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Cycling Low Idle

MNgopher

NAXJA Forum User
Location
Minnesota
I've got an issue with my '88 XJ, 4.0l, AW4, closed cooling system, 111,000 miles. The idle is currently cycling from normal to low and back again. Before the troubles, it idled pretty steady around 750 RPM, which I understand is normal.

Now, Hot or Cold, the idle starts at 750, slowly drops to 500ish over about 20-25 seconds, holds 500 (barely, sputtering a bit), and then surges back to 750 rpm, and repeats.

It does this hot or cold. EGR valve was not functioning, replaced with a new one. Fuel injector cleaner has been run (Techron) and good gas (BP) forever. Battery voltage is OK. Cleaned the throttle body and Idle Air Control with throttle body cleaner. No change. (Had a bit of the usual black gunk in there). Where to go next? I'd hate to throw random sensors at it.

Could the IAC still be lazy or sticking, causing the surge back to normal idle? Or is that how slow the Renix computer is reacting? Other ideas?
 
So it must not be working, right? There are 4 wires. Two make the motor go one direction, two make it go the other way. With an ohmeter, find out which pairs have continuity to each other. Then you could jump power to them and see if the idle changes. If not, the IAC motor is stuck or bad. If it does change, the PCM is not sending voltage (wires, connections, PCM).
 
is your defroster on?? if mine is on it will do a similar thing because it turns the ac on for a bit, then turns it off. very annoying.
 
Haven't had a chance to try it with IAC unplugged.

No, the defroster is not running when this is happening. The fan and A/C compressor cycle normally when the defroster or A/C is on. (Ice Cold R-12 Yeah!)
 
Did some more work on it tonight. Verified the IAC was being commanded to open and close by the computer. Verified that the IAC seemed to be moving. Ended up running a whole second can of Throttle body / intake cleaner down the throttle body and IAC passage. Looked at the IAC and it didn't seem "tight" to the throttle body, possibly leaking air in that would muck with idle control.

After all that, seems to idle better (not perfect though!). She'll run down to 600 rpms or so, but then go back up to 750. Smoother and doesn't feel like its going to stall.

Guess the combo of a "loose" IAC and more cleaner did the trick!
 
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