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Old 08-12-2019, 12:38 PM   #1
drj434343
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Default EJ22 low idle when warm

1990 EJ22 auto. Primary symptom is a low warm idle. 400-500 RPM, low enough that the engine exhibits a slight shake like its barely hanging on. Just off 500 rpm, everything is smooth, but I have to use the gas pedal to get there. No other driveability issues or codes. It doesn't die, but I would guess that a working AC compressor kicking in would kill it.

I've been through 3 different IAC's so far. All have been thoroughly cleaned, and resistance checked per the factory manual procedure. I get the same low idle behavior with all 3 IAC's.

Cold starts appear normal, the car idles around 1300 RPM for several moments, then slowly lowers the RPM's. After a few minutes of driving, the RPM has drifted down to its abnormally low level. It never dies though, and I believe the IAC is working.

Twisting the IAC momentarily changes the idle speed, but the ECU pretty quickly corrects back to a low idle after I make a change; again suggesting the IAC is working.

I've confirmed battery voltage at the middle terminal of the IAC connector, and proper 6 and 7 volts at the other two connector terminals per the factory manual. I've also confirmed good continuity between the ECU connector and the IAC connector per the testing procedure.

Original injectors all tested in the right resistance range, but due to the prevalence of failure on these early units, I swapped in tested red tops from a 93'. No changes in idle behavior.

I've also checked the function of the purge solenoid. Resistance is in spec, and it appears closed at idle. I've read about this causing an internal vacuum leak, but I would think that would artificially raise the idle, not lower it.

Even though I would think a vacuum leak would raise the idle, I've systematically blocked off each manifold and intake vacuum port but haven't been able to change the idle. Also did a smoke test on the intake system and didn't find any obvious leaks.

I'm looking for some other sensor or control system that could be telling the computer to shoot for an abnormally low idle. Thoughts?
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Old 08-12-2019, 12:59 PM   #2
yarrgh
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How's the voltage on the throttle position sensor?
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Old 08-12-2019, 02:06 PM   #3
drj434343
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I have measured 12V on one of the harness connector pins per the factory manual. I also measured the throttle closed switch (shorted when throttle closed, open when throttle just starts to open) with the connector unplugged per the manual. Strangely, the resistance sweeps the opposite direction than the manual indicates when probing unplugged, but this seems to be the case on both EJ22's I have, so I'm not sure what's going on there. Resistance seems to sweep steadily, and doesn't go infinite at any point.

I haven't back probed the connector for voltage yet. I didn't see that in the factory manual, but have read about it online.
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Old 08-12-2019, 07:44 PM   #4
Jessekrs123
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Had a similar issue with one of my first gens a long time ago.

Turned out to be a bad coolant temp sensor. I had a few of them hanging around and it did the trick.

Seems to be one of the few things you haven't checked yet, you seem to know your stuff too.
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Old 08-12-2019, 11:20 PM   #5
drj434343
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Yeah, that's a good suggestion, I haven't checked the coolant sensor yet. In reading the engine control theory section of the manual, I've come up with a few possibilities.

Coolant Sensor: As water temp increases, the ECU leans the mixture out.

Knock sensor: If the ECU detects knock, it retards the timing by 5 deg which can definitely lower the idle speed.

Air flow sensor or O2 sensor: either of these could erroneously lean or enrich the mixture, altering idle speed.

Its weird I'm not getting any codes, as any of the above, if truly malfunctioning, should trigger a code.
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Old 08-13-2019, 10:29 AM   #6
drj434343
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This morning after a drive, I noticed a 1 cm brass screw on the top of the throttle body. This is separate from the throttle stop screw which I know you aren't supposed to adjust.

Turning this brass screw changed the idle in a way that the ECU didn't seem to try and compensate for. I.E. I simply turned the screw 1/4 turn and the idle is now at 750!

I've not read about this adjustment in the manual, and I'm clearly not turning the throttle plate itself. It seems to be some sort of internal bypass that is only affecting the IAC.

I have no idea if this adjustment is appropriate or will stay. Has anyone seen this screw before? My 90 EJ22 manual does not have this screw.
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Old 08-18-2019, 09:57 AM   #7
El_Kameleon_G
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Nevermind, I didn't fully read the last post because I was on my phone. It does sound like you fixed it though.

I think you’ll be fine. It sounds like everything else seems to be working as it should.

Last edited by El_Kameleon_G; 08-18-2019 at 10:04 AM.
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Old 08-19-2019, 10:09 AM   #8
drj434343
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Yes, it does appear this mystery screw allowed me to adjust the idle. But what it is really meant for, and why I don't see it on my other 1990 manual is still a mystery.
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Old 08-20-2019, 09:35 PM   #9
El_Kameleon_G
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I searched “Subaru throttle body screw” in a search engine and halfway down a picture came up that called it the idle adjustment screw. It is separate from the throttle stop screw. I’m not sure why the other car didn’t have it.
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