Idle Hanging High (4+3)
Thread Starter
Racer
Joined: Jul 2001
Posts: 253
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Idle Hanging High (4+3)
Not sure what's going on here with my '86 4+3 coupe. It happens hot or cold or not at all.... :crazy:
Ok, I'm driving down the street and go to up-shift, putting in the clutch first :rolleyes: and as I watch the tach. It jumps about 300-500 rpm. I tried this again by purposely letting my foot off the gas pedal for a second or two and then push in the clutch and still watch it jump. :boxing
I might be cruising in top gear (7th? :D ) at 30 mph and take my foot off the gas and not slow down for blocks of distance. Finally, I'll push in the clutch and the rpms jump a few hundred.
Sometimes I put it in neutral as I coast to a stop and it's idling at 1300 rpm until I stop and then it comes down to 700 rpm.
You might say that this is sticking cable, but I can't duplicate this problem when sitting still. I can rev a little or to high heaven sitting still and the rpms drop like a rock to regular idle. So it's only when I'm moving. And again, not all the time.
I'm thinking Vehicle Speed Sensor? ECM?? What else could it be?? :confused:
TIA :cheers:
Ok, I'm driving down the street and go to up-shift, putting in the clutch first :rolleyes: and as I watch the tach. It jumps about 300-500 rpm. I tried this again by purposely letting my foot off the gas pedal for a second or two and then push in the clutch and still watch it jump. :boxing
I might be cruising in top gear (7th? :D ) at 30 mph and take my foot off the gas and not slow down for blocks of distance. Finally, I'll push in the clutch and the rpms jump a few hundred.
Sometimes I put it in neutral as I coast to a stop and it's idling at 1300 rpm until I stop and then it comes down to 700 rpm.
You might say that this is sticking cable, but I can't duplicate this problem when sitting still. I can rev a little or to high heaven sitting still and the rpms drop like a rock to regular idle. So it's only when I'm moving. And again, not all the time.
I'm thinking Vehicle Speed Sensor? ECM?? What else could it be?? :confused:
TIA :cheers:
Instructor
Joined: Sep 2002
Posts: 163
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From: Schererville IN
Re: Idle Hanging High (86VetteBill)
One other thing - check the shaft & housing for wear on the accelerator side. If you have a lot of slop and you try cleaning the TBI it will take out some of the build up and make it worse.
Thread Starter
Racer
Joined: Jul 2001
Posts: 253
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Re: Idle Hanging High (Grouchy67)
Wouldn't a worn TB shaft affect operation all of the time? This comes and goes... :smash:
This morning is high idled during warm up and then it was fine the rest of the 35 mile drive. :blueangel:
This morning is high idled during warm up and then it was fine the rest of the 35 mile drive. :blueangel:
Intermediate
Joined: Aug 2002
Posts: 25
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From: Wichita KS
Re: Idle Hanging High (86VetteBill)
A worn TB shaft (ie the hole is larger than the shaft) can allow air to get sucked in when its not supposed to. Depending on how much larger the hole is than the shaft will determine how often it happens. Sometimes it will seal properly sometimes it will not. (depending on where the location of the shaft in the oversized hole)
-Dave
-Dave
Thread Starter
Racer
Joined: Jul 2001
Posts: 253
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Re: Idle Hanging High (dj247046)
Thanks Dave - I'll check it out! I've sprayed carb cleaner around it before and got nothing.
Here's the weird part that makes me think it's "electronics": when I drive in low rpms - let's say 1000 rpm at 32 mph (I think) and give it a good bit of throttle, but not enough to make it shift out of OD (remember, 4+3), and then let my foot off the gas and push in the clutch, the R's go high - about 400-500 higher than it was and then it starts to come down to about 1200-1300 and stays there. As I come to a complete stop - no matter how long it takes to come to that stop - about 2 seconds later the R's come down to 700.
I'm thinking that there was no change in throttle position while I was coming to a stop, and it's the stopping of the vehicle that "initiates" the end of the high idle.
Confusing? or do you still think TB shaft?
Here's the weird part that makes me think it's "electronics": when I drive in low rpms - let's say 1000 rpm at 32 mph (I think) and give it a good bit of throttle, but not enough to make it shift out of OD (remember, 4+3), and then let my foot off the gas and push in the clutch, the R's go high - about 400-500 higher than it was and then it starts to come down to about 1200-1300 and stays there. As I come to a complete stop - no matter how long it takes to come to that stop - about 2 seconds later the R's come down to 700.
I'm thinking that there was no change in throttle position while I was coming to a stop, and it's the stopping of the vehicle that "initiates" the end of the high idle.
Confusing? or do you still think TB shaft?
Intermediate
Joined: Aug 2002
Posts: 25
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From: Wichita KS
Re: Idle Hanging High (86VetteBill)
Well it is odd that it adventualy goes back down. From my experience a worn TB shaft will want to stay at a certain RPM when it doesn't seal properly. If in a low gear the car will actually "sail" by itself without having to press the gas pedal. It generally doesn't correct itself with out tapping the gas pedal. Have you looked at your IAC and TPS to make sure they are OK? If not, there is a nice article in the "Tech Tips" section about reseting the IAC and checking your TPS for the proper voltage. Allot of "mechanics" will adjust idle screw without checking the IAC and TPS (this will really mess up your idle). You can't treat a Corvette like a Nissan.
-Dave
-Dave
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Thread Starter
Racer
Joined: Jul 2001
Posts: 253
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Re: Idle Hanging High (dj247046)
I've recently cleaned the IAC and adjusted the TPS (400 RPM w/o the IAC - after pintel is extended, then .54V and 4.65 WOT, with smooth increases in Volts while moving the throttle plates from idle to WOT).
Like you said, a worn TB shaft will stick and stay stuck until it gets a little tap fro my foot. This seems to be electronic because of its consistency in what it does WHEN it does it - it doesn't always hang high idle.
Again, if it's doing this, and I come to a stop - no matter how long it takes to come to a stop, it's a second or two until the idle comes down to a normal 700 Rs from about 1200.
That's why I was thinking of "what monitors my vehicle speed that could affect the IAC??" I'm still thinking the Vehicle Speed sensor, but I haven't read up on it yet.
Or I need a new IAC.....
Like you said, a worn TB shaft will stick and stay stuck until it gets a little tap fro my foot. This seems to be electronic because of its consistency in what it does WHEN it does it - it doesn't always hang high idle.
Again, if it's doing this, and I come to a stop - no matter how long it takes to come to a stop, it's a second or two until the idle comes down to a normal 700 Rs from about 1200.
That's why I was thinking of "what monitors my vehicle speed that could affect the IAC??" I'm still thinking the Vehicle Speed sensor, but I haven't read up on it yet.
Or I need a new IAC.....
Intermediate
Joined: Aug 2002
Posts: 25
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From: Wichita KS
Re: Idle Hanging High (86VetteBill)
Another thing that occurred to me is that I read somewhere that there are actually two idle speeds. One for moving and one for being completely stopped. Mine will idle at around 800 RPM until I completely stop, then it goes down to 600 RPM. I wish I could remember where I read this, it could be the shop manual. Its just one of those things I filled away in the back of my mind a long time ago. I would think that this is somehow controlled by the computer, but I don't have any idea how that thing works. I work on engines not computers.:)
-Dave
-Dave
Melting Slicks
Joined: Apr 2002
Posts: 3,305
Likes: 2
From: Havelock NC
Re: Idle Hanging High (dj247046)
Mine does the same thing as of recently. I had always thought that it was just trying to warm the car up, but now it does it when the car is plenty warm. Makes me so mad, I have to tap the gas to get it to come back down. Sometimes it's so bad that I can let the clutch out slowly and the car will actualy take off and I'll be able shift into second without hitting the gas, man it's makes me mad.
Drifting
Joined: Dec 2001
Posts: 1,313
Likes: 1
From: providence ut
Re: Idle Hanging High (tarheelracer)
yes......I was going the same direction your headed in trouble shooting my irractic high idle...then I oiled the trottle body shaft which had free play at the drivers side. unfortunenatly I did some other ajustments at the same time. but If it comes back I'll know its the shaft because the wd 40 won't last it will start sticking again. The one thing diffrent with my problem and yours is the idle would not come down by hitting the gas petal on mine. anyway try it an see.
Melting Slicks
Joined: Jun 2002
Posts: 2,168
Likes: 16
From: Phoenix AZ
Re: Idle Hanging High (86VetteBill)
My '86 did exactly the same thing. It WAS the worn TB shaft. I repaced it with a 52MM exchange unit from TPIS and now it works fine. Why does the idle not come down until the car stops? Just one of those unsolved mysteries, I guess. The TB closes fine with the motor off or even sitting at an idle, but once the car is moving, something causes it not to close all the way sometimes. Mine would also "hang up" then close suddenly, so I knew it had to be something mechanical. I too had thought maybe it was the ECM or even my forced air induction kit giving the MAF fits, but it was the TB.
