[OkieRover]

1993 RRC 4.2. 280,000 miles.
I have been having a weird issue with the idling. On Saturdays I often start the RRC to charge the battery. I usually let her idle until hot then take her out for a short drive. 3 of the last 4 outings when she is sitting at idle the RPMs cycle from just below 300 to surge above 1000. It will do this as long as I keep my foot off the gas pedal, in gear and in park.
The idle until she gets hot is steady with no stumbling or surging.
As always it is refreshing to come here and read all the helpful posts. I look forward to your responses.

 

[Range123]

Check all vacuum hoses for leaks. If its sucking in to much air the eco will be throwing its rattle out of the pram. I think when its cold the ECU over compensates anyway.

Take out the Stepper motor from the side of the plenum and give it a good clean also clean out the hole it unscrewed from.

 

[OkieRover]

Still having the problem. Replaced the air idler valve and cleaned the seat.
Vacuum lines look good.
Once hot, the idle goes from 500~ and slows to 0. I'm a bit stumped.

 

[RRLondon]

You may have to reset or check the base idle setting.

If you disconnect the IACV and plug the open ends of the pipes, then the (warmed and stable) idle should be set to give 750rpm I think. This will prevent the engine from dropping below this point all being well. When re-connected, the IACV should now range only from there upwards to cope with variables in use.

I don't think the IACV valve has the native resolution to control the rpm lower than the base idle figure as the steps in response are greater than the error. Also in effect that type can be manifested as you experience ie, tries to get aim idle and overshoots, ecu reads overshoot and closes down a step, rpm undershoots then loop repeats

So in essence, the system has a mechanical floor below which the ecu/IACV cannot access.

When all working properly, you get a response from the aircon position on the heater, it'll pick up from 750 to 950 rpm on mine to prevent the engine stalling when you also rotate the steering at tickover.

 

[BostonRover7]

Hi, similar problem here. Could it be the ignition amplifier module is failing?

 

[Range123]

Have you checked the brake servo housing? could have corroded to allow a leak. You may be able to hear it sucking.

The dizzy amp may be overheating. a relocation kit is relatively cheap try.

 

[RRToadHall]

One thing I don't see mentioned is cleaning the actual air intake at the butterfly. yes vacuum leaks will cause all sort of havoc. However, any build up of carbon on the seating surface of the butterfly will throw the idle way off and give incorrect IACV readings.

 

[OkieRover]

Thanks for all the replies.
Dizzy is a fresh used one. The ignition module has been relocated but is now back on the dizzy. New ignition module and coil with less then 20 hours of engine running on them.
IACV is brand new. Seat is clean. But I may pull the plenum to thoroughly clean.
I too thought the idle might be too low. But am told that is perhaps wrong. Swapped the MAF to another used one but no change.
I've got a lot to investigate.

 

[Mikieman]

While you investigate
I have thought this problem is caused from
Where the throttle body attaches to the intake if there is no gasket
After it heats up
It causes a vacuum leak
Best of luck
Mike

 

[westy1]

Hi there,If the idle air control valve is new aftermarket(Chinese),they tend to have a longer screw jack than the originals.
This effectively alters its behaviour at the lower idle setting,ie;its closed out further than the originals thus less air which destabilises the air fuel mix.
As advised, a reset of the base idle should restore that low end idle by adjusting the base air bypass using the hex screw on the throttle body.
Another option I have found successful is to use a thicker or additional washer on the iac/stepper motor to bring the screw jack back out to where the Oem originals sit.more air getting in at idle.
good luck

 

[Range123]

Problem solved?? How did you get on?

 

[OkieRover]

I've sprayed the existing MAF with cleaner to no affect. Realizing I am relying in 25 year old electronics I broke down and bought a new MAF (98$US). It will be in soon. I'll update once I've tested it.

 

[RRLondon]

If you run it with the IACV disconnected by the vacuum tubes (when it's warmed up) and both ends plugged to stop any interaction, then what does it do?

It shouldn't be able to mechanically access lower than the fixed idle set during initial adjustment.

Air leaks only make things idle faster, they never slow the motor.

 

[OkieRover]

New MAF. I broke down and bought a new MAF. On the maiden voyage she ran like a dream. If that changes, i will post back.
http://okierover.com/blog/new-maf-vlog/

 

[Mikieman]

I would say an inexpensive fix
And it’s the simple in you face fix
Awesome

 

[Range123]

New MAF. I broke down and bought a new MAF. On the maiden voyage she ran like a dream. If that changes, i will post back.
http://okierover.com/blog/new-maf-vlog/

I know its frustrating but not worth breaking down over!:dance: