Okay so here's my issue everyone. I have a 00 GT with 119k, from time to time when I throw in neutral or put in the clutch (same thing) slowing down, my idle will either stay at 1k, which is perfectly okay, or will drop down to 500, and then sometimes drop from 500 to 300 and eventually stall. I've had it happen going 80 on the highway and it scares the **** out of me. Keep in mind, this problem has been occurring before i did any modifications, so please don't say the mods are the issue. Intake wise I have a K&N CAI,Trick Flow 70mm TB, and Plenum. I figured once i did the tb and plenum install it will help it out since the stock parts were caked with crud inside, nope. So yesterday I cleaned out my IAC really good, and cleaned the electrical connections to the iac, maf, and tps. I even went and bought a new tps, still having the same issue. I bought a fuel cleaner thinking maybe it would have something to do with the injectors, i don't know, didn't help and i'm running out of ideas. I'm thinking the next step is to buy a new iac valve and see what that does. There is no noticeable vacuum leaks and all the hoses look fine, i even bought clamps for the hoses from the air intake to the iac to give it a better seal. Anyone have any thoughts or solutions? Should I just buy a new iac?

 

Your IAC could be flakey and a sticky EGR valve can cause this kind of problem. Also check your fuel pressure, either with a gauge on the shrader fitting on the end of the fuel rail or if you have or can borrow a logging tool, check the PID for the pressure drop across the injector. With the external gauge you should see about 32 psi at idle but this should jump up if you rev the engine due to the manifold vacuum decreasing, meaning the external pressure is increased to keep a constant pressure drop across the injectors.

The logged pressure drop PID should stay pretty solid at 39.5 psi.

 

Is your speedometer working? A bad VSS sensor will cause the problem you are describing as well.

 

It is possible that the TB is causing the problem and the reason it happened before was the caked on crud. The IAC operates on the idea that the throttle blade is partially open at all times allowing a pre-determined amount of air in. It simply allows more flow to adjust idle and to keep the engine from dropping RPM rapidly.

What I'm trying to say is your old one was stuck shut with gunk and the new one may be stuck shut because it needs a little adjusting. Try turning the throttle stop screw with the engine running and the IAC unplugged. A properly adjusted throttle body should allow the engine to continue to run without the IAC. Adjust the screw to where the idle should be with the IAC unplugged and then plug it back in to see how that works.

Due to the my cams I had to drill a small hole in my throttle blade like the older 5.0 engines to get enough airflow without turning the blade so far that the TPS thought my foot was on the gas.