odd idle

so i been drivin it like this a while now and it bugs me but doesnt really hurt anything. 1990 4.0 aw4 242 yada yada.....

anyway when im in say a drive thru i notice my idle sit at normal 700-750ish but it every 10-20 secs or so kinda drops idle to somewhere in the 500 range and is a little rough for a couple secs then goes back to normal. sitting there it just keeps repeating this process and is wrather annoying. the other thing it sometimes does is every once in a while, not always, when i go from goin fast to stoping at a stop light/sign the idle is at around 500 and rough also for a few secs and slowly picks back up to normal also. if i hit the gas in park/nuetral it goes back up rpm no problem but kinda goes back to low idle for a sec then normal when i let off the gas. in drive when it does the low idle thing i get a very small almost unnoticable sputter when i hit the gas, only if its doing the low idle at the time. other then that it runs great, no hesitation or sputter when i accelerate on normal occasion.

im not really sure what this would be, my iac, tb, and tps are all clean and adjusted correctly. not possitive my iac is working perfectly, but can check it tomorrow if that sounds like the likely cause

clean the IAC with electrical cleaner first

try cleaning the connectors on the MAT and MAP sensors too, that fixed a random stalling for me

well im not stalling and the iac was cleaned with electrical cleaner already(had a stalling issue befor that was caused by a bad egr transduecer flooding exhaust in when not necessary). ill check the connectors tomorrow but they should be good shes a west coast heep so not much for corrosion anywhere.

Double check your TPS setting first, adjust if necessary.

I would be a little suspicious of a "lazy" IAC. You could try and introduce some dielectric grease behind the pintle on the shaft to see if that helps.

Good suggestions here. Another thing I've seen is a "lazy" oxygen sensor slowly switching from rich to lean and taking you from open loop to closed loop and back again.

cruiser54 said:

Good suggestions here. Another thing I've seen is a "lazy" oxygen sensor slowly switching from rich to lean and taking you from open loop to closed loop and back again.

Click to expand...

Good point, Cruiser54.

try swapin out one of the iac's i grabbed from the jy a while backand see if i get anything, ill double check that tps, and try lookin into possible o2 sensor. i forgot to mention lastnight but it did seem to be really bad for a couple days til i replaced the completely plugged ccv tube that sucks air out, it would drop even lower and have an extremely rough idle up and down. i replaced that tube and this is what im left with

I have been dealing with a similar problem as well. What I have done so far is clean TB and IAC. Picked up a can of BG44K and put it in when I filled up.

Cleaning the TB and IAC seemed to improve throttle response and coast idle speed. I was still getting a little dip in RPM when coming to a stop so I picked up a can of BG44k and put it in with the gas. I still have a quarter tank of gas/bg44k left but I can already notice a difference.

ANY IAC/ICS vehicle will hunt a little on the idle--extend/retract commands from the ECU/PCM, so it will never be "perfect", however the large idle changes are way off.

The TPS tells the PCM the throttle position--so, if it is spotty the PCM will send funky commands to the IAC.

Good luck.