High idle drops when stepping on the brake...
 

Well I have been gone for quite a while and my car hasn't seen a whole lot of use in the past 2 years but just recently it has started to do this:

When I step on the gas, then let off, the throttle stays wherever it was when I let off... until I hit the brake, then it drops down to normal, but I have noticed that if I'm on the highway it won't drop down like it should when the TCC unlocks. Now I thought, and you may think that this is exactly what I just mentioned, but this also happens in Park... sitting in front of the house!!! Step on the gas a little and it idles at whatever... until I hit the brake...

Any ideas? Thanks in advance...

Kevin

Kevin,

Glad to see you're back - hopefully taking it easy.

You've got a really good one there. The first, most obvious thought is a sticking throttle or throttle cable, but tapping the brake shouldn't necessarily "reset" that. The only thing using the brake may do is lower the vacuum and place a bit more electrical load on the system from the lamps, niether of which would normally affect the idle.

Like always, I'll sound like a broken record, but a clean throttle body and IAC usually takes care of most idle speed problems. If you can't get in there and single-handedly remove the bellows and expose the throttle body for a little cleaning spray, maybe you could find a certified mechanic to help out. Even a junior mechanic in need of a little training might be beneficial - if you knew someone who was any good at training. ;)

You might want to check out the PCV valve while you're under the hood, just in case it's hanging a bit and you do have some odd sort of vacuum issue. It's also possible the brake booster is hanging up somehow and is drawing more vacuum than usual. But that still doesn't explain the original idle problem on a system that uses a MAF instead of a MAP.

What, if anything, have you done to it so far? Maybe it's just lonely.

My throttle's been sticking for years! I've removed the cable, cleaned it and rerun it, but that didn't help. I've changed the spring on the throttle--and nothing. The throttle body and throttle assembly were cleaned, too, and that didn't help. I was told by a mechanic that it's a problem caused by the cruise control system. But I've never bothered to get into that. A light tap on the pedal whenever shifting or slowing down frees it so it'll idle down.

But try this "fix," it didn't solve the real problem, but it definitely helped A LOT--I attached a spring from the cable bracket(on the plenum) to the throttle assembly. That pulls the throttle back when you let off the gas(most of the time). It's not strong enough to add resistance to the pedal, but it's just strong enough to return it when no pressure is on it.

Sounds like the throttle shaft itself is binding from being worn out. When the engine's off and there's no vacuum it works smooth as glass. But when the engine's running there's quite a lot of force being applied to it by the vacuum behind the throttle plates- suddenly it can bind and not want to return to a true curb idle.

I've also noticed that the TCC doesn't drop out of lockup when I'm in closed loop like it's supposed to... I'm going to pull off the TB this weekend and clean that and the IAC and see what happens, I'm thinking more & more that it's looking like a PROM problem... but I hope not.

TCC not dropping out? Is that when you tap the brake, or release the throttle to the closed position, or drop below 41 MPH?

Possibly, the TPS voltage is set a bit too high?

Say I'm doing 35, when I tap the brake, (and I have to tap it quite hard) it will drop down to normal for about a second then pick up where it would be if the TCC was still locked up... then when I brake to a stop the RPMs drop as my speed drops... I'll clean the TB & IAC tomorrow and let you know the results.. Thanks Vader

Kevin

Do you have an air cond solenoid.Dont know if this will help.Since my camaro is carbed,but my a/c solenoid was holding the throttle open whenever i was going around 35mph+ or in park if a revved the eng.dont know how it happened,but mine was out too far.the solenoid is adjustable.the plunger has a hexagon head that you can turn Clockwise/C.C.W to set at the proper level.This would happen regardless if A/C was on or not...

Where is the solenoid??

Kevin

On my car its by the throttle lever(Dr side).Just below the kick down/Cr.control/accelrator cables.Mine has a small bracket welded to it.That bolts to the carb body.Uses 1 wire on the back & has a rubber boot on the front where the solenoid plunger is.This plunger pushes against the throttle when its activated & holds the butterflies open to keep eng from stalling.By turning the plunger head you can raise/lower the RPMs.

Thanks 84... I'll check it in a little bit...

Kevin

OK I've removed the TB and IAC and cleaned them all really well... a little dirty, but not too bad at all. I lubed up the bushings really well and reinstalled the TB... I started the car and... Well, same thing is going on. This time I checked a little further... I moved the throttle with my hand and when it returned it stopped about 1/8" short of the stop screw. I hit the brake and I could see the throttle linkage jump back that final 1/8" to the stop screw... So then I removed the bottom cable, (the throttle cable) and actuated it by hand again, and this time it closed all the way. Just to make sure it wasn't an ECM problem, I did it some more with the engine off.... Any ideas on how to approach this throttle cable??

Kevin

Maybe your throttle cable is binding.Did you try pulling the cable by hand when removed from the linkage to see if its sticking in the cable housing.Also did you check the return spring/s on the throttle body.They might also be sticking/binding.Does sound like a mechanical rather than electrical.

I don't know... It will go back if I push it back or if I apply the brake, but if I just let off the throttle, then no. I know exactly how to make it go back to the stop screw all the way, but that's not fixing the problem... Vader, I tried some anti-sieze and that didn't do the trick... :D

Seriously... I shot some Kroil down the cable sheath and that didn't help, but I can't think of what is keeping it from backing all the way in.

Kevin

I think I know what your problem might be. This is reaching a bit but the engine vacuum can pull the throtte assy in the tb back jsut far enough to cause the butterflies to bind in the bores. The vacuum (err, atmospheric pressure on the other side) puts a load on the throttle shaft and it can cause it to bind. I had a similar problem. The tb would return jsut fine with the engine off:cable on, engine on:cable off, but not with the cable on and engine on. The cable provided that little bit of extra resistance that stoped it from fully returning. Probably the application of the brake lowers the vacuum jsut enough to allow it to unstick and come back. Try to move the end of the throttle shaft where the cables hook up forward and back with the throttle open a bit and see how much play there is.