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Old 11-12-2019, 01:42 AM   #1
ZenBoy
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Member#: 12524
Join Date: Nov 2001
Chapter/Region: MWSOC
Location: Minneapolis, Minnesnowta
Vehicle:
2004 STi
WRB/Gold

Default Relp! 04 STi Runs Great on Highway - Suddenly Won’t Idle

Hopefully some of the geniuses on the boards here can offer their input on this….

I’ve got an 04 STi w/~135k miles, owned since new (July 2003) and Cobb Stage II with a GodSpeed pro tune since 40k miles. A couple of weeks ago I was out running errands, about to prep my 04 STi for winter storage with a dose of Sea Foam and a fresh oil change, as I do every year. The car was running normally, smooth as ever, and driving / idling normally with no issues.

I made a stop for about 10-15 minutes, 1/4-tank of gas (pre-Sea Foam), and then when I started the car, suddenly it wouldn’t idle. Feathering the gas keeps it running, and driving around it runs smoothly and pulls strongly with no apparent misfires until I let it idle, when the engine again tries to quit. Adding Sea Foam and filling the gas tank made no difference in behavior, even after ~15 miles of driving.

Highway driving doesn’t trip a code, but the misfiring at idle does. The dealer checked and found a code for a cam timing issue, but after clearing the codes it didn’t come back. Diagnostics showed misfiring on cylinders 1, 2, and 3.

Inspection of the timing belt showed that cam timing was spot-on, and the belt (replaced @ ~95,000 miles) was in great shape. Spark plugs look normal.

I had a similar behavior 3 years ago, when on a cross-country road trip the fuel pump and sending unit went out; the car would run great for hundreds of miles at a stretch, but wanted to die anytime I stopped for fuel and let the RPMs fall. Sending unit and pump got replaced at the time and the problem.

For my current issue, the dealer mechanic ran a compression test, and while they’re below 140 (averaging around 130), all are within 10% of each other. Dealer wants to rebuild the valve train (to the tune of $3k-4k) to eliminate carbon deposits as a cause.

In my mind - however off-base - it seems to me that if carbon buildup on the valves was the cause, it wouldn’t be a sudden-onset, acute idling issue; it would have developed gradually over time. In my case, it was like someone flipped a switch in the midst of my errands, with no backfires or incidents to disrupt the engine, just tame around-town driving.

Does anyone have any suggestions for a troubleshooting approach that might be fruitful, and cost less than a valve train?
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Old 11-12-2019, 02:46 PM   #2
straitstrider67
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Join Date: Dec 2016
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Location: Illinois
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2014 WRX STI Hatch
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Well with the cold weather im wondering if it could have caused a small vacuum leak somewhere if a hose or fitting got brittle hence why that small leak would make a massive difference only at idle when only a small amount of air is going into the engine but once you open the throttle that leak is adding a much smaller percentage of uncalculated air to be added to the system. I highly doubt its your valve system or any other major internal components since your compression is normal but a leak down test could confirm this.
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Old 11-14-2019, 05:44 PM   #3
sig
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haha its definitely not carbon deposits or valves, to me its sounds like the MAP/MAF sensor could be dirty or maybe the throttle body. there's the Idle Air Control Valve on the throttle body that can get dirtied up and cause bad enough idle to make the car shut off. Check this thread out and see if it sounds like what youre dealing with.

https://forums.nasioc.com/forums/sho...d.php?t=781242
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Old 11-18-2019, 04:23 AM   #4
drewdogg
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Quote:
Originally Posted by sig View Post
haha its definitely not carbon deposits or valves, to me its sounds like the MAP/MAF sensor could be dirty or maybe the throttle body. there's the Idle Air Control Valve on the throttle body that can get dirtied up and cause bad enough idle to make the car shut off. Check this thread out and see if it sounds like what youre dealing with.

https://forums.nasioc.com/forums/sho...d.php?t=781242
I second this. I'd start with the MAF.
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Old 12-29-2019, 10:56 PM   #5
ZenBoy
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Join Date: Nov 2001
Chapter/Region: MWSOC
Location: Minneapolis, Minnesnowta
Vehicle:
2004 STi
WRB/Gold

Default

Quote:
Originally Posted by straitstrider67 View Post
Well with the cold weather im wondering if it could have caused a small vacuum leak somewhere if a hose or fitting got brittle hence why that small leak would make a massive difference only at idle when only a small amount of air is going into the engine but once you open the throttle that leak is adding a much smaller percentage of uncalculated air to be added to the system. I highly doubt its your valve system or any other major internal components since your compression is normal but a leak down test could confirm this.

Quote:
Originally Posted by sig View Post
haha its definitely not carbon deposits or valves, to me its sounds like the MAP/MAF sensor could be dirty or maybe the throttle body. there's the Idle Air Control Valve on the throttle body that can get dirtied up and cause bad enough idle to make the car shut off. Check this thread out and see if it sounds like what youre dealing with.

https://forums.nasioc.com/forums/sho...d.php?t=781242

Quote:
Originally Posted by drewdogg View Post
I second this. I'd start with the MAF.

Thanks, all! After I pushed back on "diagnostic dismantling of the engine" (to the tune of $1,000+), the Service Manager involved the Shop Foreman - huge props to him for saving the day - and the problem was found.

The passenger-side cylinder head’s oil control valve had become blocked because its plastic screen had disintegrated, and a small piece of it got wedged in the valve, jamming it.

The valve timing was unable to adjust, hence the difference in drivability between normal driving RPMs and idling RPMs with the clutch disengaged.

Whew. That was quite a bit of troubleshooting $$.
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Old 12-30-2019, 08:33 AM   #6
horsePOOGAS
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When I bought my 04 STI off of the original owner... The first thing I did was remove the oil screens out of the Banjo bolts because of this reason. Good thing for you it didn't do any permanent damage.
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