occupa

Maybe another if it ain't broke don't fix it but...

I have noticed that I have a bit of a lag getting back up to "speed" after maneuvering a series of gates or turns.

This lag is reduced if I two foot the car. Braking with the gas on, but a not a speed. It really pulls exiting the turn etc.

I noticed that someone mentioned setting the idle speed to 1500 RPM for autocross. A Corvette post not sure if it was for the 84.

The question is... What happens if I set the idle to 1500 rather than its current setting at 600 RPM. Stock Cross Fire Injection, I will remove the tamper proof weld.

What does the computer think is happening and will it try to "fix" it?

What happens to the IAC motors?

What does the TPS send to the computer?

and it I move it up and turn it off. Can I re start it in this condition?

Tom400CFI

To do this, you'd simply turn up the throttle stop screw to get your desired RPM, and NOT reset the TPS, then put it back, after the event.

*Computer will "think" you're stepping on the throttle slightly.

*IAC's will do what ever they do, when you step on the throttle slightly, which is typically to open up some

*Car should start just fine with a cracked throttle.
^In other words, no harm will come of it. You could always unplug your IAC's too, for the event and setting. I actually ran MY CFI car w/IAC's unplugged all the time and bottomed (closed) in order to get what I considered to be better manual trans shifting behavior (it eliminated "hanging RPM" during shifts).

You will lose a lot of engine braking, but the theory has merit. It will likely make power on/off transitions much smoother.

occupa

Thanks Tom400CFI

and good point about the engine brake and for autocross I hope that will be of minimal impact.

I think based on your suggestion I will take following approach.
Get car to operating temperature
RPM has been consistent w/o AC at 600 RPM (tach on dash)
Turn off
Remove cover, air filter, and lower plate
Unplug IACs
Start car
Record RPM on external tach to allow resetting
Adjust idle screw with external tach to 1500 RPM
Leaving IACs unplugged replace lower plate, air filter, and cover

When the event is over,
Restore to Idle RPM value recorded above
Plug IACs back in

I think that this will take the IAC out of the loop and prevent them from being over driven, closed.

Again thanks for you help

antfarmer2

How old is your o2 they get old and slow.

Tom400CFI

Sounds like a good plan!

occupa

Regarding the O2 sensors, I believe one is new and the other unknown.

I have only had the car not quite two years

occupa

Hmmm...
I tried it.

and I thought I had it.

600 RPM before unplugging the IAC

1400 RPM after unpluging the IAC.

The idle screw did not seem to have any effect

and after a couple of runs the idle went back to 600 even with the IAC unplugged.

Did the computer "learn" and the correct? Even without the IAC?

Tom400CFI

No. It can't do that. The only way the ECM can control the idle speed is w/the IAC's.

Did you unplug both of them?

occupa

Yes unplugged both.

It does appear that the Idle screw has any effect.

I moved it a couple of turns each way and it did not have any effect.

I will look at it again before the next event on Sunday Sept 6th

Tom400CFI

If the IAC's are unplugged, that throttle stop screws should absolutely have an effect. Something seems odd here. Are you SURE you're unplugging the IAC's?

On my CFI car, I deliberately disabled the IAC's, to eliminate "throttle following/hanging RPM's during shifts. I bottomed them, unplugged them, then set the idle speed to the spec I wanted w/the throttle stops. It behaved like a carb.

Only down side was when COLD (like below 20*F, cold start), I'd have to depress the throttle slightly for start up and for the first 10 seconds of running, of so.

I'll be interested to see what you find.

occupa

Makes more sense now.

I had two issues.

1. I did not plug the vacuum line to the air cleaner
2. I did not under stand the "off" position of the IAC

The first was easy to solve, plugged the line

I did not realize that the IAC fully opened when the car is off. I thought they stay in the last position. Your comment "bottomed out" now makes more sense

So when I unplugged them they went fully open and that is why the Idle screw had not effect.

I do now know how to bottom out the pintles an the IAC, so I plugged the passage on the Throttle Body.

The plug "bottomed out" the IAC and reduced the idle to 500 RPM and from there I was able to adjust the idle to around 1300-1500 RPM using the idle screw.

The plug I used was some wadded up bicycle inner tube. I was not sure that it would withstand the temperature.

So, how do I bottom out the pintle on the IAC or will the inner tube withstand the temperature.

Or other solutions

whalepirot

The '84 has one, left exhaust manifold.

Tom400CFI

There are two text book ways that I am aware of;
1. You can buy specific, IAC plugs, from Thexton. It is a "Thexton 368", GM IAC plug. You'd need two of course;



2. Take a paper clip and open it up and form it into a big "U" shape. Insert the clip ends into the ALDL connector in the ‘A’ and ‘B’ pins.

Turn on the ignition, but don’t start the engine. Wait 30 seconds. Jumping the A to B pins w/the Key on commands the IAC's to bottom out all the way. 30 seconds ensures that has happened. Next, remove the connectors from the IAC's.

Other ways include building plugs, like you have already done. That can be very effective. Another "hack" but effective method is to introduce a massive vacuum leak, such as a brake booster line disconnected, w/the engine running. ECM will try to compensate my motoring the IAC's all the way bottomed out, then unplug the IAC motors, then fix your vacuum leak.

ONE of those options should work for you!

occupa

Sorry for the late reply

and

Thanks

occupa

I was able to plug the IAC.

Set the idle to 1200 RPM

and ran the Autocross morning session plugged.

In the afternoon I ran back to normal.

My impression I was a second faster in the afternoon

May not be a definitive test...

But I am satifisfied to leave as is