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Low idle and stall

25515 Views 9 Replies 4 Participants Last post by  FDdragon
Hello, so I just bought a used 2004 STI off a friend of a friend. I was out of town for a while and when I get home I start taking a look at my car to do any work that needs to be done.
well, first thing I notice is that the car was idling low around 400-500. (wasn't doing this the day my wife bought the car). the battery was dead for a couple of weeks so I replaced it and I was thinking that maybe the car was relearning. after warmup I press the gas and there is a little delay...the rpms drop to about 100 or less and the car stalled a few times. (if I press the gas enough the car wont die, obviously, but the rpms drop as soon as I let go of the gas. then it was struggling to get back to a stable idle) I added new gas and cleaned the MAF sensor thinking maybe things were dirty. So it felt like the car was getting stable after a full day of cleaning, test driving, etc. Then I changed the spark plugs last night and the idle went back to dropping to around 100-200 after I touch the gas pedal. The RPMs drop that low when the car is coming to a stop as well. I even stalled at a stop light and I tried to slowly move forward (the whole touch the gas pedal and the RPMs drop thing)

Now I am thinking that since I removed the battery it has something to do with it (it was twice that the car was reset and both times same issues)....or maybe my plugs weren't gapped right.....or maybe there is something else wrong with it that I need to look into. I also noticed that when im driving the car, it doesn't respond right away and feels like the delay I was talking about earlier.....(when im in park and I step on gas and the car drops rpms for a second or two) but it feels good in between gears when im moving fairly quick.

I am also reading -20 vacuum on my boost guage at idle.

Mods are:
COBB Accessport with custom tune ( I don't know how to use it yet but if I have to make changes then ill get some help )

like I said, my wife and friend didn't mention anything about it when the car was bought so I am assuming it was fine. (my friend would have told me something) any help is appreciated! Thanks guys.
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Relax. You have a vacuum leak. Get some zip-ties and gently tighten up all of your vacuum lines (post MAF).

...so, double check all ancillary hoses that connect to your Intake Manifold, BPV, IC, etc...and tighten them all down and zip tie, them.

there are loads of suggestions on nabisco, awd pirates, iwsti, etc. ...but the consensus is that low idle is typically caused by a vacuum leak.

good luck and happy leak hunting.
Ok, here are my guesses from previous issues with other cars. Vacuum leak could be the issue, slight leak could cause small issues with idle and stalling. Large leak would cause almost instant stall out. Could also be a damaged MAF sensor, they are incredibly sensitive and it could be more than dirty. A mechanic friend said the ones in my MINI ran at like .5 volt so just a little off will break them. It could also be a timing issue causing the engine to lose time and stall itself out of have to rev up quick to keep itself from stalling. My MINI would sit at a light and the rpms would drop to about 250 and almost stall before catching back up and it was a bad timing chain tensioner(3 times). If your battery died then the new one reset the computer and the computer is looking for new sensors with proper values, your old sensors are probably putting out bad numbers and they are disagreeing with each other. I would do a leak down test and a smoke test to make sure you don't have any leaks and go from there. I'm not a mechanic so I could be completely incorrect but I have had to deal with all those things that cause similar issues.
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Put this into a Google search and click on your very first link. Read the whole post and make a good educated guess. The vacuum leaks are the most common and EASIEST to fix. Good luck!

low idle 2005 STI vacuum leak
If its a leak you should be throwing a code unless the sensor was bypassed by the tuner. Not saying its not a leak. Either way try to flash the ecu with the map and let it relearn its idle and see if it helps and check for leaks at the charcoal canister in the rear of the car. You should see a big change when you re-flash the computer.
cool, ill check for any leaks. ill keep you guys posted. thanks
"...If its a leak you should be throwing a code unless the sensor was bypassed by the tuner...."

Not necessarily true. My '05 STI recently started behaving similarly, with rough idle, low RPM's and even stalled coming up to a stop light (clutch pedal pressed in).

Not a single code was thrown by my AP or even OpenECU. I ended up using zip ties on all lines to and from the intake manifold, intercooler, bpv, etc. Although most issues seemed to have disappeared afterwards, I still noticed a slight roughness. I ended up replacing the BPV gasket and now my car is as smooth as butter.

Vacuum Leak. Call it a day and enjoy your new beastie.
so I checked what I could with the tools I have and couldn't find anything. im taking it in to my mechanic tomorrow.
So I got the car back and I didn't have any leaks. Im guessing its because it was sitting for a while and the car needed some driving around for it come together....if that makes sense. ill keep an eye on it. thanks guys.

now Im hearing a rattling noise when I drive...but when I push the clutch the noise goes away. Gotta check that out now.
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