At idle it will slowly creep up to 160 and max out and stay maxed out at idle, but during cruising it will drop to around 70 or so. I've cleaned it and the TB completely after it started happening but I still have the problem. It holds idle pretty well, but if I turn on the a/c it will drop the rpms ~300-350rpms(from 850 to around 500-550). I do also have a slight "flutter" at low rpms and part throttle, which is very noticeable at 45-48mph. You think the IAC bad or something else causing the problem?

 

Do you have the stock throttle body?

That's an indication of the idle bleed air not set right. So the engine is compensating by opening the IAC more, but it can only open so far to make a difference.

If you have the stock throttle body, there is a hole in the IAC channel that allows air to bypass the IAC. Make sure this isn't plugged up.

If you have installed an aftermarket throttle body, set the idle bleed air correctly or drill a small hole in the throttle plates to allow more air in. Do this until your IAC is around 50-60 counts or so.

Since you have an aftermarket cam and heads, you almost need to adjust the bleed air. (the bleed air is the amount of air that can enter through the throttle body when the throttle is closed. Either you can turn the adjustment screw to open the plates a little, or drill a hole in the plates to allow the air in)

-Dave C. '97 Z28

 

It's a stock TB. I'm going to check today when I get home to make sure the passenges aren't blocked, but I don't think they were last time I looked at them. How can I adjust the adjustment screw?

 

On the stock throttle body I think there is a plug you have to remove. If you look at the top of there the linkage hooks up, where it meets the front of the throttle body, you can see the end of the screw sticking out, stopping it. The plug is opposite this. I can't remember off the top of my head what kind of screw it is, but it's probably torx or allen.

-Dave C. '97 Z28

 

Ok, I know what screw your talking about, so how do I get the plug out? I'll look at it closer when I get home.

 

I'm not sure...you'll have to figure that out. I think you can punch it out with a screwdriver and hammer.

-Dave C. '97 Z28

 

So in other words its not going back in then, lol.

 

If this is something happening all of the sudden, you don't want to be messing with the throttle stop screw. You have a rich condition that is causing the computer to compensate by raising the IAC counts to let in more air.

 

Would the computer open the IAC to lean it out? I'd figure the PCM would reduce the injector duty cycle.

-Dave C. '97 Z28

 

We're talking about idle. The whole purpose of the IAC is to control idle speed by letting in various amounts of air. It's the electronically controlled version of idle air screws on a carburetor. If the PCM only had to change the injector duty cycle, it would not really need the IAC.

 

It's done it ever since the cam was installed back in March, I'm just now trying to fix it because I'm tired of it randomly idle'ing down and dying with the a/c on.

 

The injector duty cycle sets the mixture. The PCM doesn't change the idle speed by leaning the car out. The IAC sets the idle speed by the amount of air that is let in, and then the injector pulse width is adjusted to set the mixture. They have IAC because they did not have the PCM control the throttle body butterflies, like how you adjust idle speed on a carbuerated engine.

Also, the idle mixture screws on a carb adjust the idle fuel flow, not the air flow. If you need more air at idle, you usually have to open or drill the butterflies unless you have a carb like a Speed Demon with Idle-Eaze or equivalent. Normally you want to drill the butterflies rather than open them too much for a low-vacuum cam because you don't want to activate the primary metering circuitry.

 

After reading this again, it looks like we may be talking about the same thing, only in a different way.....

But, I can't understand why you say he has a rich condition which is causing the PCM to try to let more air in. In that case it would reduce the injector duty cycle to clear the lean condition, and only if the car didn't maintain the idle speed would it ramp up the IAC.

-Dave C. '97 Z28

 

OK, I just adjusted the set screw some and then I unplugged the tps and turned the car on for 10 seconds then off and plugged it back in, then on for 10 seconds and then off. (not sure if that was necessary or not but I saw it posted on ls1tech) Then I started the car. It now idles in park where it is supposed to be(right now at 1050), Prior to just doing this it would idle at 900-950, even though desired idle was 1050. In gear it idles at 750-800, but should be at 950. With the a/c on it drops to around 500-750. IAC at all times in park is 160, cept once when I turned the a/c off it dropped to 158 then back up to 160 in a matter of seconds. Cruising the iac is around 90 to 100 at all times. Can I email you guys my datamaster log I just did? Think I need to adjust it out more?

 

Might want to talk to Bryan about the tune, I had a similar problem with mine where it would load up so bad it would about die, then rev up and start all over. My problem was fuel reversion from the cam, the tune was off putting more fuel than needed to be into the cylinders, the cam had enough overlap to push some of the fuel back into the intake, it would start building up, the o2 sensors would swing so bad the ecm was getting false readings and putting more fuel to try and keep it running.

my truck is gone, can't remember exactly what we did to fix it, but Bryan should be able to help you out.

good luck.