Did you check the IAC to see if it is working? The scanner just tells you how many "counts" the IAC motor is supposed to step open. It doesn't tell you the actual IAC position. If the valve is damaged, or the passages are plugged up, it will repsond exactly as yours has done..... the PCM keeps setting the "counts" higher to get the right amount of air, but no extra air comes in because the IAC valve is not moving, or because the IAC passages, from the hole between the blades, through the baseplate and the IAC mounting point, are plugged up.

Shoebox has an instruction for checking the IAC valve on his website.

Obviously, with a cam, and with such a high target idle RPM, the PCM is going to need to set higher IAC counts to get the desired results, and adjusting the "closed throttle" blade opening may be the only way to accomplich this. But check the valve and the passages first. If you have altered the throttle blade stop screw setting, you should check the TPS "closed" voltage, to make sure it is not too high.

 

Idle screws on a carb let in both air and fuel.

Since a vacuum leak will send the IAC counts to near zero, conversely, I would think a rich condition would cause the IAC counts to go up.

 

A vacuum leak causes the IAC to go to zero because the it does the same thing as an opened IAC; it lets more air in, thus raising the RPM. The PCM trys to slow the engine back down by closing the IAC valve, but it can only go to fully closed.

-Dave C. '97 Z28

 

I think I know that.

 

If the IAC motor isnt busted or stuck closed like Fred was saying then there is a simple and effective fix. There is a small hole that leads from the TB to the IAC passage in the intake.. its actually an IAC bypass hole. All you need to do is drill out that hole letting more air bypass the IAC so it doesnt have to work as hard. The stock TB is set up for a stock motor so it makes sense youll need to make some adjustments when you start adding ported heads/cam because of the increased air flow. My car use to stall out on me when id turn the AC on at a stop light but after doing this mod I no longer had any trouble. IAC counts are now down in the 20-30 range instead of 60-80 too which wasnt near as bad as what you are experiencing.

 

Why dont you just replace your IAC and see what happens from there. My car was idleing funny and fluttering. IAC was very high at times and then it would be back to normal. Very off and on. Swapped it out and cleaned my TB thorougly and now my car runs great.

 

Bryan is aware of the problem and he suggested opening the TB up some. When I get paid tomorrow I'm gonna pick up a new IAC and see if that fixes it, if not I'll keep everyone updated. Thanks for the help all.

 

i don't know how much a new IAC is gonna cost you but i just pulled my stock 48mm off for my new 58mm. My old one idled great and has been drilled out. I only want $45 for it. It might be cheaper than a new IAC i don't know what they cost.

 

New they are only ~$65 from autozone. I'm gonna try to pick one up and see if it helps or not, hopefully I'll get time to do it this week.

 

I don't think that's actually the case. I can sit and watch my car idle on datamster, and all of my fuel trims go to the full rich end with the computer taking as much fuel out as it can, but my IAC stays parked right where it is, and the idle holds too.

Just an FYI, but when I was playing with my tune, I originally had the idle RPM set to 800, and the IAC was fine. On a new tune the guy doing the tunes set the idle RPM to 1050, and that caused my IAC to completely max open. I HAD to play with the throttle stop screw. It can only compensate for so much, and a change in target idle RPM that large seems to be too much for the IAC.

 

Shoebox: I didn't mean it to sound sarcastic or disrespectful.

What I meant is that a vacuum leak on a port fuel injected motor isn't the same as a vacuum leak on a carb'd motor because on a carbed motor the air from the leak is bypassing the fuel mixing device, so it's just additional air. In a Port fuel injected motor (except for certain circumstances where the leak is at the intake to head gasket) the air is metered by the MAP whether it comes through the throttle body or the vacuum leak. So the PCM will just keep closing the IAC trying to bring the engine idle speed under control. But sometimes at fully closed, this is not enough if the leak is big enough, even though through all of this the mixture is just fine.

On a TBI motor vacuum leaks are squirelly because although the leaked air is measured by a MAP (usually), depending on the type of leak it is the extra fuel doesn't make it into the intake to mix with it because the air is already ahead of it, whereas a port injected motor it can be squirt in soon enough to set the mixture properly.

-Dave C. '97 Z28