help low idle rpm, no code thrown

First I'd like to say hi to everyone here. Not really a car forum guy in the past so kind of nervous for a first post.

My 03 galant has been having this symptom for a while. The idle rpm is too low (<500) and sometimes can stall when braking. By far I only tried a few things that easy to do. Cleaned MAF sensor, throttle body and IAC valve. Seems only helped for the first time but not now. Never see the engine light one making it harder to diagnose. When it was colder i actually noticed the idle rpm go up and down periodically. When the rpm drops to ~500 it will bounce back to ~2000 as if I was hitting the gas (while actually i didn't) .

Was thinking about vacuum leak, haven't tried testing though. Just wondering due to the age of this vehicle, how likely that this is caused by old spark plug, fuel filter, fuel injector, etc. Or could it be something more serious. Any input is appreciated!

You didn't say if it's the 2.4L or 3.0L engine. Possible vacuum leaks is good thinking but most likely,(typically), it's the IAC valve. They're expensive... like $190 - $225 aftermarket,(Rock Auto). First remove the IAC valve and throttle position sensor and wipe them off dry. Don't get them wet! Then I'd clean the MAF sensor and Throttle Body with the appropriate cleaners. If you've disconnected the battery Mitsubishi's can take a long time to relearn. Like 15 minutes of idle time and up to 100 miles driving time. HTH

If the IAC is anything like the IAC's found in DSM's and 3000GT/Stealths they can be tested if they are bad or not.

All you need is a multimeter.

Test the continuity across the pins. There will be six pins, 3 in each row.

123
456

Test continuity across pins 1 and 2, pins 2 and 3, pins 4 and 5, and pins 5 and 6.

On the 3000GT's/Stealths continuity should be around 28-33 ohms if its an old style beige ISC. It could be different on these, but main thing is consistency. If you get no continuity between any two pins then the ISC is shot.

Thanks @DeltaP @Stealthee for the heads up.

My galant is 2.4L. I cleaned MAF sensor with MAF sensor cleaner, cleaned throttle body and IAC valve with throttle body cleaner. The IAC valve after taken off seems pretty shiny since I did cleaning not long ago (but there is a little bit rust like color right on top, those were cleaned with the cleaner). I haven't done anything to throttle position sensor yet (and i never did that before). I'll try that as it would be easy.

Also I will try test the IAC valve as Stealthee suggested and see how the reading look like.

But one thing I notice today while playing with my car is that, I noticed that there is hissing sound come from area near the exhaust, should be somewhere underneath the exhaust cover. Another thing is that after I crank her up, the rpm is really low like 300rpm. Then I used my hand to open the throttle body, which brings up the rpm. As i let it down the car stalled, and right after stalling I heard a very obvious hissing (or like breathing) sound right around the throttle body. Not sure if those hissing sound indicate anything but definitely does not sound right.:rolleyes:

The hissing near the TB could be a vacuum leak, but a vac leak would likely make the idle high and erratic.

As for the IAC being clean, that doesn't matter if the IAC has failed internally. Ironically an IAC issue is what brought me to join a forum originally back on 03, and then eventually find this forum in 04.

Check your TB gasket for a leak. You can buy a new paper one for $2 at autozone, and some gasket sealant does wonders.