Trizney

I had my throttle body rebuilt, and it was bored to a 54mm. I matched the new bore both on the gasket and the intake, so there is perfect flow. I replaced all the gaskets, and the IAC and TPS sensor. The car now idles at about 1100 RPMS, and I will sometimes get a surge up to 2000RPMs when the car first turns over. I have searched, and I can find posts on L98s, where there is a screw, but I see no screw anywhere on the LT1 TB.

How does one adjust the IDLE on the LT1?

VenkmanP

The idle can be adjusted by reprogramming the computer. I don't know if this is the solution to your problem however.

bogus

1) disconnect the IAC harness (lower right of the TB).

2) turn on the key. This will put the IAC into "reset mode".

3) turn off the key and reconnect the IAC harness

4) Start the car, buy holding your foot LIGHTLY on the gas. This will reset the IAC. Run for 5-10 seconds.

5) turn off the car and release the gas.

6) restart, and all should be ok. If not, the IAC may be gummed up. I had this problem with bastet44's 87 coupe after cleaning it and then putting it on the shelf whilst I finished the engine rebuild. I sprayed a touch of PB Blaster into the IAC hole. Over a day or 2 it loosened right up.

500hp

Make sure the TPS is installed correctly or try using the old one. Back when I had a bored out 52mm I had to elongate the mounting holes to get the .55 to .60 volts usually preferred for proper operation. I say this because I think it could partly be the fault of the mass produced plastic sensor. There is a semi-hidden screw that will open or close the throttle body and affect the idle, but cleaning and resetting the IAC first is probably your best bet until you can locate a scanner and get some readings for pinpointing the cause. I can give you a bunch of numbers, but they won't do you any good without a scanner/diagnostic linkup.

96GS#007

1990 and later cars "self-zero" the throttle position as long as the TPS voltage is less than ~.95v

The screw in the throttle body is used to adjust the minimum idle air. While it will affect the idle speed for a short time after messing with it, the PCM will ultimately adjust the IAC valve to compensate. Bogus outlined a procedure for resetting the IAC valve.

To reset the minimum idle air do the following:

1) Turn the ignition key to "Run" but do not start the car
2) Disconnect the IAC harness from the IAC valve
3) Start the engine
4) Adjust the screw in the throttle body until the idle speed drops to ~500rpm
5) Turn off the engine
6) Reconnect the IAC harness
7) Restart the engine and let idle. The PCM will adjust the IAC position for the programmed idle speed (this may take a couple of minutes, the engine may idle low or high initially, and the engine may surge. Give it time).

Trizney

Where exactly is this? When you say in the TB, you mean taking the rubber shroud off and look in the TB?

I tried the way bogus recommended on Sat, but that didn't work. Your steps differ in that the key must in the Run position before the IAC is unplugged. I will try this and also look for this screw.

I have the GM shop manual for my car, and it talks about using the Tech1 to put the computer into learn mode for the IAC. Can I do this with TunerCat?

One other thing, the shop manual talks about the pintle length of the IAC. Does this really matter and can it effect idle?

96GS#007

The screw in the throttle body is on the drivers side, top. It's usually blocked by a plug unless it's been tampered with previously. Unless you have the correct tools, you'll need to unbolt the throttle cable bracket from the intake manifold in order to access it. You do not need to remove the rubber bellows from the throttle body.

I'm not familiar specifically with the Tech1 ('96s use a Tech2). I do know that TunerCar does not have bi-directional control capability like a Tech1.

The shop manual discussion regarding the pintle is in regards to a new one. If you try and adjust the existing pintle, more often than not it breaks the mechanism inside the IAC and you get to buy a new one.

Yes, pintle length has an impact on idle. It's what the IAC motor is moving to control the idle, so it's a rather critical part.

tempest

to the above.

a supplement to bogus' procedure, between steps 5 & 6, give the ECM at least 10 seconds to shut down before restarting.

like Jim said, the idle stop screw on the LT1 (the stock one at least) has a hidden "head". i adjusted mine gently with needle nose pliers. not a proper way to do it, but it worked in my case after failing to find any vacuum leaks.

a side note, if you ever have the IAC valve out of the TB, unless it is a new one do not push or pull on the pintle, unless it is a new one (in which case you can push in gently).

also i would not keep the IAC valve connected to the harness while not installed in the TB. if the valve goes into a reset mode (i.e. after a battery disconnect) the pintle jumps out. guess how i know...

also, the Tech-1 is only needed for the EBCM to learn the new min TPS. why the ECM doesn't need to do this while the EBCM does doesn't make sense, but it does not surprise me either for a car built in the early 90's/late 80's.