High idle after full throttle?
Thread Starter
Melting Slicks
Joined: Jun 2011
Posts: 2,096
Likes: 15
High idle after full throttle?
A week or so ago I took my 82 out and ran it full throttle and noticed that the idle was very high and couldn't get it to come down. I thought maybe the linkage was bound but it wasn't so then I thought too much air was being sucked through the throttle shaft so I decided to put back on my 2" bored throttle bodies. After install started up and idled great. Took around a couple blocks and perfect ..I thought till tonight. I took her out on the rural highway and opened her up. The first full throttle 1/4 mile was good after let up on the accelerator but after the second it was the same as if I had the original throttle bodies on. I pulled off the side of the highway to check under the hood and now it was idling around 2000 rpm. Pulled the air cleaner and everything looked fine. I pulled on the throttle linkage a few times and it started to drop rpm. After a few more times it was now back to normal. I drove back home not getting into the throttle. So what could be causing this because whatever it is it's the same issue that caused it on the other TBs. The IACs are fairly new AC Delco units and there is no binding on the linkage. I tell you it's always something. 
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
A week or so ago I took my 82 out and ran it full throttle and noticed that the idle was very high and couldn't get it to come down. I thought maybe the linkage was bound but it wasn't so then I thought too much air was being sucked through the throttle shaft so I decided to put back on my 2" bored throttle bodies. After install started up and idled great. Took around a couple blocks and perfect ..I thought till tonight. I took her out on the rural highway and opened her up. The first full throttle 1/4 mile was good after let up on the accelerator but after the second it was the same as if I had the original throttle bodies on. I pulled off the side of the highway to check under the hood and now it was idling around 2000 rpm. Pulled the air cleaner and everything looked fine. I pulled on the throttle linkage a few times and it started to drop rpm. After a few more times it was now back to normal. I drove back home not getting into the throttle. So what could be causing this because whatever it is it's the same issue that caused it on the other TBs. The IACs are fairly new AC Delco units and there is no binding on the linkage. I tell you it's always something.
Any vacuum leak will cause a higher-than-normal idle and that's why I have only used RTV to seal the lid to the manifold. Although I've never had it happen I have heard a bad PCV can allow excessive air into the intake and cause a high idle. As cheap as PCV's are I suggest installing a new one and also replace the short 10mm hose from the PCV to the manifold just to eliminate that possibility. Also the 7mm hose going to the MAP sensor should be replaced with a piece of 1/4" fuel hose that has the internal braid because the original hose is very prone to splitting where it plugs onto the left throttle body.
As long as the throttle is definitely closing and there are no vacuum leaks the only other thing that can cause a high idle is the IAC motors which are controlled by the various ECM inputs. The O2 sensor is one of the inputs and it needs to be replaced every 30,000 miles so if you have never replaced yours now would be a good time to start fresh and log it in your maintenance record book.
Thread Starter
Melting Slicks
Joined: Jun 2011
Posts: 2,096
Likes: 15
Any vacuum leak will cause a higher-than-normal idle and that's why I have only used RTV to seal the lid to the manifold. Although I've never had it happen I have heard a bad PCV can allow excessive air into the intake and cause a high idle. As cheap as PCV's are I suggest installing a new one and also replace the short 10mm hose from the PCV to the manifold just to eliminate that possibility. Also the 7mm hose going to the MAP sensor should be replaced with a piece of 1/4" fuel hose that has the internal braid because the original hose is very prone to splitting where it plugs onto the left throttle body.
As long as the throttle is definitely closing and there are no vacuum leaks the only other thing that can cause a high idle is the IAC motors which are controlled by the various ECM inputs. The O2 sensor is one of the inputs and it needs to be replaced every 30,000 miles so if you have never replaced yours now would be a good time to start fresh and log it in your maintenance record book.
As long as the throttle is definitely closing and there are no vacuum leaks the only other thing that can cause a high idle is the IAC motors which are controlled by the various ECM inputs. The O2 sensor is one of the inputs and it needs to be replaced every 30,000 miles so if you have never replaced yours now would be a good time to start fresh and log it in your maintenance record book.
Thread Starter
Melting Slicks
Joined: Jun 2011
Posts: 2,096
Likes: 15
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
Thanks for your quick reply toobroketoretire. Everything you have mentioned has been done. Also I'm running a Renegade intake. The 02 sensor doesn't have 1000 miles on it and the fact that the idle comes back down to 650 and stays there is the puzzling part. Now I'd like to check the operation of the IACs but not sure how to do so. Or the ECM could be going bad and that doesn't have maybe 1500 miles on it. I'm scratching my head on this one...
So you're saying it'll only idle fast after a full throttle for a while? Like a few seconds or a few minutes? When an engine is run at a high rpm and the throttles are suddenly closed vacuum can spike to as much as 25" Hg so the abnormally high vacuum for a moment MAYBE is collapsing the MAP sensor hose (and that's a big maybe). If you haven't done so already replace the MAP sensor hose with a piece of 1/4" fuel hose because the fuel hose has the internal reinforcement.
To check the function of the IAC motors take a strong flashlight and look down into the 3/4" X 1/2" triangular shaped holes next to the air cleaner studs (in the tops of the throttle bodies) and you can see the tapered tips of the IAC pintle valves opening and closing when the throttle is blipped.
And don't bother checking the distributor mechanical advance springs because the '82's don't have a mechanical advance. Being ECM controlled there isn't any mechanical advance in the '82 HEI's.
Last edited by toobroketoretire; Aug 7, 2015 at 04:08 AM.
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Thread Starter
Melting Slicks
Joined: Jun 2011
Posts: 2,096
Likes: 15
So you're saying it'll only idle fast after a full throttle for a while? Like a few seconds or a few minutes? When an engine is run at a high rpm and the throttles are suddenly closed vacuum can spike to as much as 25" Hg so the abnormally high vacuum for a moment MAYBE is collapsing the MAP sensor hose (and that's a big maybe). If you haven't done so already replace the MAP sensor hose with a piece of 1/4" fuel hose because the fuel hose has the internal reinforcement.
To check the function of the IAC motors take a strong flashlight and look down into the 3/4" X 1/2" triangular shaped holes next to the air cleaner studs (in the tops of the throttle bodies) and you can see the tapered tips of the IAC pintle valves opening and closing when the throttle is blipped.
And don't bother checking the distributor mechanical advance springs because the '82's don't have a mechanical advance. Being ECM controlled there isn't any mechanical advance in the '82 HEI's.
To check the function of the IAC motors take a strong flashlight and look down into the 3/4" X 1/2" triangular shaped holes next to the air cleaner studs (in the tops of the throttle bodies) and you can see the tapered tips of the IAC pintle valves opening and closing when the throttle is blipped.
And don't bother checking the distributor mechanical advance springs because the '82's don't have a mechanical advance. Being ECM controlled there isn't any mechanical advance in the '82 HEI's.
Thread Starter
Melting Slicks
Joined: Jun 2011
Posts: 2,096
Likes: 15
Thread Starter
Melting Slicks
Joined: Jun 2011
Posts: 2,096
Likes: 15
Ok I might be on to something. I think it's somewhere in the linkage. I just went out and made the same passes as I did yesterday and from a rolling start it was find, idle came down like it should. But then from a dead stop and dropping the gear selector in 1st ( I made the mod to the detent in the shifter) it stuck running high again. I pulled into a farmer's shed drive and put the car in "P" to take a peak. This time it was revving about 3500 rpm. I lifted up on the throttle cable but nothing. Then I pushed down on the driver's TB throttle shaft where the lineage is hook up to and felt movement and the rpm settled back down. So it's something with that but I've been messing around with it and can't replicate what it was doing...So I'm still unsure what's causing it and have a general area from where it's from.
Race Director
Joined: Jan 2000
Posts: 12,399
Likes: 247
From: San Jose CA
What's the throttle shaft condition like on the throttle bodies? I know you have two pairs, but if one of them is causing the throttle to hang this could happen.
Also, not familiar with TBI/CFI but are there throttle return springs and if so, are they in good condition?
Also, not familiar with TBI/CFI but are there throttle return springs and if so, are they in good condition?
Le Mans Master
Joined: Jul 2006
Posts: 8,117
Likes: 104
From: charlotte north carolina
The butterflies could be sticking in a slightly open position. I rebuilt a qjet once and I had the same issues you are having. I reset the primary butterflies and all was OK.
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
Well yesterday the idle stayed at 2000 rpm for several minutes while I kept stabbing the accelerator to get the idle to drop then I got out to look under the hood. And gradually the rpms started to drop but it wasn't quickly. A few minutes after that it was idling back at 600 or so rpm. Very weird. The IACs still look brand new and do function as you describe. I will give it another GOOD once over and take it for another spin.
On a cold start my '82 will fast idle for as much as 5 minutes depending on how cold the air temperature is. You may have a temperature sensor going bad (the two-terminal sensor that screws into the front of the intake manifold right under the smog pump). The O2 sensor also has to reach operating temperature (closed loop) to slow the idle speed down. As long as your throttles are closed the idle speed is completely dependent on the ECM and the inputs from the several sensors.
Thread Starter
Melting Slicks
Joined: Jun 2011
Posts: 2,096
Likes: 15
I found the problem. I had the wife in the car depressing the accelerator seeing if I could spot something. Well right when I was about to give up and look somewhere else I noticed the driver's throttle shaft moved slightly to the side. I grabbed the shaft and was able to move it from side to side but not much maybe 1/8". Turns out the butterfly screws that attach it to the shaft were loose. And of course can't get to them with the injector tower on. I thought this would be a good time to tear it down just to make sure it was bushed. And it was so back together, added medium strength Lock-tite and back out for a test drive. Hurray!!! that was the culprit. Thanks everyone for all your help.
I do have one question though, when the pedal is to the floor I'm able to manually open the butterflies just a smidge more. I haven't found any way to adjust the cable unless the carpet is holding it back. I don't think it would even make a difference but thought I'd ask
I do have one question though, when the pedal is to the floor I'm able to manually open the butterflies just a smidge more. I haven't found any way to adjust the cable unless the carpet is holding it back. I don't think it would even make a difference but thought I'd ask
Race Director
Joined: Jan 2000
Posts: 12,399
Likes: 247
From: San Jose CA
It depends on your definition of smidge - are the throttle blades at 90*? That's where peak flow is. A smidge more wouldn't help then. If they're less than 90*, you're probably leaving the slightest of slight HP on the table. Could be tens of HP though if you're running a higher HP mill.
Thread Starter
Melting Slicks
Joined: Jun 2011
Posts: 2,096
Likes: 15
It depends on your definition of smidge - are the throttle blades at 90*? That's where peak flow is. A smidge more wouldn't help then. If they're less than 90*, you're probably leaving the slightest of slight HP on the table. Could be tens of HP though if you're running a higher HP mill.