Engine surges when a/c cycles off
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Engine surges when a/c cycles off
I have a 2004 2wd Escape with 6 cylinder engine.
When the engine is idling and the a/c is either turned off or cycles off the engine surge to 3000+ rpm. If I've turned the a/c off I can generally get the idle to return to normal by turning it back on, but not 100%.
I've replaced the IAC since this was mentioned by a mechanic as a potential cause but it didn't have any impact.
Can anyone help me diagnose this issue?
Thanks, Mike
When the engine is idling and the a/c is either turned off or cycles off the engine surge to 3000+ rpm. If I've turned the a/c off I can generally get the idle to return to normal by turning it back on, but not 100%.
I've replaced the IAC since this was mentioned by a mechanic as a potential cause but it didn't have any impact.
Can anyone help me diagnose this issue?
Thanks, Mike
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From: Warren, MI
Idle speed is controled by the PCM or computer so something is confusing it or it's confused. The extra load of the A/C causes the RPM to drop so the PCM raises it but should lower it when turned off.
Isn't your vehicle part of the recall?
Isn't your vehicle part of the recall?
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From: Warren, MI
Unplug the IAC while running if you can. The engine should die. That'll tell you if it's being commanded by the PCM to idle faster. This might cause a CEL/SEL in which case you'll have to remove the neg. battery cable to clear it or just wait till it clears on its own.
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I just ran the engine and waited for the a/c compressor to cycle on at which point I noticed the small increase in idle speed that's expected. I turned the a/c off and the idle RPMs started to climb.
So I unplugged the IAC and the idle returned to normal and the engine did not die. I plugged the IAC back in and RPMs shot back up. I repeated this a couple of times and it was very consistent.
I also heard what sounded like it might be a vacuum leak, but I can't really tell where it's coming from.
So I unplugged the IAC and the idle returned to normal and the engine did not die. I plugged the IAC back in and RPMs shot back up. I repeated this a couple of times and it was very consistent.
I also heard what sounded like it might be a vacuum leak, but I can't really tell where it's coming from.
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From: Warren, MI
I just ran the engine and waited for the a/c compressor to cycle on at which point I noticed the small increase in idle speed that's expected. I turned the a/c off and the idle RPMs started to climb.
So I unplugged the IAC and the idle returned to normal and the engine did not die. I plugged the IAC back in and RPMs shot back up. I repeated this a couple of times and it was very consistent.
I also heard what sounded like it might be a vacuum leak, but I can't really tell where it's coming from.
So I unplugged the IAC and the idle returned to normal and the engine did not die. I plugged the IAC back in and RPMs shot back up. I repeated this a couple of times and it was very consistent.
I also heard what sounded like it might be a vacuum leak, but I can't really tell where it's coming from.
Try that without turning your A/C on.
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I just went and played with it some more. With no A/ the idle did not change when I unplugged the IAC.
After running the A/C and turning it off the RPMs surges to over 3000. When I turn the A/C back on the idle returns to normal. The high RPM idle is directly controllable by the A/C.
When the idle is high I can turn the car off and restart the engine and the idle is fine. But if I turn the A/C on, wait for the compressor to cycle on, and then turn off the A/C the high RPM idle issue starts again.
What should I try next?
After running the A/C and turning it off the RPMs surges to over 3000. When I turn the A/C back on the idle returns to normal. The high RPM idle is directly controllable by the A/C.
When the idle is high I can turn the car off and restart the engine and the idle is fine. But if I turn the A/C on, wait for the compressor to cycle on, and then turn off the A/C the high RPM idle issue starts again.
What should I try next?
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Joined: Jan 2004
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From: St Louis
Just because a part is new doesn't mean it's good. I learned that the hard way a long time ago.
Your symptoms are of a bad or ineffective IAC. For example, if the TB plate is stuck partly open or there is a large vacuum leak the IAC cannot control the air coming into the intake.
Your symptoms are of a bad or ineffective IAC. For example, if the TB plate is stuck partly open or there is a large vacuum leak the IAC cannot control the air coming into the intake.
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From: Warren, MI
I just went and played with it some more. With no A/ the idle did not change when I unplugged the IAC.
After running the A/C and turning it off the RPMs surges to over 3000. When I turn the A/C back on the idle returns to normal. The high RPM idle is directly controllable by the A/C.
When the idle is high I can turn the car off and restart the engine and the idle is fine. But if I turn the A/C on, wait for the compressor to cycle on, and then turn off the A/C the high RPM idle issue starts again.
What should I try next?
After running the A/C and turning it off the RPMs surges to over 3000. When I turn the A/C back on the idle returns to normal. The high RPM idle is directly controllable by the A/C.
When the idle is high I can turn the car off and restart the engine and the idle is fine. But if I turn the A/C on, wait for the compressor to cycle on, and then turn off the A/C the high RPM idle issue starts again.
What should I try next?
The IAC is controlled with a PWM(pulse width modulated) signal which is best viewed with a scope although a high end DMM that can read duty cycle is a second choice. The newer IAC's may be a bit different too.
I used to have access to all of Ford's online manuals but they found their mistake and plugged the hole, so I can't help any shop manual stuff from '04.
There have been threads in other forums about PCM replacements but I never followed them too close.
Here's one thread to read: http://www.fordforumsonline.com/foru...r-failure.html
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From: Warren, MI
What the interaction of the A/C in the symptoms?
Hotshot
Joined: Jan 2004
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From: St Louis
When the computer commands AC compressor operation (compressor clutch operation is moderated by the PCM under software control), it also issues a leading command to the IAC to open more to compensate for the extra load the compressor places on the engine.
When the compressor is disengaged, the PCM removes the "extra" bias from the IAC control circuit which should allow the idle speed to drop back to normal. If the IAC, for example, is sticky, it will remain in the position that allows too much air past the throttle plate and will cause higher RPMs or loss of idle speed control.
When the compressor is disengaged, the PCM removes the "extra" bias from the IAC control circuit which should allow the idle speed to drop back to normal. If the IAC, for example, is sticky, it will remain in the position that allows too much air past the throttle plate and will cause higher RPMs or loss of idle speed control.