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Old 05-05-2019, 12:41 PM
ululi1970 ululi1970 is offline
Learning to use the forums
Join Date: May 2019
Posts: 1
Default Erratic idle problems

I have '98 with the 2.5L engine, manual transmission.
My issue with the idle is this.
Say I am approaching a red light. I put in neutral and start coasting.
The idle stays high, and as a matter of fact often goes higher, approaching and even surpassing 2,000 RPM, the slower the car goes.
Only when the wheels come to a complete stop, the idle drops
back to its normal value.
The maddening thing is that it does not always do that! It may work fine at one light, and the next will rev up until the car stops.

I replaced the IAC, and tried the trick to use electronic cleaner but that does not fix the problem. Also, I may go weeks without experiencing it, and the comes back.

I am pretty sure it is not a vacuum leak, but rather the ECU somewhat opening the IAC. I know that because if I unplug the IAC, I never have the problem. Of course, the engine idles rough with the IAC unplugged.

Also, it never does it when the engine is cold.

Other than that, I cannot find any other pattern to it. It does it when it's cold outside or warm, rain or sunshine, with AC on or off.

Aside for the annoyance, it also does it when the car is in gear.
Sometime, when I let the foot of the gas, but still in gear, I feel the
engine pulling the car.

So this is not a consistent high idle problem. When the wheels are not moving, the idle is always good.

Any help is appreciated.
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Old Yesterday, 06:26 PM
Harbor_Handed Harbor_Handed is offline
Ford Tough
Join Date: Dec 2016
Posts: 231
Default Re: Erratic idle problems

I hear ya. In my own anecdotal experience, the IAC is one of the components that is most prone to malfunction. It's hard to find a "good one" that just does what it's supposed to do AT ALL TIMES.
2003 Mazda B2300
2.3 L DOHC
4 Cylinder, Automatic Transmission
157,000 miles on Odometer as of Nov 2018
25 mpg on mixed hwy/city driving

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Old Today, 04:36 AM
tomw0 tomw0 is offline
Ford Ranger Driver
Join Date: Aug 2015
Posts: 958
Default Re: Erratic idle problems

If you get a OBD-II device you can find out if the computer is telling the IAC to bump the idle speed or the IAC is doing this due to grunge/dirt/crud(hanging).
Using FORSCAN software on a ipXd, tablet or phone you can connect to the scan tool and find a lot of information. The scan tools are available online for ridiculous prices, and FORSCAN(non-pro) is no cost.
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