Strange issue with my SR: idles at 1.5K to 2K, but normal when AC is on!

M

mike5

Adventurer
I've never seen such an issue on ANY car....and I've owned a few European cars. Basically, the car is idling at 1,500 to 2,000 RPM. This is after it is warmed up or even after a 40 mile drive on the freeway. It could be in gear, park or neutral. The idle in park or neutral weel creep up to 2K, then work down slowly to 1,500. When it roughly hits 1,500, it will jump up to 2K again and work down...and keep repeating.

BUT, if I turn the AC on, it idles at 800rpm.

This is a 96 SR, OBD2. I would think if the computer thought something was wrong, I'd get a check engine light. But as of now, no light. I've been driving it for a couple of days.

What could it be? What can I check? I'd posted here yesterday that I noticed a clip missing on a harness. I thought that was the problem...but it is not. Wiggling that harness results in no difference and it feels super tight even without the cliip.

Thanks again!
 
M

mike5

Adventurer
I do have a generic scanner...but doubt it would be useful. I'm just trying to get an idea...otherwise back to the mechanic:-(
 
O

off-roader

Expedition Leader
I'm having similar issues but haven't spent the time to look into it yet. Also try asking this on 4x4wire where there are a lot of technically inclined owners.
 
M

mike5

Adventurer
I know vacuum leaks normally go with weird idle problems. But, why is it perfect with the AC on? Thats why I don't think it is a vacuum leak.
 
M

mike5

Adventurer
I was reading over at 4x4wire about idle issues that were tied to the IAC. I'm just thrown off by the fact that with the AC running, its perfect. You'd think the IAC would not work properly whether the AC was on or not. So my guess would be it is not the IAC. I'll see if I can remove it and maybe have a look and make sure it is clean anyway.
 
O

off-roader

Expedition Leader
mike5 said:
I was reading over at 4x4wire about idle issues that were tied to the IAC. I'm just thrown off by the fact that with the AC running, its perfect. You'd think the IAC would not work properly whether the AC was on or not. So my guess would be it is not the IAC. I'll see if I can remove it and maybe have a look and make sure it is clean anyway.
Click to expand...

On mine the orig. IAC/ISC was bad. I replaced it with a known good unit and now I've got this idle issue. I suspect a prior owner adjusted the base idle to mask the iac problem so now it needs to be adjusted back.
 
Patman

Patman

Explorer
The computer will compensate pretty well for a vacuum leak. If it is bad enough it will cause a high idle. The added load of the AC will pull the idle down. The computer will not tell the IAC to compensate (like it should under perfect conditions) because the idle is already within spec. The idle leak acts like the IAC is letting in more air.

Seen it before on other cars. Doesn't mean its your problem, but it sounds like a good place to start. Easy enough to test for vac leaks
 
M

mike5

Adventurer
Thanks Patman, valid points you make. How do I check for a vacuum leak? I'm not a mechanic and I don't have a vacuum gauge to hook up. I'll try and double check the vacuum lines that I do see.

So this evening, I took the IAC out, cleaned it and put it back in. No difference. However my helper and I noticed something. When I turned the AC on with the car idling high, the throttle body cable moved automatically, I suppose the car knows there is more load so it basically applies a little gas. This got us thinking that when the AC is on, the TB plate opens up just a little bit, allowing air to flow through the TB. This of course means that the IAC is no longer in play. Hence when the AC is on, IAC is in effect bypassed and the issue is resolved. Of course this theory would suggest a bad IAC.

For those that have no idea of what I'm saying, the IAC gets its air through two rectangular holes in the TB, just before the TB plate. And the IAC in effect is a solenoid. So when the TB is closed because no gas is being applied, the TB is bypassed with the help of the IAC pulling air through those two openings.
 
O

off-roader

Expedition Leader
I measured the coil resistances on the IAC using a multimeter to tell for sure. There are ohm ratings in the FSM. I knew it was bad because it wouldn't idle properly when the engine was cold or when the AC was turned on.
 
D

Deleted member 13060

Guest
mike5 said:
So this evening, I took the IAC out, cleaned it and put it back in. No difference. However my helper and I noticed something. When I turned the AC on with the car idling high, the throttle body cable moved automatically, I suppose the car knows there is more load so it basically applies a little gas. This got us thinking that when the AC is on, the TB plate opens up just a little bit, allowing air to flow through the TB. This of course means that the IAC is no longer in play. Hence when the AC is on, IAC is in effect bypassed and the issue is resolved. Of course this theory would suggest a bad IAC.

For those that have no idea of what I'm saying, the IAC gets its air through two rectangular holes in the TB, just before the TB plate. And the IAC in effect is a solenoid. So when the TB is closed because no gas is being applied, the TB is bypassed with the help of the IAC pulling air through those two openings.
Click to expand...

There is nothing between the gas pedal and the throttle body on that car that can move the cable like you saw. The IAC is a controlled vacum leak to adjust idle speed and other than that the computer has NO control of the throttle during normal operation.

The cruise control could do it....

96 SR's came with either vacum or electronic accuators for the cruise control cable. Disconnect the cruise control cable at the throttle body and see if the problem goes away......
 
O

off-roader

Expedition Leader
LaOutbackTrail said:
What is an IAC?
Click to expand...
IAC=Idle Air Controller

Also IAC is usually interchangeable with ISC.

ISC= Idle Speed Controller (what I usually call it).

It's a common failure point on Montero's if they aren't kept clean. What usually happens is dirt/dust from the engine intake collects on the unit and causes the ISC/IAC to work harder when it actuates. Eventually it kills one of the ISC/IAC's coils (electrical open) and the ISC/IAC simply stops working.

One word of caution. It's been suggested by some of the companies that rebuild ECU's that one cause of ECU failure is when some ISC/IAC's die, they send a voltage spike to the ECU that fries it as well.:Wow1:
 
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