68FBMustang

My sons 1997 GC Limited 5.2 (318 for us old guys) hardly runs after it warms up.
Runs fine until the temp reaches @ 195-210 degrees. After initial start up I can get it to idle smooth @ 700 RPM. When the A/C is turned on it remains at 700 RPM while still below 195 degrees.
While in Park I run the RPM up to about 2K and it runs and idles great until it reaches running temp. Once warmed up the RPM will drop waaay down and then recovers to @ 2K and continues to cycle like that. The idle is down to 200-300 RPM.
I have replaced the coil to no avail. (Should have read resistance prior to troubleshooting by replacing).
I have removed the battery cables in hopes of clearing the trouble codes and have the following codes after re connecting and running:
12- Battery power to PCM was disconnected. Yep, did that.
42- An open or shorted condition detected in the auto shutdown relay circuit. Don't know what the auto shutdown relay is or how to check it.
21- O2 sensor input voltage maintianed above the normal operating range. Ya, I had previously disconnected the battery to clear the check engine light to pass state inspection. Normally dip in muratic acid to clean them off but can't get the da*m thing out.
55- End of codes.
My next "Guess" would be in the distributer. I think the fuel pump would manifest itsself upon start up, same with strainers.
Was going to check fuel pressure but no test outlet on the fuel rail so I'm sol on that.
So, what do the Jeep gray beards have to say?
Thanks.

tiwvr1

my jeep was doing somewhat the same thing, and the o2 sensor has SHORTED out the asd relay in the past on other peoples rigs. I would replace the o2 sensor, the front one. Inspect the harness closly for burns on the 02 wiring. And replace the auto shutdown relay "ASD". Also pull the IAC idle air control on the back of the throttle body and clean well. Should make a nice diffrence. O2 sensors ware out overtime, even if they don't throw a code, replacing every 10 years or 100k miles is good maintaince.

Bustedback

The CTS may be a problem, not giving the correct temp info to the PCM so the PCM thinks the engine is cold and dumps more fuel.

68FBMustang

First, I hate when someone posts a problem and other members post possible solutions to said problem, but you never hear from him again. Did the suggested corrective actions work or not? I’m not that guy.
The short story.
The O2 sensor and ASD Relay replacements corrected the problem as stated in post #1.
tiwvr1 you are the winner, thank you! Bustedback, thank you too, I also appreciate your input.
OK, the long story.
Changed the known bad O2 sensor. Started the jeep and within 10 seconds the RPM started to oscillate from 2K to about 750. Said a few bad words at this point.
Previously, the same type of oscillations only began after the Jeep was at operating temp 195-210 degrees.
If I had done my research correctly I would have realized that the ASD relay does not accept any information from the upstream O2 sensor until the vehicle has reached operating temp (back to the 195-210 degrees I reported in post #1) duh. This should have pointed me to the ASD Relay in the first place. Until operating temp is reached the ASD relay will not forward the upstream O2 information to the PCM and the PCM is operating on a pre programmed schedule.
Initially, after replacing both components the vehicle was doing the same thing (more nasty words were spoken at this point) and the ABS light came on! CRAP. Not only is it not fixed, but we have additional problems.
We started it a couple more times and it settled out, no oscillations. I guess the PCM/ECU was just getting used to the new components. Pulled the ABS fuse and re set it, restarted and the ABS light was out.
Anyway, that was the fix. Thanks again tiwvr1.
I did get to spend some quality time with my son and if nothing else, that was worth it.
My other son who is gone in the Army has a 95 GC that we put a Pro Comp 3 ½” lift on and has been eating passenger side hub’s like they’re Girl Scout cookies, but I will post that on another thread.

Thanks.