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1991 Subaru Legacy Surging, stalling at idle when weather I

Tiny
DRKWC
  • MEMBER
  • 1991 SUBARU LEGACY
  • 4 CYL
  • AWD
  • MANUAL
  • 17,000 MILES
1991 Subaru Legacy 2.2l AWD wagon:

When the weather is below about 45 degrees outside, my Legacy won't idle smoothly or reliably. Instead, it surges between 0 RPM and about 1,300 RPM, up and down, until it stalls. The colder it is, the worse the problem.

When I take my foot off the accelerator while driving, if the outside temp is in the 20s or 30s (or lower), idle drops to nothing and engine stalls.

Both of these related behaviors happen both while engine is warm and while cold, though at startup wihen the car is cold, it actually will run smoothly for about 20 to 30 seconds before the surging begins.

I replaced all vacuum lines, cannister, mass airflow sensor, throttle position sensor, purge solenoid control valve. No change, except that I no longer get a diagnostic code indicating that the purge solenoid control valve is stuck open.

Since that is apparently NOT the problem, I'm hoping someone will recognize it. It is definitely tied to outside air temperature. On those rare wonderufl winter days here when it's 50 or 60 degrees out, the car runs perfectly smoothly, as it should, no idle issues at all. Soon as the outside temps drop, the surging at idle issue begins.

Suggestions?
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Monday, March 1st, 2010 AT 2:52 PM

10 Replies

Tiny
KHLOW2008
  • MECHANIC
  • 41,815 POSTS
Hi drkwc,

Thank you for the donation.

The components affected by temperature are the Engine Coolant Temperature (ECT) sensor and Idle Air Control (IAC) Valve.

Since no codes are present, you would need to test their performance to confirm them to be within specs.
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Tuesday, March 2nd, 2010 AT 12:23 PM
Tiny
DRKWC
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Can you point me to where the idle air control valve is? That seems a likely suspect. I believe the coolant temp sensor is most likely OK as the temp gauge is working properly.

What tests would I run on the idle air control valve? Is this have an electronic sensor as well?
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Tuesday, March 2nd, 2010 AT 1:47 PM
Tiny
DRKWC
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Never mind. I found it in my manual and will do the diagnostics and/or grab another at the scrap yard.
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Tuesday, March 2nd, 2010 AT 5:08 PM
Tiny
2 LINSEY
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Hi, Thank you for using 2carpros. Com, we appreciate your donation and look forward to helping you in the future. Have a great day!
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Wednesday, March 3rd, 2010 AT 12:14 PM
Tiny
DRKWC
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Actually, I need just a bit more help with this.

On the idle air control valve, I see where the hoses connect but I am not seeing where this is bolted to the throttle body or engine casing.

I do see a small cover that can be removed. To remove the whole valve assembly from the car, do I need to open that cover and will I find the bolts in there with which it is attached to the car?

If not, can you point me to a diagram that shows the bolts that hold this thing on (or explain where they are so that I can find them). I'm going to go to the junkyard and grab one off a Legacy, once I figure out how to get it off.
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Wednesday, March 3rd, 2010 AT 12:33 PM
Tiny
KHLOW2008
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Sorry for the delay in replying. Seems the automatic notification process goofed again.

The IAC is known as the Bypass Air control Valve for this model and here is a location diagram. It is bolted on at either end and aftr removing the bolts, it can be pulled out. Quite often cleaning the valve with carb cleaner would fix the symptome described as there are no trouble codes.


https://www.2carpros.com/forum/automotive_pictures/192750_IACV91LegacyFig06_1.jpg



Here are the procedures for testing the component.

BY-PASS AIR CONTROL VALVE (TBI)
1 . With engine running, disconnect electrical connector. Engine speed should drop (a little with engine warm, a lot with engine cold). Reconnect electrical connector. Ensure engine speed returns to previous speed.

2. If not, turn engine off. Turn ignition switch to ON position. Ensure 10-12 volts are available at Black/White wire of air control valve electrical connector.

3. Turn ignition off. Ensure resistance of Air Control Valve (ACV) is 7.3-13 ohms at -4 to 176 °F (-20 to 80 °C). If not, replace ACV. Check ACV solenoid for any short to ground. If present, replace ACV.

4. Attach electrical connector to ACV. Disconnect connector at ECU. Turn ignition on. Ensure 10-12 volts are present at terminal No. 45 (Green/Red wire) of ECU. If not, check harness between ACV and ECU.

5. Turn ignition off and reconnect ECU connector. Turn ignition on. Backprobe terminal No. 45 (Green/Red wire) of ECU. There should be one volt or less for about one minute after ignition switch is turned to ON position. After one minute, there should be at least 10 volts. If not, check for a bad ground, problem with ECU circuits (sensors) or ECU.

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Saturday, March 6th, 2010 AT 7:04 AM
Tiny
DRKWC
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Thank you very much. Am going to head out in just a few minutes and see if I can pull it and clean it. If that doesn't do any good, then I'll learn how to do the ohms testing. (Which it's high time I learn, anyway.)
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Saturday, March 6th, 2010 AT 4:19 PM
Tiny
DRKWC
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OK. I pulled it. Didn't remove the coolant lines attached, but did shoot carb cleaner liberally through the air lines and connectors -- one of which was particularly crudded up inside.

It's 60 degrees out today. Nice warm northern New Mexico March evening.

I'll have to wait 'til the outside temp drops into the 30s to see if I've accomplished anything.

At least I understand now how to R&R the thing and know what tools to carry to the junkyard if I need to grab oine. Also, I discovered at least two more air or vacuum lines that I can replace.

Thanks again for the help. After I've driven it at low outside temps, I'll report back whether or not that did the trick.
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Saturday, March 6th, 2010 AT 6:38 PM
Tiny
DRKWC
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That was it!

Winter is back, so I went out this a.M. When it was snowing and about 30 degrees out and started the car. Let it run for about 10 minutes. Kicked down the idle.

Idles steady. No surging.

So just pulling and cleaning the Bypass Air Control Valve did the trick. After all those other parts I needlessly replaced.

Thanks!
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Wednesday, March 10th, 2010 AT 12:23 PM
Tiny
KHLOW2008
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Sorry for not getting back to you earlier. The notification process failed yet again. I was browsing my questions when I came upon the numbr of replies that you have made.

Glad to know you have fixed the problem.

Thank you for using 2carpros.

Have a great day.
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Thursday, March 11th, 2010 AT 9:39 AM

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