'91 exessive idle?
Le Mans Master
Joined: Dec 2000
Posts: 7,839
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From: San Diego, Ca
Don't know if monkeying with minimum idle is going to do anything (and a lot who do end up with other problems). Absent a scanner, might want to see if the ECM can control the IAC. Disconnect it, turn on the key and ground the diagnostic link. Then use a test light to ground on each terminal. Steady or blinking light shows control - No light usually an open, but make sure there's 20 ohms between terminals A & B and C & D.
Check TPS at the ECM - short to voltage in the return (Dk Blue), may not show up at the connector.
Check CTS at the ECM. Signal is 5 volts and may be close to that number on a cold motor. Whatever it is, it should drop as it warms up and the resistance in the CTS decreases.
Check TPS at the ECM - short to voltage in the return (Dk Blue), may not show up at the connector.
Check CTS at the ECM. Signal is 5 volts and may be close to that number on a cold motor. Whatever it is, it should drop as it warms up and the resistance in the CTS decreases.
Race Director
Joined: Sep 2000
Posts: 17,298
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From: The Top of Utah
Originally Posted by SunCr
Don't know if monkeying with minimum idle is going to do anything (and a lot who do end up with other problems).
Originally Posted by 77vetteuk
The BBK closes alot tighter-you can feel it in the gas pedel when you brake oben the butterflys-stiff!
RACE ON!!!
Thread Starter
Instructor
Joined: Oct 2005
Posts: 143
Likes: 0
From: Lincolnshire,ENGLAND
Originally Posted by CFI-EFI
This sounds to me like the throttle stop screw isn't contacting the throttle arm and that the throttle plates are jambing and sticking in the throttle bores.
SunCr- Ill check the IAC controll as you describe. So should there be a steady/blink light from EACH terminal at the connector when grounded?
TPS & CTS Checked and are 100%working.
THANKS.
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Originally Posted by 77vetteuk
Throttle blades sticking closed because of the vacuum lock-no holes in these bledes like the originals.
Originally Posted by 77vetteuk
Im still playing with the idle adjust screw,wanna get it set-up just right.
RACE ON!!!
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
Steady and/or flashing is good - no light is bad.
Minimum idle is set by turning on the ignition, grounding the diagnostic link (you should be able to hear the IAC motor running), then disconnect the IAC and start it up. The pintle should be fully extended and the only air getting pass the throttle blades will be determined by how far they are opened. Adjust it by turning the screw until it's at 425 +/- 25 rpms. If it's close, I'd leave it alone, but if you do adjust it, reset the TPS to .54.
If it won't adjust - either air is getting in from somewhere else, the pintle is jammed or the IAC motor didn't get pulsed by the ECM. Check wiring as previously posted.
Minimum idle is set by turning on the ignition, grounding the diagnostic link (you should be able to hear the IAC motor running), then disconnect the IAC and start it up. The pintle should be fully extended and the only air getting pass the throttle blades will be determined by how far they are opened. Adjust it by turning the screw until it's at 425 +/- 25 rpms. If it's close, I'd leave it alone, but if you do adjust it, reset the TPS to .54.
If it won't adjust - either air is getting in from somewhere else, the pintle is jammed or the IAC motor didn't get pulsed by the ECM. Check wiring as previously posted.
Thread Starter
Instructor
Joined: Oct 2005
Posts: 143
Likes: 0
From: Lincolnshire,ENGLAND
Originally Posted by SunCr
Steady and/or flashing is good - no light is bad.
Minimum idle is set by turning on the ignition, grounding the diagnostic link (you should be able to hear the IAC motor running), then disconnect the IAC and start it up. The pintle should be fully extended and the only air getting pass the throttle blades will be determined by how far they are opened. Adjust it by turning the screw until it's at 425 +/- 25 rpms. If it's close, I'd leave it alone, but if you do adjust it, reset the TPS to .54.
If it won't adjust - either air is getting in from somewhere else, the pintle is jammed or the IAC motor didn't get pulsed by the ECM. Check wiring as previously posted.
Minimum idle is set by turning on the ignition, grounding the diagnostic link (you should be able to hear the IAC motor running), then disconnect the IAC and start it up. The pintle should be fully extended and the only air getting pass the throttle blades will be determined by how far they are opened. Adjust it by turning the screw until it's at 425 +/- 25 rpms. If it's close, I'd leave it alone, but if you do adjust it, reset the TPS to .54.
If it won't adjust - either air is getting in from somewhere else, the pintle is jammed or the IAC motor didn't get pulsed by the ECM. Check wiring as previously posted.
Im having exactly the same problems with both throttle bodys though- car never idles lower than 900rpm, after revs have been taken up it will stick at around 2000rpm (most of the time) and a blip of the throttle will bring it back down to 900-1000rpm.When it sticks the TB linkage is a couple of mm away from the adjustment screw.
THANK YOU.
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Race Director
Joined: Sep 2000
Posts: 17,298
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From: The Top of Utah
Originally Posted by 77vetteuk
Im having exactly the same problems with both throttle bodys though- car never idles lower than 900rpm, after revs have been taken up it will stick at around 2000rpm (most of the time) and a blip of the throttle will bring it back down to 900-1000rpm.
THANK YOU.
THANK YOU.
RACE ON!!!
Cruising
Joined: Nov 2005
Posts: 14
Likes: 0
Had the same problem on my ex-wife's 89.Drove me crazy for a couplke weeks off and on before I talked to a local guy, something like 15 years GM corvette division experience. He still works on Corvettes only. He told me to replace the relays under the hood. MAF burnoff, fuel pump, fan, etc. All of the relays are the same #. He said they may check good but at the cost of only about $6.00 he said to replace them. At least one of them must have something to do with the MAF and TPS I guess because....Bingo!!!!! Problem solved!!!! His name is Jim Watson and he is the go to guy on Corvettes in the Atlanta area. Lost his phone # in the divorce.
Last edited by rockrash; Dec 6, 2005 at 12:29 PM.
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
'91 is MAP not MAF so the Relays don't exist. If the throttle isn't returning to the stop - disconnect the cable and see if it does. If so, I'd replace the cable. Otherwise, the IAC is simply a stepper motor with 256 steps (or counts in automotive speak). It controls idle by retracting the Pintle to allow more air through the throttle body. 0 steps or counts is no air - 256 max. At idle, most are around 15 to 20 with the a/c off. Once the car is cruising, TPS above .62, the IAC becomes a dashpot and retracts to about 150 to 160 counts. That way it doesn't stall when the throttle shuts - though it's return to idle should be seamless. It will also park itself at this number once it shuts down - that insures plenty of air for a quick restart and if you note the RPM at shutdown, and then immediately restart it, that restart RPM will be initially higher than when you turned it off. Usually, if it takes a perceptible amount of time to return to normal idle, it's gummed up or the motor is bad or the pintle is jamming in the bore, or there's high resistance in the wiring. With a new throttle body hanging up the same as the old one, sounds like a funky cable on this one, though in my experience, GM cables usually just snap in two.
Thread Starter
Instructor
Joined: Oct 2005
Posts: 143
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From: Lincolnshire,ENGLAND
Checked ecm signal to IAC- Steady/almost blinking light at each terrminal when earthed, 19.something ohms at a-b & c-d.
Set the minimum idle-got it down to 500 rpm before the screw was backing away from the plate.Reset tps/iac, took it for a drive,TB still not returning at around 2000 rpm. Also noticed at driving speeds it will hold its rpm and not slow down at around 2000-1500 rpm when i back of the gas pedal.
Checked the throttle cable (again) and feels super smooth,TB returns all the way when engine is off.Gas pedal not catching on carpet etc.
Same problem with cruise cable dissconected.
Set the minimum idle-got it down to 500 rpm before the screw was backing away from the plate.Reset tps/iac, took it for a drive,TB still not returning at around 2000 rpm. Also noticed at driving speeds it will hold its rpm and not slow down at around 2000-1500 rpm when i back of the gas pedal.
Checked the throttle cable (again) and feels super smooth,TB returns all the way when engine is off.Gas pedal not catching on carpet etc.
Same problem with cruise cable dissconected.
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
500 RPM mininum air is out of specs, but what did you use to monitor RPMS (the dash readout isn't that reliable). The most likely culprits would be a pintle that's hanging up in the bore or a faulty ECM. At this point, probably best to troubleshoot with a scanner - you can then see what's being commanded (counts vs. targeted RPM and host of other data such as the Engine Coolant Temp Signal).
