I had a head gasket replacement on 2008 Legacy, 2.5L, automatic. The engine was out of car about one week. On test drive, the idle was lower than before. I have researched that the engine idle is factory set, but that the computer is supposed to adjust for load.

 

This was never an issue before the gasket replacement. Using the air conditioner at idle wasn't a problem. Now, after the replacement, the engine idles so low that the AC compressor won't kick in. The RPM gauge shows idle exactly halfway between 0 and 1, I presume that represents around 500 rpm.

 

The head gasket replacement was performed at an authorized Subaru shop. Since the battery was disconnected for an extended time, I understand the car has to relearn my driving habits. Why doesn't it 'learn' the idle is too low?

Edited September 13, 20169 yr by George W

The electronic throttle body controls idle speed by opening or closing the throttle plate slightly. If the throttle body is coked up with carbon it will have a harder time re-adjusting.

It sounds like the plate may be getting stuck, which would indicate the throttle bore is coked and needs to be cleaned. Throttle body cleaner is the best to use.

There should be some videos on YouTube or elsewhere on how to properly clean the throttle body.

I ran into this issue on a 07 forester (i think it was an 07) when i cleared the codes with my shops scan tool it messed up the idle (also may happen when you disconnect the battery for a long period of time and the ECM loses all of it's juice). We tried everything to fix it, because while the computer does control it, if it loses it's memory it can't fix it by itself. 

 

I don't know the procedure (it's a really weird procedure and we couldn't find it anywhere except on iATN fourms) 

If you don't get it fixed by Monday I'll bring the sheet home and take a picture and post it here.

 

Once I did that it idled perfectly. Before I did it would idle @ 500 rpms and if i turned the A/C on it would nearly die from the load. 

 

 

The electronic throttle body controls idle speed by opening or closing the throttle plate slightly. If the throttle body is coked up with carbon it will have a harder time re-adjusting.
It sounds like the plate may be getting stuck, which would indicate the throttle bore is coked and needs to be cleaned. Throttle body cleaner is the best to use.
There should be some videos on YouTube or elsewhere on how to properly clean the throttle body.

 

I agree with what you're saying, but it wouldn't really make sense for it to suddenly not idle after the HG was replaced. 

Edited September 10, 20169 yr by golucky66

subscribed for golucky's pic

 

Edited September 10, 20169 yr by 1 Lucky Texan

Fairtax is right. The throttle plate gets build up on it and sticks after an ECU reset. Cleaning of the throttle plate and body remedies this problem.

Double post.

Edited September 10, 20169 yr by Caboobaroo

Thank you for your experiences with your Subaru. Mine is still at the repair shop and won't see any further action until Monday. I will be on the phone asking their tech to try the throttle body cleaner.

 

golucky66. If it is not an inconvenience, I'd would like to forward a snapshot of that procedure to the tech. Thank you.

These are the words I didn't have to explain what I thought about the problem with my 2007 Outback automatic, 4cylinder engine

The check engine light comes on and the cruise control quits.  Computer reader says code 171, running lean. After it's reset, it does it again when I'm on the road.  The interval had become short, and I'm worried about TO WHAT the sensor is responding. Recently replaced an O2 sensor, but a week later it happened again, just driving around town.  It's hot, but that's no excuse, is it? 

I'd sure like for this to be fixed, and hope you people have some ideas.

Why doesn't it correct the air/fuel mixture? Then maybe the sensor would stop complaining.

a large vacuum leak may let in enough unmetered air that the ECU has exhausted it's ability to compensate. Or a fueling problem is present - failing pump, etc.

 

you may need someone to log live data and do other diagnostic procedures to find the problem.

 

I'ts common fro cruise control and traction control and other similar systems to be deactivated on modern cars when a DTC is set.

The electronic throttle body controls idle speed by opening or closing the throttle plate slightly. If the throttle body is coked up with carbon it will have a harder time re-adjusting.

It sounds like the plate may be getting stuck, which would indicate the throttle bore is coked and needs to be cleaned. Throttle body cleaner is the best to use.

There should be some videos on YouTube or elsewhere on how to properly clean the throttle body.

Hey the one thing is you have no way to control the throttle from under the hood with the ETC right? That's the one thing I hated.

These are the words I didn't have to explain what I thought about the problem with my 2007 Outback automatic, 4cylinder engine

The check engine light comes on and the cruise control quits. Computer reader says code 171, running lean. After it's reset, it does it again when I'm on the road. The interval had become short, and I'm worried about TO WHAT the sensor is responding. Recently replaced an O2 sensor, but a week later it happened again, just driving around town. It's hot, but that's no excuse, is it?

I'd sure like for this to be fixed, and hope you people have some ideas.

Why doesn't it correct the air/fuel mixture? Then maybe the sensor would stop complaining.

I have had this stump me in the past. Replaced the front 02 sensor, checked for intake leaks, tried a few other things. Come to find out, it was a bad mass air flow meter causing the ECU to think it's running lean so it tries to add as much fuel as possible. Cleaning might help but expect replacing it for a proper fix.

Heres a link of my sheet on how to teach the ECM to relearn the idle.

 

​The link isn't working, I'll post it when I get home.

Edited September 12, 20169 yr by golucky66

Ok, here is a link that I hope works.

 

To get the idle to learn properly, make sure you follow each step carefully. Even the ones that seem pointless/common sense. 

 

https://1drv.ms/i/s!AmalRfJxLmkYhahKi_RtsvSBaGF44g

Edited September 13, 20169 yr by golucky66

golucky66,

 

Thank you for effort and time to link this procedure. I printed a copy to take to the auto shop this morning. Going to keep my fingers crossed...

Golucky66.

 

I don't exactly know how, but it seems to have helped. I didn't even complete the procedure correctly because my fans wouldn't turn on(guess I should not have parked under the shade tree). After 30 minutes of 'no fan' idle, i turned on the A/C to force the fans on. I let the car idle this way for about a minute before turning the engine off.

 

At first i thought I had messed things up because the A/C didn't blow cold air right away. After about 3 minutes it kicked in, and the idle is back to the way it used to be (So far). After a half-dozen stop lights, the idle kicks 'up' when the A/C cycles on, not 'down'. The RPM needle used to take a downward dip towards a stall, but now it goes north when the compressor turns on. The idle speed still reaches down to 500rpm, but now the load of A/C doesn't seem to drag anything down.

 

THANK YOU THANK YOU for your help and generosity.

Glad it helped. If you run into that low idle issue again, try the procedure again but let the fans come on (don't force it).

 

But, seeing as it helped! Glad your car is back to normal! No body likes a broken car that nobody knows how to fix.

My next question: How to eliminate that tacky residue that forms on the dashboard from hydro-carbon sweating over 8 years....(rhetorical question).

Tuff-stuff upholstery cleaner.

Tuff-stuff upholstery cleaner.

got a recommendation?