The 1947 - Present Chevrolet & GMC Truck Message Board Network







Register or Log In To remove these advertisements.

Go Back   The 1947 - Present Chevrolet & GMC Truck Message Board Network > 47 - Current classic GM Trucks > The 1973 - 1987 Chevrolet & GMC Squarebody Pickups Message Board

Web 67-72chevytrucks.com


Reply
 
Thread Tools Display Modes
Old 04-07-2009, 12:19 AM   #1
Square84
I don't think these pills work
 
Square84's Avatar
 
Join Date: Apr 2008
Location: burkburnett,Tx
Posts: 626
tbi help

i need to find a good tbi mechanic or a good website with tbi info. i have replaced all the sensors on my throttle body and it idles fine when its cold but as it warms up it starts to idle lower and lower. and when it is fully warmed up it idles at 500 in park and 300 in drive and when your on the brakes it tries to die. so any info would be greatly appreciated? thanks in advance.
__________________
SQUARE84
90 SUBURBAN
Square84 is offline   Reply With Quote
Old 04-07-2009, 03:02 AM   #2
heavychevy79
Cheyenne Super
 
heavychevy79's Avatar
 
Join Date: Nov 2008
Location: Dallas Texas
Posts: 545
Re: tbi help

Sounds like the idle control motor is stuck. The extra fuel on cold start up
may be keeping it running until it goes into closed loop, then it starves for
air and fuel.

A clogged PCV may add to the problem.

How long has it been doing this? Was it progressive, or just started doing it
one day?
heavychevy79 is offline   Reply With Quote
Old 04-07-2009, 11:26 AM   #3
Square84
I don't think these pills work
 
Square84's Avatar
 
Join Date: Apr 2008
Location: burkburnett,Tx
Posts: 626
Re: tbi help

i did a complete tune up on it (everything from pcv valve to fuel filter, plugs and wires and so on ) then one day about two months after the tune up it wouldn't start unless you put your foot on the gas like a carbed motor. so i put a new idle air control valve, throttle position sensor, and coolant temp sensor. after that it worked great for about two weeks. then it tried to die at a stop light and the rpm's dropped and the volt gauge dropped so i figured it was the alternator could not carry the load but after replacing it, it still does the same thing?
__________________
SQUARE84
90 SUBURBAN
Square84 is offline   Reply With Quote
Old 04-07-2009, 01:16 PM   #4
heavychevy79
Cheyenne Super
 
heavychevy79's Avatar
 
Join Date: Nov 2008
Location: Dallas Texas
Posts: 545
Re: tbi help

Carbon buildup in the IAC passages? Or maybe defective IAC?
heavychevy79 is offline   Reply With Quote
Old 04-07-2009, 03:51 PM   #5
rfmaster
Registered User
 
rfmaster's Avatar
 
Join Date: Oct 2003
Location: OC CA
Posts: 1,374
Re: tbi help

Quote:
Originally Posted by Square84 View Post
i did a complete tune up on it (everything from pcv valve to fuel filter, plugs and wires and so on ) then one day about two months after the tune up it wouldn't start unless you put your foot on the gas like a carbed motor. so i put a new idle air control valve, throttle position sensor, and coolant temp sensor. after that it worked great for about two weeks. then it tried to die at a stop light and the rpm's dropped and the volt gauge dropped so i figured it was the alternator could not carry the load but after replacing it, it still does the same thing?
I am assuming it is your '90 burban. I would look at the following two areas since all sensors have been replaced and assumed to be good (false hope as it is all too common to get DOA aftermarket replacement parts, especially if they were made in China)

1) One of the most common problems that cause grief to TBI owners is a low fuel pressure. GM equipped factory TBI systems requires 9 to 13 PSI of FP, and plenty of fuel volume to be happy. FP check is not that difficult to do as most auto parts stores will rent FP check set.

2) Idle Air Controller - IAC is a dual winding bipolar stepper motor controlled by ECM, however ECM does not know were the IAC pintle (cone shaped head) is positioned within TB. From ECM perspective IAC position is measured in steps (0 to 160). One step moves the pintle a very small amount. When set to zero steps the pintle is fully extended, closing off all IAC bypass air flow which corresponds to lowest commanded idle speed. As the IAC steps are increased, the pintle is retracted from the orifice, allowing more air to bypass the throttle blades. Just remember that an IAC retraction (or an increase in steps), opens the air passage to raise the engine speed.
When ECM command the IAC motor to a minimum step value for idle speed check it pulses each IAC circuit at a rate of 40 hz/ 50 percent duty cycle. IAC failure is relatively uncommon, but to check it requires specialized equipment - either GM IAC noid inline LED test fixture or DSO (digital storage oscilloscope). A short or an open in the wiring harness will render IAC inoperative. A shorted IAC will damage ECM driver circuits, so replacing IAC may not fix idle problem. All of the above is assumed that IAC passages are clear of carbon and crud build up .


//RF
__________________
"The Beast"

1975 Chevrolet C20 longbed
350/700R4! with 3inch body lift
Dual Flowmasters Super 40's!
TBI retrofit completed (2007-07-29)
New 383CID (+030) 08-304-8 9.5:1CR x36,005 (2012-12-17)
rfmaster is offline   Reply With Quote
Old 05-04-2009, 09:58 PM   #6
Desert1957
Registered User
 
Desert1957's Avatar
 
Join Date: Apr 2009
Location: New Freedom Pa.
Posts: 1,335
Re: tbi help

Hey Guy,
I know by now you have propably fix your truck but , my 87 TBI did the same thing , turned out to be a weak fuel pump. Low fuel pressure on TBI's is a big problem. Just for future problems and performance tuning , I installed a walbro fuel pump and converted the regulator to adjustable. What a difference! These engines respond to fuel pressure adjustments 1/2 pound above factory spec makes a huge difference, You can keep tinkering with the pressure until it runs poorly then back it off. Note you will need to purchase the inline gauge adapter fitting and adjustable regulator cup/spring for all fine tuning adjustments. Good Luck...

Desert
Desert1957 is offline   Reply With Quote
Old 05-04-2009, 11:12 PM   #7
Square84
I don't think these pills work
 
Square84's Avatar
 
Join Date: Apr 2008
Location: burkburnett,Tx
Posts: 626
Re: tbi help

Quote:
Originally Posted by Desert1957 View Post
Hey Guy,
I know by now you have propably fix your truck but , my 87 TBI did the same thing , turned out to be a weak fuel pump. Low fuel pressure on TBI's is a big problem. Just for future problems and performance tuning , I installed a walbro fuel pump and converted the regulator to adjustable. What a difference! These engines respond to fuel pressure adjustments 1/2 pound above factory spec makes a huge difference, You can keep tinkering with the pressure until it runs poorly then back it off. Note you will need to purchase the inline gauge adapter fitting and adjustable regulator cup/spring for all fine tuning adjustments. Good Luck...

Desert
thanks for the info i have actually not fixed it yet no time to work on it so for now i just live with it
__________________
SQUARE84
90 SUBURBAN
Square84 is offline   Reply With Quote
Old 05-05-2009, 01:55 AM   #8
pig rig
Registered User
 
pig rig's Avatar
 
Join Date: Feb 2009
Location: washington the state
Posts: 730
Re: tbi help

hey square84 is it dropping rpms because its loading up with to much fuel if thats the case a quick simple and half way cheep fix is change your coolant temp switch its the one right by your thermostat if it goes bad it can tell the ecm that that the engine is running cold. it works kind of like a choke
__________________
my sports car rolls on six wheels only because I'm to fat to get in and out of new corvette


http://hubgarage.s3.amazonaws.com/ph...ped_detail.JPG
pig rig is offline   Reply With Quote
Old 05-05-2009, 01:28 PM   #9
Damien
Registered User
 
Damien's Avatar
 
Join Date: Mar 2007
Location: Farmington, MN
Posts: 928
Re: tbi help

I also say CTS. It is a common thing to go bad. I have a friend that unplugged one injector and the TPS so he could limp the truck home when his went bad. I thought that was pretty ingenious.
__________________
I can still count my vehicles on two... wait three hands.
Damien is offline   Reply With Quote
Old 05-05-2009, 06:55 PM   #10
Square84
I don't think these pills work
 
Square84's Avatar
 
Join Date: Apr 2008
Location: burkburnett,Tx
Posts: 626
Re: tbi help

ok the coolant temp sensor, coolant temp switch, thermostat, and fan clutch are all new. also the tps, iacv, and egr valve are all new. what it is doing is when i first start it up it idles fine 800-park and 600-drive. now when it warms up the idle drops and it sits at 500-park and 300-drive with it idleing so low it stutters and tries to die but it never actually shuts off. the check engine light is on but it is a code for the vss and it has been on since i got it. the other day it was running great the idle never dropped or anything so i thought it might have corrected itself but then when i put it in park it started surgeing from 800-1500 rpm. and now it is idleing low again so i don't know this is my first tbi motor everything else was carbed and easy to fix.
__________________
SQUARE84
90 SUBURBAN
Square84 is offline   Reply With Quote
Old 05-05-2009, 11:28 PM   #11
rfmaster
Registered User
 
rfmaster's Avatar
 
Join Date: Oct 2003
Location: OC CA
Posts: 1,374
Re: tbi help

Did you reset IAC when you installed a new one????
To reset IAC take the paper clip and open it up and form it into a big "U" shape. Insert the clip ends into the ALDL in the 'A' and 'B' pins.

Turn on the ignition, but don't start the engine. Wait 30 seconds. Now, go remove the connector from the IAC.

Start engine. You are now going to adjust "minimum air". There is a Torx screw on the side of the throttle body. This is what needs to be turned to adjust minimum air, or more commonly known as minim "idle speed". It comes from the factory with a protective metal cap over it. If the cap is still there, use a small punch to knock it out. Set the idle speed to 450 rpm, rotating the Torx screw clockwise to raise rpm, and counter-clockwise to lower rpm. Once the idle rpm is set, turn off the engine. It is also worth while to check your base timing after setting minimum idle speed.

Re-connect the connector onto the IAC. Start engine. Idle speed is now once again governed by the ECM, but your idle should be smooth and steady, approximately 600 rpm in Drive (for unmodified cars or trucks).

If you set an SES light by having the IAC disconnected, then after shutting down the engine disconnect the negative battery terminal. Wait 5 minutes. This will clear the ECM of all trouble codes. Re-connect the battery and drive the truck for about 20 minutes to allow the ECM to relearn VE table corrections driving style.

//RF
__________________
"The Beast"

1975 Chevrolet C20 longbed
350/700R4! with 3inch body lift
Dual Flowmasters Super 40's!
TBI retrofit completed (2007-07-29)
New 383CID (+030) 08-304-8 9.5:1CR x36,005 (2012-12-17)
rfmaster is offline   Reply With Quote
Old 05-05-2009, 11:36 PM   #12
Square84
I don't think these pills work
 
Square84's Avatar
 
Join Date: Apr 2008
Location: burkburnett,Tx
Posts: 626
Re: tbi help

ok i will try that because the book i read just said to turn the key on for 30 seconds then start the truck and drive it till its at operating temp then kill it and wait till its cooled down and it should be ok. but all these problems seem to be pointing to an iacv problem so i will try it and let you know. thanks
__________________
SQUARE84
90 SUBURBAN
Square84 is offline   Reply With Quote
Old 05-05-2009, 11:41 PM   #13
trixter99
I have no time for this
 
trixter99's Avatar
 
Join Date: Dec 2002
Location: Fort St John B.C. CANADA
Posts: 434
Re: tbi help

chek the vaccum line from the back of the throttle body to the map sensor thats what was wrong with mine.
__________________
1986 Chevy short box ( PROJECT )
1994 Chevy Blazer lifted 6"
2009 Gmc 2500 HD
1999 Hugger orange Camaro SS
2007 BMW X5 Sport

Last edited by trixter99; 05-05-2009 at 11:41 PM.
trixter99 is offline   Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -4. The time now is 07:11 PM.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
Copyright 1997-2022 67-72chevytrucks.com