djb383

.....as in while sitting at a traffic light, all of the sudden the motor goes from a perfectly smooth normal idle to a very rough idle lasting 5-6 seconds then instantly returns to a perfectly smooth idle. Just started doing this 3 days ago and does it 2 or 3 times throughout a days worth of around town driving. ScanGauge shows no codes, no CEL acts up only at idle, not while driving.

EZEARL

No gods around right now. Just us peons.

Heat soak maybe? Just can't get use to that term. Sounds like tropical condition. I like vapor lock better.

Plug or wire breaking down. They'll sually smooth out at higher rpms.

djb383

Bump.

Thanks EZ.......plugs look perfect, wires checked for continuity and ohms resistance, new cap/rotor. Runs perfect off idle. Like flipping a switch, it idles rough for 5-6 seconds, then like flipping a switch again, idles perfectly smooth. Does this 2-3 times during a days worth of driving.

cruiser54

Can you read live data with that scanner?

You know me. I'd clean every ground I could in the engine bay and refresh every connector just cuz.

djb383

The ScanGauge is currently set to display water temp, volts, mpg and throttle position. Started checking electrical connections yesterday....working our way around slowly but surely, more slowly than surely. LOL One idle hiccup again this AM.

cruiser54

Did you get all the connectors done yet, including PCM?

djb383

Started at the PCM, working our way around the IAC, TPS, MAP and heading toward the PDC.

cruiser54

I wonder if someone has a grounding diagram....

CCKen

No diagram, but if you go here and look at post #49 there's pics of the engine bay grounds.
https://www.cherokeeforum.com/f2/gre...e-6558/index4/

All sensors are grounded inside the PCM.

CCKen

Does your "ScanGauge" have the ability to show O2S activity and Short Term Fuel Trim? If so, watch it when the problem occurs. You'll probably see the STFT peg out pulling fuel off, meaning the PCM has seen that fuel is being dumped in the motor for some reason and the PCM is correcting, and when the fuel trim stabilizes the problem goes away (for awhile). A glitch in some sensor input to cause adding too much fuel.

This has happened to me on several occasions and I happened to catch it in the act with my scanner one day. I can't explain why or how to fix it. I'm waiting for a hard failure.

cruiser54

Thanks Ken!!

djb383

Thanks for the help/suggestions......kiddo and I have quit for the day, hot and tired.......a real head scratcher for us. Another idle hiccup about an hour and a half ago. No way to read fuel trim. Have check a bunch of plugs/grounds but I'm sure we missed some. What r your thoughts on the IAC, TPS, MAP sensors.......ever had one go bad?