I've read this 100 times - that the best you can get is .999

Why? What do you lose if it's .998 or 1.0?

And I'm sure the Ford Manual states to set it at .999 but why? what's the benefit and how do these things work? What exactly do they control?

If I was dyno tuned with it at .899 will my tune be screwed up if I adjust it up to .999?

 

It's because the PCM has "modes" of throttle positions. Closed throttle, Part throttle, and WOT. If you are at closed throttle, but the PCM thinks you are at part throttle, then it will alter the fuel and spark differently.

On the old cars like the 5.0s, 1.0 volt was the CT-PT break point. So you wanted to keep it under 1.0 v. You try to get it as close to 1.0 without going over it, so that there is no "lag" when you step on the throttle, because nothing changes in the PCM from say, .76 to .99 volts. So that becomes literally wasted throttle movement.

On the Gen 2 however, the break point is either 1.05 or 1.15, I forget. In all honesty, the TPS setting thing really does nothing for performance. It affected the older cars much worse, and is just one of those things carried over from the 5.0 days.

But it is smart to check it after removing the TPS or changing TBs.

 

Thank you Sal!

I'll check it and get it close as I can.

 

I agree with Sal. Before I changed my throttle body I checked the TPS voltage on the stock Ford throttle body and it was reading a little over one volt. I believe it was between 1.05 and 1.10, somewhere in there. Anyway, according to what I read in the Ford Service Manual the PCM relearns the closed or idle throttle position each time the engine is started so if the voltage is anywhere in the ball park it should work fine. The Gen II PCM seems to be pretty flexible in the reguard.