tbi help

[Square84]

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.

[heavychevy79]

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?

[Square84]

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?

[heavychevy79]

Carbon buildup in the IAC passages? Or maybe defective IAC?

[rfmaster]

Originally Posted by Square84

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

[Desert1957]

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

[Square84]

Originally Posted by Desert1957

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

[pig rig]

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

[rfmaster]

Originally Posted by pig rig

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

CTS is easy to check - with engine cold (not running) disconnect harness connector from CTS and measure sensor resistance. At about 73F (20C) you should see about 2800 to 3000 Ohm for a good sensor (a cheap DVM will do the trick). In reality CTS is not a choke, but a sensor. ECM performs injector pulse duration calculations based on about half a dozen sensor inputs with CTS being one of them. Having an open CTS should set SES light and will also set DTC code 15 (Shorting pins A&B of ALDL will force SES light to blink DTC codes).

//RF

[Damien]

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.

[Square84]

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.

[rfmaster]

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

[Square84]

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

[trixter99]

chek the vaccum line from the back of the throttle body to the map sensor thats what was wrong with mine.