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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? :confused:
 

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From what you wrote agree. Not your valves and not carbon. Could be valve train. did you check the timing belt yourself?

Far more likely causes: MAF and small leak.

You have an AP. Log. What are your A/F Learning values.


Could be a tooth off. Could
 

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Discussion Starter #3
From what you wrote agree. Not your valves and not carbon. Could be valve train. did you check the timing belt yourself?

Far more likely causes: MAF and small leak.

You have an AP. Log. What are your A/F Learning values.


Could be a tooth off. Could
Good question. The dealer mechanic reported that the belt was fine and not the cause. He showed me the belt condition, and it's like-new, still showing the installation marks.

I have an AP v1...no logging that I can see. (?)

I did offer to re-flash the ECU back to stock, just in case it's masking any codes we need to see, or interfering in any way. I've offered several times, in fact. Service advisor has yet to acknowledge or take me up on that offer.

As for A/F learning values - no clue as I'm not experienced in tuning maps, but that was part of my reasoning for re-flashing back to stock, since I only have a catted DP and then a cat-back; if we eliminate the tune as a factor, wouldn't it help? :notsure:
 

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Given the age and miles anything is possible. Before you go any further, i would suggest you have a vacuum test done first. Make sure there arent any air leaks in the engine. A vacuum leak large enough can cause misfires at idle since the engine is pulling the most vacuum then.
 

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I would also suggest taking a look at your cam sensors and crank sensor. I had a somewhat similar issue with my car where it would randomly stall and sputter at idle. It wouldn't give a code either. Turns out one of my cam sensors went bad. It seems that if the engine doesn't see the cam position, it won't provide spark, but it also won't throw a code. I know you mentioned you had codes for cam timing issues, so that had me thinking it could be something to do with the sensors.
 

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I would also suggest taking a look at your cam sensors and crank sensor. I had a somewhat similar issue with my car where it would randomly stall and sputter at idle. It wouldn't give a code either. Turns out one of my cam sensors went bad. It seems that if the engine doesn't see the cam position, it won't provide spark, but it also won't throw a code. I know you mentioned you had codes for cam timing issues, so that had me thinking it could be something to do with the sensors.
I second this! This is another great suggestion. Cam to crank position is very important. On other cars, i have seen bad crank sensors cause all kinds of issues like stalling, no starts, and rough running.
 
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