1997 Ford Ranger idles high only with A/C on

Tiny
ZAWARTKAY
  • MEMBER
  • 1997 FORD RANGER
  • 6 CYL
  • 2WD
  • MANUAL
  • 134,000 MILES
3.0 L
car runs and drives perfectly.

but when, and only when I turn the a/c on, or turn on a climate control function that requires a/c, the idle goes through the roof to 2,000 rpms and stays there like a cruise control.

sometimes in the morning if its not terribly hot out the problem doesnt happen. The a/c is properly charged and blows cold. I have replaced the idle control valve, and to rule out a possible vacuum leak I drove the car with the IAC removed and the IAC intake holes plugged. The problem didnt happen then. I have studied my ford service manuals searched the internet, asked friends, and got nowhere. Seems like a rare problem. I also scanned the obd and have not found any codes nor has the check engine light ever came on.
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Saturday, July 24th, 2010 AT 4:27 PM

9 Replies

Tiny
KHLOW2008
  • EXPERT
Hi zawartkay,

Thank you for the donation.

Problem is related to the IAC and it could be :

1. Faulty ECM. The engine control would increase the voltage to the IAC to open it more when the A/C is turned on. The higher the voltage the more the IAC would open.

2. Wrong temperature signal. The ECM could be receiving the wrong signal from the Engine Coolant Temperature sensor and increase the idling speed when it thinks the engine is still below operating temperature. This could be due to a faulty ECT.
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Sunday, July 25th, 2010 AT 2:12 AM
Tiny
ZAWARTKAY
  • MEMBER
Thank you for the reply.
I tested the ect with an ohmeter and at normal operating temperature it is 2.5 k ohms.

Also, when this problem happens the engine overheats, which I am guessing is a fuel/air mixture problem as a result of the iac is being told to open too much.

If I go out and drive the car and never turn the a/c on it is fine. But after running the a/c and experiencing the problem, if I turn the a/c off, the problem still occurs and will not go away until I shut the car off and restart it.

If I am running the a/c and experiencing the problem, ocassionally if I stop at a light, it will go away, then when I get moving again and work the gas pedal a few times it will come back.

I always thought a pcm rarely goes bad.
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Sunday, July 25th, 2010 AT 11:10 AM
Tiny
KHLOW2008
  • EXPERT
PCM rarely goes bad but the still do.

Since you have overheating issues, then that is the most likely cause.

Have you checked the coolant in system?

Is the IAC equipped with coolant hoses?
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Sunday, July 25th, 2010 AT 3:00 PM
Tiny
ZAWARTKAY
  • MEMBER
Coolant level is normal, no the iac does not have coolant hoses, only an electrical connection.

Is there any possible circuit that is on when the a/c is on that tells the pcm to increase the idle? Could this circuit be giving the pcm too strong of a signal causing the problem?

Otherwise I would agree the problem has been narrowed down to the pcm.

This was previously a florida car and has seen tremendous a/c usage.
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Sunday, July 25th, 2010 AT 4:24 PM
Tiny
KHLOW2008
  • EXPERT
When A/C or other electrical components are turned on, the additional load would cause the engine rpm to drop.
This extra load is detected by the PCM, which would compensate by increasing the air intake via the IAC.

If it is the electrical load detector, then the rpm would also increase if the headlights are turned on.

As the IAC is not equipped with any coolant hoses, the ECT would inform the PCM the engine temperature. If the reading says the engine is cold, the IAC would increase the idling speed.
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Sunday, July 25th, 2010 AT 4:36 PM
Tiny
ZAWARTKAY
  • MEMBER
What has happened recently is the idle would be doing this at all times. I have read the article about high idle where it says

"I can tell you this, the oxygen sensor (02), MAF or MAP and coolant sensors are prone to this type of sensor failure."

what happened the other day is this:
i finally got a check engine light. Pulled into vatozone
and this is what it said. P0420, catalyst efficiency below threshold bank 1. Perhaps my o2 sensors, or catalytic converter may have been causing the computer to run in "open loop mode" or "closed loop" not sure which one, but a high idle condition.
Is this a possibility?
I suspect this has been causing my problem

thanks!
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Sunday, August 15th, 2010 AT 7:17 PM
Tiny
KHLOW2008
  • EXPERT
P0420 is usually a fault with the catalytic converter.

For the idling to increase, something has to work to allow increased air flow into the manifolds and the O2 sensor is not likely to be the one. If the MAF is faulty, it wuld usualy stall rather than increase the rpm as it needs increased air flow otherwise it would be running rich.

Have you checked for intermittent restrictions to the throttle body? Somthing that might cause the throttle to remain partially opened?
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Sunday, August 15th, 2010 AT 8:53 PM
Tiny
ZAWARTKAY
  • MEMBER
You are not going to believe what the problem was and it couldnt be guessed a hundred times. The battery!
The battery was old as hell. It was unable to team with the alternator and support the a/c compressor load, plus cabin fan high load. So the computer goes into some kind of rescue mode, and cranks the rpms. But it went so haywire. The computer would tell the iac to go full blast.
Replacing my battery and spending the 90 bucks was the magic pill! My electrical system can now support the a/c and
it now idles so perfectly I can engine brake, and shift smoothly.

Thankfully the only other money i've thrown at the problem has been on:
4 huge ford factory service manuals, a multimeter, and a mass air flow sensor.
Oh and the iac valve I bought but then used and returned cause it didnt fix the problem. Vatozone!

Well I really feel great that I didnt go buying a new ECM. Because I was coming really close to narrowing everything else down and renderring out all other possibilities.

I just cant believe that the battery was the culprit.
Please pass this along and put it in your tech section or
things to check first section.
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Wednesday, September 15th, 2010 AT 6:22 PM
Tiny
KHLOW2008
  • EXPERT
Thank you for the input.

I did mention about the charging but did not think about checking the charging rate and you did not indicate any starting problem so it just got through.

Glad to know you have fixed the problem.

Have a nice day.
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Thursday, September 16th, 2010 AT 5:57 AM

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