plastic rail housing before putting in the new wiring (to allow more room to flex) and taped up the suspect area with good electrical tape. No problem since and that was years ago! Hope this helps.

cherokeexj21

Ill check those now thank you.

'90Cherokee

Shorted and open circuits are totally different and kinda the opposite of each other. Just saying so you'll use the right terminology, it could matter some day.

cherokeexj21

So according to this


everything is in working order..gonna have to order a PCM, process of elimination