2002 Mercury Marquis high engine idle speed

2002 MERCURY MARQUIS
85,000 MILES • 4.6L • V8 • 2WD • AUTOMATIC
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HARRY SINGLETON
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Lowest idle speed obtainable is 800 to 850 rpm hot in neutral. Idle speed at rest in gear 650 to 700. Idle speed always increases when shifting to neutral while in motion. For example, idle speed increases from 1400 rpm to 2200 rpm at 60 mph coasting in neutral and stays high until reducing vehicle speed. Long downhill runs in gear are accelerator against brakes.

Disconnecting the IACV drops neutral resting hot idle to approx 550 rpm. The computer command to the IACV seems to be incorrect.

IACV replaced and system reset was performed to no avail. Local Ford dealer says all readings within spec.

60 degree F startup idle rpm 1400.
181 degree F run rpm 800

I am comparing this car to a 2001 just like it that showed none of this problem. Fuel economy is normal.

I think a sensor is giving the vehicle computer wrong information.

Help please? I don't have enough hair left to justify tearing any out dealing with this problem.

Harry Singleton
May 4, 2015 at 9:08 AM
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HMAC300
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you might have a sticky throttle plate so try cleaning it on both sides and iac hole with choke cleaner they get gunked up from oil coming in from breather. try that first also check for vacuum leaks and all grommets on engine as well as duct work leading to engine having no holes and connected properly don't be to concerned about idle in drive or neutral because you can't adjust it anyhow. I think the cleaning will help immensely
May 4, 2015 at 9:42 AM
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HARRY SINGLETON
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Thank you. I will try those things.

Additionally, I just tried disconnecting the IACV and driving the car at operating temperature and it runs perfectly. It idles up and down just fine. About 575 rpm idle, running just like it should, like it used to, and like the other car I had just like it. Check engine lite was on, of course.

Is it not possible that a speed or oxygen or throttle or air flow sensor or something is outputting incorrectly and fooling the computer? It seems to me that the computer is trying to make the car run too fast. If the problem is linked to a crud buildup, would not the problem still be there with the IACV disconnected?

Thank you again.

Harry
May 4, 2015 at 10:40 AM
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HMAC300
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if a sensor is bad then the check engine light would be on and from your description yousaid it wasn't. sohave an auto parts scan for codes for you.
May 4, 2015 at 2:15 PM
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HARRY SINGLETON
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I thoroughly cleaned the throttle plate and throttle bore, removing all coking deposits. I also cleaned the MAF sensor. (The IACV has been changed and is new). No change in engine idle speed or vehicle performance was evident. No DTCs are displayed, but the idle remains high. Surely this vehicle is not supposed to idle down the roadway at 15 mph? Or sit idling in the driveway at 800 to 850 rpm.

Again, the vehicle runs perfectly with the IACV disconnected at a hot idle of approx 575 rpm, purring like the proverbial kitten.

Is it not possible that a sensor might not indicate a defect, but is enough out of spec to fool the vehicle computer into idling too fast?

I have am ELM adapter for my Iphone connected to the OBD2 port. No troubles are displayed.

This vehicle idled properly 2 months ago.

Harry Singleton

May 6, 2015 at 10:07 AM
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HMAC300
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if you are still having the same problem then it's either an eger plugged passage or egr not closing all the way or a vacuum leak 800 isn't a bad idle speed but check those and make sure all vacuum lines are connected and not leaking.
May 6, 2015 at 3:33 PM