91 Idle Screw Issue/Tampering
Thread Starter
Racer
Joined: Dec 2003
Posts: 349
Likes: 0
From: Hong Kong
91 Idle Screw Issue/Tampering
I asked my garage to clean my throttle body and IAC. The idiots managed to damage the IAC some how, and in doing so decided to mess with the idle screw to increase the idle, which i understand is not to be adjusted on a 91.
I ordered a new IAC and replaced and set by holding the throttle on startup for 5 secs. Upon regular starting the car then surged to 2500rpm for 5 seconds on startup until the IAC can find its correct position, and allow the idle to drop back down.
I checked the TPS voltage, it was .63, so I screwed the idle screw in until the TPS voltage read .55 (closest I could get to .54), and now the car starts up much better, but still not as it used to.
Im wondering if I am completely screwed here, or did I take the right steps to fix the problem, since I couldn't find anything in the FSM about the idle screw on this car. I also understand the TPS is not adjustable, so was I correct to trust this value and set the idle screw according to that?
If I was not correct, can someone please point me in the right direction of achieving normal idle.
Cheers and thanks for your help in advance.
I ordered a new IAC and replaced and set by holding the throttle on startup for 5 secs. Upon regular starting the car then surged to 2500rpm for 5 seconds on startup until the IAC can find its correct position, and allow the idle to drop back down.
I checked the TPS voltage, it was .63, so I screwed the idle screw in until the TPS voltage read .55 (closest I could get to .54), and now the car starts up much better, but still not as it used to.
Im wondering if I am completely screwed here, or did I take the right steps to fix the problem, since I couldn't find anything in the FSM about the idle screw on this car. I also understand the TPS is not adjustable, so was I correct to trust this value and set the idle screw according to that?
If I was not correct, can someone please point me in the right direction of achieving normal idle.
Cheers and thanks for your help in advance.
Team Owner
Joined: Dec 1999
Posts: 34,873
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From: Lacey WA RVN 68-69
NCM Sinkhole Donor
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
You have no idle speed screw. The idle speed is controlled by the computer manipulating the IAC. The throttle stop screw is for making the minimum air adjustment, which you should see in the FSM. 1990 and up Corvettes have non a adjustable TPS. Any reference you may find to adjusting the TPS won't be in YOUR FSM and applies to 1989 and earlier cars.
With all due respect for vader, who has printed and published a lot of good information for C4 owners, when in doubt, stick with the FSM. The same goes for advice from me. Unless someone can give you a GOOD reason to deviate.
RACE ON!!!
With all due respect for vader, who has printed and published a lot of good information for C4 owners, when in doubt, stick with the FSM. The same goes for advice from me. Unless someone can give you a GOOD reason to deviate.
RACE ON!!!
Thread Starter
Racer
Joined: Dec 2003
Posts: 349
Likes: 0
From: Hong Kong
The FSM says the minimum idle speed is set at the factory, and thats it, no information on how to properly reset the minimum idle speed stop screw.
Does anyone know how to do this on a 91?
Thanks
Does anyone know how to do this on a 91?
Thanks
Burning Brakes
Joined: Oct 2004
Posts: 898
Likes: 1
From: Pearland Texas
Vaders site got my idle where its supposed to be. Beside messing with the TPS, the information I followed worked great. I did buy a Autozone TPS made by Wells, and I had problems with it. The AC/Delco TPS worked and I wished I would bought it the first time.
Thread Starter
Racer
Joined: Dec 2003
Posts: 349
Likes: 0
From: Hong Kong
My throttle body doesnt have a plug that covers the screw, its just solid shut. I had to use pliers and wrap the screw with a cloth to twist it back in...
so you think vaders method is going to work in a 91, with a non adjustable tps?
so you think vaders method is going to work in a 91, with a non adjustable tps?
Burning Brakes
Joined: Jul 2003
Posts: 873
Likes: 2
From: Whittier ca
With all due respect, for my '91, Vaders instructions are good until the TPS section. CFI-EFI is correct about TPS adjustment, there isn't enough movement to change the numbers. Before you start the engine, turn up the idle speed, then after it's warmed up some, bring down the idle to the recommened settings. I found that the idle speed would surge some, so I found a happy medium.
Make sure the timing is set at 6* before anything else, then your good to go for it.
Good luck
Subman
Make sure the timing is set at 6* before anything else, then your good to go for it.
Good luck
Subman
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Melting Slicks
Joined: Apr 2002
Posts: 2,118
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From: St. Charles, MO Route 66 Corvette Club
Here is a procedure on adjusting the min air or base idle from Gordon Killebrew that is very easy to follow. However, the procedure does not mention that you should reconnect the EST after completing the procedure. I guess that he assumed anyone who removes something should know to reinstall it 
Melting Slicks
Joined: Apr 2002
Posts: 2,118
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From: St. Charles, MO Route 66 Corvette Club
for your year car ...... and do not forget to reconnect the timing connector after the procedure Thanks for the HK photo as I have seen this view with my natural two cameras
Race Director
Joined: Sep 2000
Posts: 17,298
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From: The Top of Utah
Originally Posted by dailo
Thanks Sam Lam, will give it a go! I'm assuming that whatever the screw position is for 450rpms will give the correct reading for the TPS voltage?
I'll post back when its done.
I'll post back when its done.
Originally Posted by dailo
The FSM says the minimum idle speed is set at the factory, and thats it, no information on how to properly reset the minimum idle speed stop screw.
in this thread are fine. Just skip step 8. Don't frustrate yourself trying to mess with the TPS.
RACE ON!!!
Le Mans Master
Joined: Sep 2006
Posts: 6,050
Likes: 147
From: Phoenix AZ
2021 C4 of the Year - Modified Finalist
Tps
I purchased a Wells tps, and when I washed my engine the thing went nuts.
turns out, the Wells TPS wasn't sealed like the factory one, and ingested water. The water shorts out part of the TPS, and makes the ecm think your giving it gas.
mine wouldn't idle below 1500 RPM.
turns out, the Wells TPS wasn't sealed like the factory one, and ingested water. The water shorts out part of the TPS, and makes the ecm think your giving it gas.
mine wouldn't idle below 1500 RPM.
Melting Slicks
Joined: Apr 2002
Posts: 2,118
Likes: 0
From: St. Charles, MO Route 66 Corvette Club
I do not think it matters which gear the trannie is in when you make the adjustment.I would get the engine to operating temperature and make sure the AC is OFF before you make the min air adjustment.
BTW ..... what are you paying for a liter of gasoline in HK.....and convert this to US$ for me.....please!
Thread Starter
Racer
Joined: Dec 2003
Posts: 349
Likes: 0
From: Hong Kong
Hahahahahahahaha!!!! Funny!
Price per Litre HK$ 9.50 (US$ 1.22) , plus govt tax per litre HK$ 6.06 (US$ 0.78)
Total price at pump is now HK$ 15.56 (US$ 1.99) / litre
Now in gallons... US$ 7.55 / gallon...
Price per Litre HK$ 9.50 (US$ 1.22) , plus govt tax per litre HK$ 6.06 (US$ 0.78)
Total price at pump is now HK$ 15.56 (US$ 1.99) / litre
Now in gallons... US$ 7.55 / gallon...
