Intermitant high idle

[puddy]

I sometimes find that after I start the car are drive it a short period maybe a mile or two to a gas station. Then when I restart it after filling the tank, the idle rpm will go up to around 2200-2500 rpm.

I thought the AAR was working fine. I had some issues with it last year and some wires crossed but seems to functioning now. I have a freshly rebuilt BL WUR. What should I look for to try and fix this?

[RarlyL8]

This is not uncommon. As the air metering devices age they become sluggish and sticky. Most often it is the AAR that stays open during a short drive. To test this you can disable the unit or use a pair of large plyers and pinch the hose when it is acting up. If disabling the unit you have to plug both inlet and outlet hose then use your right foot to increase the idle RPM until the engine warms. If the AAR passes then move to the AAV.

[puddy]

So if its idling high and I pinch the hose, the idle should drop back down to around normal? If it stays at same idle then there's a problem with the AAR?

[Jeff NJ]

Originally Posted by puddy

So if its idling high and I pinch the hose, the idle should drop back down to around normal? If it stays at same idle then there's a problem with the AAR?

If you pinch the hose and the idle drops, the AAR is staying open too long and causing the high idle. Pinching the hose replicates the AAR closing. Either removing the AAR as Brian described, or fixing the slow response of the AAR is the solution. If it stays at a high idle, I don't think the AAR is the issue.

This has happened to me on occasion, but while removing it would stop it from happening, it would then cause the motor to not idle at all when stone cold, and you would have to hold the throttle open with your foot until it will idle on its own. To me, this is a much bigger PITA, since it would happen every time you start it, vs a high idle on occasion when specific criteria are met. So you are effectively replacing an occasional issue that doesn't need any intervention to keep the car running with a daily issue that requires you to manually keep the car running. Not worth it to me.

I took my AAR apart and its very simple inside, maybe clean and lube the mechanism before deciding to remove or live with it if you find the AAR is the issue?

[puddy]

My car did not have the AAR installed when I bought it so i'm familiar with it not wanting to idle when cold, not convenient! So I definitely do not want to remove the AAR. I'll take it apart and make sure its good and clean and see if it happens again.

[Alan L]

My SC did the same thing - after filling at gas station. 1500-200rpm. Once I get driving, all back to normal. It eventually went away all on its own. Hopefully the AAR is your fix.
Alan

[RarlyL8]

I did not say remove it, I said disable it. If you get a hang nail you don't cut your finger off. Once the problem is diagnosed you can decide how to proceed. If it is the AAR you can take it apart and fix or replace it with a good one.

[JFairman]

You can also install a BMW 320i heater valve in the hose between the AAR and throttle body. Fits perfect.
Then hook up a generic choke cable to it to operate it during those times the CIS idle speed is way too high.
Run the cable back over the tin and forward towards the cabin. There's a number of places to install that depending on length of the cable.

[mark houghton]

Kids, feel free to correct me if I'm wrong, but this symptom is not uncommon on a correctly operating AAR. After initial warm up, your idle settles down to normal (let's say 950) but then after a few minutes of being shut down to go into the store for whatever you need, when fired back up she idles high. Then, down the road for a couple minutes and she idles back to normal.

The AAR needs to be hot to close the window that lets air into the intake to raise the idle. Engine heat alone is not enough to close her up; that's why there's an internal heating element to speed up the heating process and close the regulator. When you shut off the engine, the AAR will slowly open up as it cools off and result in a high(er) idle when you start her back up. It is for sure annoying, but that's what it does. You should only see a few hundred rpms increase....if more than that, then perhaps the innards are in need of cleaning. It's a damn archaic system to assure high idle at initial cold startup, but we get the added bonus of the gift that keeps on giving.

[JFairman]

It probably helped keep the smogged out motors running with the retarded timing, catalytic converter, etc.. to pass emissions.

[puddy]

Thanks Fellas, now that I know its not an uncommon thing i'm not as concerned about it. But i'll give it a good cleaning and see if it helps.

[puddy]

Still having issues. The car, when cold, will start at approx 2400 rpm then slowly creep up close to 3000 rpm. Which is pretty obnoxious with a straight pipe in a quiet neighbourhood while in trying to diagnose problems. When I pinch the hose between the AAR and the intercooler it goes to normal idle simulating the door closing inside the AAR. So perhaps my AAR is faulty. I did spray some cleaner into it last fall and blew it out with compressed air. Perhaps I also caused more harm than good? I recently replaced the 2 pin plug connector to the AAR as the previous wires were badly worn almost to the point of breaking. I hoped that was part of my problem but it didn't seem to make a difference.

I also have a voltage issue that appeared last fall. The low voltage gauge lights come on when car starts. I've tested the alternator, regulator & battery they're all fine. God I wish I had the time to learn more about electronics :/

[puddy]

I've been doing some googling and seeing some posts about faulty AAV and what I've seen in other pics matches what I have in my pic below. When I look up the part # it comes up as a vacuum limiter switch or valve. Can I test this by unplugging hoses etc?

[Rawknees'Turbo]

Yep, PuddyPuller, that is the vacuum limiter (aka - the deceleration valve) - it is a strange device that causes RPMs to hang up when the throttle is suddenly closed - running without it results in much better throttle response, and more exhaust crackling during closed throttle.

To test if it is leaking around or through its diaphragm, unhook and plug the big hoses .

[puddy]

Ok thanks Rawknees! I'll test'er out but would prefer to eliminate completely if it's not going to affect things greatly.

[Rawknees'Turbo]

I eliminated mine many years ago - much prefer the normal throttle response and don't mind the additional crackling. If you remove it, you need to cap the throttle body NIPPLE, too, buttofcourse.

[puddy]

Hey Rawknees, Last night I pulled the small hose off the unit and idle speed increased, then when I plugged the small hole in the hose the idle speed went back to what it was before I pulled the hose off the unit. So Does that mean its working properly?

[Rawknees'Turbo]

^^^

Pud, nope. The idle increased when you did that because the small hose is attached to a vac nipple that is below the throttle plate, and by removing the hose and running the engine with it open, unmetered air entered the primary intake manifold (acted like a second, throttle air bypass/idle screw).


When that hose is attached, and if the vac limiter is working correctly, during closed throttle, the diaphragm in the vac limiter is pulled back in such a way that unmetered air is allowed to pass into the primary intake manifold (by way of the two bigger hoses), after the throttle plate - this is what causes the engine rpm to drop so slowly during closed throttle. Usually when they go bad (tear in diagram, corrosion inside, etc.), air leaks past at all times and you end up with a high idle that you can't adjust - usually worse when engine is hot.

[puddy]

So I removed the deceleration valve today and plugged all the open ports. After doing so nothing seemed to charge on the start up and warm up process. The idle stayed at around 2200 rpm until the engine was warm and didn't change. So I guess the AAR is remaining open. Either it's faulty or when I resoldered a new plug on it perhaps I got the wires back wards but I highly doubt it. So I've just eliminated the AAR for now. I don't really drive the car in colder months anyways so I don't think it'll affect me to much. The AAR wasn't installed when I bought the car. Maybe I'll bump the idle up a little to around 1100 rpm.

I like that it's simpler in the engine compartment. When I get my RU5163 K&N next week I'll ditch my big ugly intake box too.

[flightlead404]

Originally Posted by Rawknees'Turbo

...that is the vacuum limiter (aka - the deceleration valve) - it is a strange device that causes RPMs to hang up when the throttle is suddenly closed - running without it results in much better throttle response, and more exhaust crackling during closed throttle.

Can you elaborate on "better throttle response"?

AFAICT the vacuum limiter has absolutely no effect on increasing RPM, that is when you put the boot in it. It only delays the drop in RPM when you take your foot out.

So how is that "better"? Please define best.

Doesn't preventing a sudden drop in RPM help keep the turbo spinning during rapid upshifts and therefore reduce turbo lag? Isn't that the "flat footed shifting" feature in modern ECU's?