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Old 01-04-2018, 09:52 AM   #1
57NAPCO
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Help, High idle problem TBI, how the heck do you get it down.

I have a 1988 Chevrolet V30, 7.4, 400 transmission. I have a high idle problem. On startup its idles way to high and never really drops down. Truck has low original miles on it. Changed...iac, tps, rebuilt TBI and new base gasket, pulled the egr and its not froze up, new fuel pump. Never touched the timing, could timing cause it on a TBI ? Sorry for the stupid question but Im not a technician, just a parts changer...
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Old 01-04-2018, 09:58 AM   #2
b454rat
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Re: Help, High idle problem TBI, how the heck do you get it down.

One of mine idled high, I turned the idle screw out some. But for whatever reason they have a plug that covers up the screw. I think there is a way to “reset” the IAC and TPS but not 100% if that adjusts idle speed or not.
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Old 01-04-2018, 05:30 PM   #3
Tiger Joe
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Re: Help, High idle problem TBI, how the heck do you get it down.

Coolant temp sensor will make it idle high too.

On the front of the intake. Our 92 454 rib would idle way high and that solved it
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Old 01-04-2018, 08:14 PM   #4
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Re: Help, High idle problem TBI, how the heck do you get it down.

Did you replace the gasket under the adapter plate?
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Old 01-04-2018, 08:20 PM   #5
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Re: Help, High idle problem TBI, how the heck do you get it down.

check all the vacuum lines for cracking...high idle is usually a vacuum leak....
also as someone stated check the coolant temp sensor AND the connector....
but usually if either one is bad you will get a rich condition in addition to a high idle...if the pcm sees a cold engine condition it will dump in extra fuel....
also check the IAC for proper operation....this controls the amount of air going in at idle...
and check the TPS voltage at idle...you may need to tweak it a little.

read here...http://www.gmtuners.com/tech/TPS_IAC.htm
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Old 01-05-2018, 04:48 AM   #6
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Re: Help, High idle problem TBI, how the heck do you get it down.

Quote:
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Did you replace the gasket under the adapter plate?
Yes I did, also rebuilt the TBI. Thanks
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Old 01-05-2018, 04:50 AM   #7
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Re: Help, High idle problem TBI, how the heck do you get it down.

Quote:
Originally Posted by A1971Blazer View Post
check all the vacuum lines for cracking...high idle is usually a vacuum leak....
also as someone stated check the coolant temp sensor AND the connector....
but usually if either one is bad you will get a rich condition in addition to a high idle...if the pcm sees a cold engine condition it will dump in extra fuel....
also check the IAC for proper operation....this controls the amount of air going in at idle...
and check the TPS voltage at idle...you may need to tweak it a little.

read here...http://www.gmtuners.com/tech/TPS_IAC.htm
Good stuff, Ill order the coolant sensor, they are cheap.

Good reading on the link, thanks.
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Old 01-05-2018, 03:52 PM   #8
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Re: Help, High idle problem TBI, how the heck do you get it down.

If you rebuilt it, follow the steps for the minimum idle setting.

Also, get a ALDL cable and download a scanning app and you won't need to guess about whether sensors are good.
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Old 01-06-2018, 10:31 AM   #9
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Re: Help, High idle problem TBI, how the heck do you get it down.

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Originally Posted by DieselSJ View Post
If you rebuilt it, follow the steps for the minimum idle setting.

Also, get a ALDL cable and download a scanning app and you won't need to guess about whether sensors are good.
Thanks for the info, I certainly need the ALDL set up but I ditched all my PC stuff and drank the apple coolaide
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Old 01-06-2018, 07:52 PM   #10
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Re: Help, High idle problem TBI, how the heck do you get it down.

Where the heck is the coolant temp sensor anyway ?
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Old 01-06-2018, 10:35 PM   #11
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Re: Help, High idle problem TBI, how the heck do you get it down.

I said on our 92 it’s right by the thermostat front of the intake not sure if 88 is the same know it’s a different manifold
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Old 02-06-2018, 08:14 PM   #12
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Re: Help, High idle problem TBI, how the heck do you get it down.

So I finally had a chance to get back to working on the truck. Ordered the coolant temp sensor and it was the wrong one, mine is a 2 wire and I got a single wire. I ordered the right one and it should be here in a few days. Until it arrives I took the IAC off and started the truck and it shot up to 3000 Rpms or higher. I wanted to plug the big hole ( where IAC is) to see how it affected the idle but the idle was too darn high so I turned it right off. I read where if you plug that hole it should barely idle.

With the IAC passage completely plugged the engine should barely be able to idle, fully warmed up with all accessories off.

If it still idles fast with the IAC passage plugged there is a vacuum leak somewhere. Extra air getting in the engine will speed up the idle. Check all the vacuum hoses.

I used carb cleaner all around the vacuume lines and base gasket before removing the IAC and no change in the...to high of an idle.

Dang Im frustrated
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Old 02-07-2018, 12:19 PM   #13
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Re: Help, High idle problem TBI, how the heck do you get it down.

Quote:
Originally Posted by 57NAPCO View Post

With the IAC passage completely plugged the engine should barely be able to idle, fully warmed up with all accessories off.

If it still idles fast with the IAC passage plugged there is a vacuum leak somewhere. Extra air getting in the engine will speed up the idle. Check all the vacuum hoses.

:
The IAC passage you're trying to plug in that procedure is on the top of the throttle body. The IAC is just a controlled vacuum leak from the top of the throttle body to below the throttle blades. By removing the IAC you created a huge vacuum leak. That's why the rpm shot up so high.

This is part of the IAC reset procedure in the link posted above:
1) With the IAC valve connected, ground the diagnostic (ALDL) terminal (same as you would do to flash trouble codes thru the check engine light).

2) Turn ON the ignition, but do NOT start the engine. Wait at least 30 seconds.

3) With the ignition still on, disconnect the IAC electrical connector.

If you stop right here, you should be able to look into the IAC passage on the top of the throttle body and see the end of the IAC pintle as this procedure drives the IAC fully closed. If you can't see the end of the pintle, you've probably found the source of your high idle as the IAC is not fully closing and cannot control your idle.
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Old 02-07-2018, 09:15 PM   #14
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Re: Help, High idle problem TBI, how the heck do you get it down.

Thank you, I may just head back over to the shop and do this procedure.
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Old 02-08-2018, 03:08 PM   #15
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Re: Help, High idle problem TBI, how the heck do you get it down.

Quote:
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If you stop right here, you should be able to look into the IAC passage on the top of the throttle body and see the end of the IAC pintle as this procedure drives the IAC fully closed. If you can't see the end of the pintle, you've probably found the source of your high idle as the IAC is not fully closing and cannot control your idle.
Did this and the pintle is clearly visible inside the throttle body. So I went ahead turned ignition off and pulled my paper clip grounding device out and for the heck of it started the truck with the IAC still unhooked. The truck started up and idled a lot lower, probally close to where it wants to be. Is this telling me I have a vacuum leak for sure? Or still maybe some other gremlin issue?
Thanks!

Would it hurt anything to drive it with the IAC unplugged ?

Last edited by 57NAPCO; 02-08-2018 at 04:17 PM.
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Old 02-09-2018, 04:00 PM   #16
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Re: Help, High idle problem TBI, how the heck do you get it down.

Did you do the minimum idle setting yet? You are just chasing your tail if you didn't start there.
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Old 02-10-2018, 10:37 AM   #17
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Re: Help, High idle problem TBI, how the heck do you get it down.

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Originally Posted by DieselSJ View Post
Did you do the minimum idle setting yet? You are just chasing your tail if you didn't start there.
I understand but my truck is 100% factory original with only 20,000 original miles on it. All of my reading says do not knock out the idle adjustment cap to ajust the idle.
Thanks for the tip.

...I’m trying figure out what changed to make the idle go too high. Ran and started fine for a few years. Thanks again for all the help y’all!

Last edited by 57NAPCO; 02-10-2018 at 10:43 AM.
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Old 02-10-2018, 12:09 PM   #18
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Re: Help, High idle problem TBI, how the heck do you get it down.

I read through the posts but I may have missed this: Did the truck just all of a sudden start idling fast or was there some work or parts replacement done that caused it to idle fast?
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Old 02-10-2018, 02:13 PM   #19
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Re: Help, High idle problem TBI, how the heck do you get it down.

It just started one day, I did drop the tank and replace a faulty fuel pump.

Been driving it for a coupe days now with the IAC unplugged and it runs great starts fine and idles right. The IAC is new, A/C delco part.
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Old 02-10-2018, 04:10 PM   #20
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Re: Help, High idle problem TBI, how the heck do you get it down.

If it's running fine with it unplugged there's bound to be a leak somewhere. Is it possible that a vacuum line from the tank may have broken or come unplugged? http://repairguide.autozone.com/znet...528005c9bd.gif

It could also be the intake manifold gasket or other place where air may be getting in. The problem with running with it disconnected is that when starting or when engine is cold it may have trouble idling properly -sometimes when coming to a stop.

There is a possibility that the IAC is new defective. These days even OEM parts are not super reliable. I can't begin to count the new parts I've returned in the past for being defective.
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Old 02-10-2018, 04:58 PM   #21
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Re: Help, High idle problem TBI, how the heck do you get it down.

Thanks!
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Old 02-11-2018, 01:54 AM   #22
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Re: Help, High idle problem TBI, how the heck do you get it down.

Quote:
Originally Posted by 57NAPCO View Post
Changed...iac, tps, rebuilt TBI and new base gasket, pulled the egr and it’s not froze up,
Let's go through these one by one:

1. IAC--did you make sure you removed all the old gasket material/replace the IAC gasket when you changed this? That is a potential vacuum leak. Also, did you have the idle problem before you changed the IAC?

2. TPS--did this affect your idle in any way? Unless defective, it shouldn't.

3. Rebuilt TBI--Did you rebuild your throttle body or install a rebuilt unit? The BBC's had two different base footprints over the years. If the base footprint of your throttle body doesn't match the gasket you used and the mounting surface on the intake manifold that could cause a big vacuum leak.

4. Pulled the EGR--Did you replace the gasket when you reinstalled?

5. Have you tried capping off each vacuum tap from the throttle body/intake manifold one by one? If you have a leak in one of the lines, doing this will help you find it because with the IAC unplugged as you have it now you would cut off the air and the engine should almost stall.
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Old 02-11-2018, 07:02 AM   #23
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Re: Help, High idle problem TBI, how the heck do you get it down.

Gonna get this feller sorted out, one way or another

Quote:
Originally Posted by arrg View Post
Let's go through these one by one:

1. IAC--did you make sure you removed all the old gasket material/replace the IAC gasket when you changed this? That is a potential vacuum leak. Also, did you have the idle problem before you changed the IAC?

Yes, rubber O ring, yes the problem existed before changing IAC

2. TPS--did this affect your idle in any way? Unless defective, it shouldn't.

Did not

3. Rebuilt TBI--Did you rebuild your throttle body or install a rebuilt unit? The BBC's had two different base footprints over the years. If the base footprint of your throttle body doesn't match the gasket you used and the mounting surface on the intake manifold that could cause a big vacuum leak.

I rebuilt it with a factory kit, no difference.

4. Pulled the EGR--Did you replace the gasket when you reinstalled?

No on the gasket, but no difference after checking it

5. Have you tried capping off each vacuum tap from the throttle body/intake manifold one by one? If you have a leak in one of the lines, doing this will help you find it because with the IAC unplugged as you have it now you would cut off the air and the engine should almost stall.

Not done this yet, I will work on doing that next.

Thanks a bunch for your input.
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Old 02-12-2018, 12:44 PM   #24
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Re: Help, High idle problem TBI, how the heck do you get it down.

Hello When I did the head gaskets I cleaned and rebuilt the TBI and replaced with new gasket. Now years later doing the intake manifold gaskets I found that the TBI bolts were loose. I wondered why the engine would surge. That probably was the problem. So now after I get the intake squared away I will always check the TBI bolts.
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Old 02-12-2018, 02:43 PM   #25
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Re: Help, High idle problem TBI, how the heck do you get it down.

Quote:
Originally Posted by 57NAPCO View Post
I understand but my truck is 100% factory original with only 20,000 original miles on it. All of my reading says do not knock out the idle adjustment cap to ajust the idle.
Thanks for the tip.

...Im trying figure out what changed to make the idle go too high. Ran and started fine for a few years. Thanks again for all the help yall!
Ignore the manual. If you rebuilt the TBI you *MUST* reset the minimum idle setting.

http://forum.73-87chevytrucks.com/sm...?topic=21039.0
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