Low idle

[2007imp]

Hey guys great site,l just picked up a 2005 legacy limited w/140k and it seems to have a problem with really low idle speed. When you come to a stop sometimes the car will die or idle really low,but after driving the car for a while the problem seems to go away. No check engine lights or any stored codes. Any ideas. Thanks

[OB99W]

[...]after driving the car for a while the problem seems to go away. [...]

Welcome to the forum.

 

Please define "a while". Does that mean "when the engine has warmed"?

 

If you watch the temperature gauge as the engine warms, what's the position when the problem goes away? About how many miles and how long does that typically take?

[2007imp]

When i first got the car it wouldn't run with out one foot on the brake and the other on the gas.The stall/low idle problem usually happened within the first 1/2 hour of driving, it also had a miss with the cruise on at like 70mph.Now it seems like it is alot better,some oil on plugs wires,don't know when or if ever changed.Possible arc/grounding issue? Bad gas? I filled with high octane gas and threw in some injector cleaner.

[OB99W]

If you don't know the maintenance history, it would probably be a good idea to replace general "tune up" items. That would include spark plugs (get originals), ignition wires (suggest Subaru only), fuel filter, air filter and PCV valve (clean the tubing as well).

 

If you don't know that the timing belt was changed on schedule, it and probably the idlers, perhaps tensioner, and water pump should be considered. The engine is an interference type, and if the belt breaks there will be internal engine damage.

[Olnick]

+1^. Definitely the place to start. Let us know how it goes and if things improve.

 

Good luck.

[polychromeugand]

The suggestions so far are good tune up ideas. If they don't resolve the problem, the problem could be either the idle switch or the idle air control valve, and it could also be the coolant temperature sensor.

 

When the engine control unit senses that the throttle is closed via the idle switch (part of the throttle position sensor) it uses the idle air control valve to adjust the amount of air entering the engine to set the idle RPMs. The ECU uses the coolant temperature sensor to decide if the engine is cold and if so, it runs the engine fuel-rich to allow it to start and idle despite the poor fuel vaporization. Below 30F it can be very difficult to start the engine if the coolant temperature sensor is defective and at 0F its all but impossible.

 

A defective coolant temperature sensor should cause a stored code but may not always light the MIL (malfunction indicator lamp or check engine light). Its a fairly common part failure and difficulty starting a cold engine and poor idle when cold are typical symptoms. BTW - The temperature gauge on the instrument cluster has an independent sensor, that never seems to break. Unfortunately the ECU can't read it. The part is usually somewhere between $20 and $30 and its reasonably simple to change.

 

The idle switch is the top two contacts on the four pole connector on the opposite side of the throttle body from the throttle cable. To test it disconnect the wiring harness, and using an ohmmeter verify that the contacts open and close whe the throtle is moved. You shouldn't have to test this manually... If this were defective, the MIL (malfunction indicator lamp aka check engine light) should be lit and a code should be set.

 

The idle air control valve gets gunked up pretty easily, but its also fairly easy to clean. All you need is a can of throttle body cleaner. The idle air control valve gets air through a small hose that taps into the large air intake hose that runs from the air filter to the throttle body. With the engine off, remove the air intake hose from the throttle body open the throttle and spray all that clean and reassemble. Next, detach the small hose that leads to the IAC (right next to the throttle body) from the air intake hose. Spray throttle body cleaner down the hose until it just about drowns and reconnect it. Start the engine. You may have to open the throttle a little to get it started, and it may die a couple of times as it pulls the cleaning fluid through. I find it takes two to three cycles of drowning it in cleaner and starting the engine to get the IAC clean when its sludgy. You'll know its wokring correctly when the engine idles correctly again...

[2007imp]

Hey thanks for all the good info.This morning when i started the car the idle was higher than it had been on previous starts aroung 1000-1500 rpm. No stalls or hesitation.Later this afternoon i tried again and it had the same low idle issue drove for maybe 5 min and cleared up. Put in new airfilter and new plugs,old ones didnt look bad at all,some oil in plug wells though. Next will be new plug wires.

[2007imp]

Cleaned the throttle body plate,no more issues,thanks for the help!

[ferret]

If your problem returns, here's a link to a problem I just fixed. Only applies if you have the newer electronic EGR valve as the thread shows. Something else to consider if your problem returns.

 

http://www.subaruforester.org/vbulletin/f88/low-idle-stuttering-erratic-jerking-various-rpms-76450/index3.html#post887065

[porcupine73]

Glad it's working. Does that have drive by wire throttle? If so those idle air control valve details don't apply; since the ECU is controlling the throttle in the drive by wire it simply uses that to control idle air as well.

 

Also you might want to see if there are any ECU reflashes available. It seems right around the time drive by wire came out the number of ECU reflashes began to increase quite a bit.

[2007imp]

Thanks for the replies,this car does have the drive by wire throttle control.Has anybody had their car reflashed/updated at the dealership, if so any idea of cost. I am also looking to install new struts front and rear,I was thinking of putting in kybs,anything special i need to know,direct fit or need some sort of mount kit.I know most are custom fit for each vehicle. Thanks