Idle creeping to 2K
 

Well my son and I put the nitrous kit on last weekend and tested it to make sure it worked. Went out on a highway and armed - man this reminded me of my ole hot rodding days!

Our 88GT's idle will slowly go from normal to idling around 2K. We can kill the motor and restart - idle returns to normal, but will slowly creep back up to 2K.

Any suggestions? I believe our TPS is in pretty good shape .87 at idle.

Any other symptoms or strange behavior? I don't know if that's a proper TPS reading or not but that problem does sound like a sensor to me... Can't imagine it's the IAC valve.....or whatever the hell they're called on Fords...Air Bypass Valve is it? Uhm.....a shot in the dark here (which is what all of my guesses on here are...I'm used to Chevy not Ford)....vacuum leak?

give the IAC valve a good cleaning as well as the TB itself. I am not sure, but I think a reading of .86 on the TPS is too low, check your chiltons. Also, if your 88 is speed density, that may be your problem. I have numerous friends with speed density cars that idled so poorly they had to sell them, and refused to be fixed. You may want to consider switching to mass air, better for the Nitrous anyway.

But there shouldn't be anything wrong with the speed density system unless the Manifold Absolute Pressure sensor is shot, or the vacuum line to it is bad....or there are other fried intake sensors that the computer uses for input. True, the MAP tables will only support up to certain flow numbers and horsepower numbers, but those numbers should be fairly high and not a problem for street cars... But I do agree that SD isn't good for nitrous..once the horsepower gets up there, the charts the computer uses just can't sustain power...it'd probably start throwing the AF ratios out of whack..I assume to the lean side but I could be wrong.

I'll look in the manual on the TPS voltage - but I've seen posts before that I was in the o.k. range. What is the "IAC valve " that you guys are saying to clean?

As far as nitrous with speed density it's only a 75 shoot, I know that 88workcar runs SD with a little shoot of funny gas. We do have plans to convert to MAF in the future when we change heads, cam etc.

the IAC valve is the silver cylinder looking thing attached to the throttle body at an angle. Take it off, clean it, check the voltage, etc.

TPS setting
 

Your TPS reading should be between 0.95 & 0.98 but not over 1. What's your timing at? Is your check engine light on? If so, check what codes you're pulling.

Timing was set at 14 - just backed it back to stock 10 because we've got the nitrous kit installed. The idle crept up before nitrous, and before we set timing to 14.