1997 Subaru Impreza Intermittent High Idling

Tiny
JAGULAR1785
  • MEMBER
  • 1997 SUBARU IMPREZA
Engine Performance problem
1997 Subaru Impreza 4 cyl All Wheel Drive Manual

So I have had this problem for a while. My outback sport likes to idle high in cooler, colder and damper weather, even when at full operating temperature. I have replaced the MAF sensor, seafoamed it, cleaned the IAC and throttle body, checked every single vacuum hose, hosed the motor down with starter fluid while running to check for other leaks and reset the ECU many times.

I am 98% sure that it is some sort of sensor/electrical issue giving the ECU bad readings but there is no CEL and no codes that I have been able to find. The reason I think this is because when I reset the ECU this problem went away for a while. It drove better, seemed "peppier" and all around was happy-for about 30 minutes of in-town driving.

What gets me is that the idle will shoot up to well over 2 to 2.5k rpms when cold, and when fully warm it will be at 1.25k rpms. But as you coast to a stop, the idle will be at 1.5k until you are under 12 mph, the exact value that the speed sensor 1 uses on the ECU. And it doesn't matter if you use brakes or not, if you speed up with the motor or roll down the hill, it switches at 12 mph from 1.5k rpm to 1.25k rpm and vice versa.

Again, I have replaced the MAF sensor with a junkyard MAF sensor, to no effect. I would guess that it could either be the throttle position or engine coolant sensors, but unplugging the TPS did not change the idle speed. Coolant sensor is next on the list I guess. I just don't understand how if it was a bad coolant reading, the idle would be affected by the vehicles speed.
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Saturday, February 27th, 2010 AT 12:29 AM

7 Replies

Tiny
MMPRINCE4000
  • EXPERT
If you have checked all vacuum lines and are SURE there is no leak, then, Fast Idle Control Device is suspect.

FICD solenoid is mounted in-line with fresh air intake duct and intake manifold. When A/C request is present, FICD solenoid opens and allows fresh air to by-pass throttle valve, increasing idle speed for the extra engine load.
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Saturday, February 27th, 2010 AT 8:13 AM
Tiny
JAGULAR1785
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Well that seems like it would be the case, but I have not seen any other places that air would be allowed in the intake. The IAC is separate on this particular model, strange I know. It takes a separate air tube to a hole in the intake manifold. I was under the impression that the idle air control valve did all the idle speed alterations. Perhaps not?

I will read through the manual again, to see how if I can find how the a/c increases idle speed.

EDIT: I have just found the device you are talking about, however, this is only on the 1800cc model, I have the 2200cc. I should have specified, I'm sorry.
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Saturday, February 27th, 2010 AT 10:09 AM
Tiny
MMPRINCE4000
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So you found the FICD?
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Saturday, February 27th, 2010 AT 12:33 PM
Tiny
JAGULAR1785
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Sorry again, no. I found the device you are talking about in the manual. But the car is not equipped with one.
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Saturday, February 27th, 2010 AT 7:50 PM
Tiny
MMPRINCE4000
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Then I would try an IAC from a junkyard, or remove, inspect and clean old one.
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Sunday, February 28th, 2010 AT 8:00 AM
Tiny
JAGULAR1785
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I have already cleaned and inspected this device; it did not have an extraordinary amount of buildup on it when I took it out. I cleaned it and made sure it worked properly ie, closes and opens without hinderance. I do not think this to be the problem, because it was doing the high idle before and after I cleaned/inspected it.
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Sunday, February 28th, 2010 AT 9:10 PM
Tiny
MMPRINCE4000
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I would probably take to a shop and have a smoke test performed to eliminate vacuum leaks as a cause.

Intermittent problems can be hard to isolate.
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Thursday, March 4th, 2010 AT 8:41 AM

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