Car falls flat on it's face at 4,200 rpm

badstang123 said:

If I mess with that screw on the throttle body that links to the IAC will it raise the idle for good or will the computer adjust it back down? Thanks
- Justin

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It depends upon how the idle was set. My preference is to set the hot idle with the IAC disconnected. If you did it this way, you can turn that screw/shank to raise the idle manually.

If the idle was set another way (some people use a method of setting the cold idle) then the IAC might be able to play around with any adjustments.

There is a bleed screw on the IAC that can be used to raise the idle. Note that this is a controlled vacuum leak and if the screw is loosened too far, it can create vac-leak-like symptoms (hanging idle, etc).

For many alternators, if you get idle RPMs to about 750 or a hair more, the output with a load is alright. I've also had to fine tune my idle just to accomodate electrical load.

Good luck.

badstang123 said:

So you just make sure the car is up to operating temperature, then disconnect the plug on the IAC while the car is running, and finally tune accordingly?

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If you like to fart around with things, you can do this. The simple answer is to just raise the idle up without disconnecting the IAC and see how it does. I mostly posted the other stuff in case you knew how the idle had been set before (the cold idle setting procedure is very popular on here - I just personally dont use it).
If it turns out that this simple adjustment doesnt work (your IAC is being utilized for the hot idle now, so adjusting the stop makes the puter close the duty cycle on the IAC and lower the idle), you can disco' the IAC and set it that way (so the computer cant lower the idle after you set it).

I like to record all idle adjustments so I know where I am relative to where I started - you can always go back to where you are now.

Wow, that post is clear as mud.