drivablity issue?

Recently I noticed that between 1250-1385 rpm(digital tach) that when im trying to hold a steady speed the rpm's will drop and the car will start decelerating until i slightly press on the gas. Its like the IAC is reacting when its not suppose to. Above or below that rpm the car is fine. This happens in town avg of 25mph. When Im on the hwy and try to get the car to act up it doesnt do it at the faster speeds.

Im not sure if this is a sensor issue or a tunning issue. The guy that did the tune was trying to clean up the idle, he did like the loop it had from the cam. The only problem is I dont remember noticing it till a month or 2 ago. I had the car tuned in April?

Any thoughts.

Motor is out of a 89sc but its in a 88 mustang, but that shouldnt matter since its a recent issue..

Doug

At that low of throttle position, you may actually be triggering idle mode with the EEC since the voltage of the TPS may be dropping below the part throttle threshold. I had to tweak mine down just a tad to get rid of that same issue as you have.

Only way to know for sure is to check the voltage reference for idle in the tune and then measure the voltage from the TPS at that point.

Fraser

makes sense. I have the PP Tbody and only one of the hold downs is tight? Maybe the top one is working its way loose and the tps has moved. Ill have to recheck the voltage. Ill drop it down alittle. I think I had it set at .97

You can only lower the reference voltage in the tune. Lowering the TPS voltage by adjusting it won't cure the problem. When you first turn the key on, the computer takes a reading of the current voltage of the TPS and then everything throttle wise is "referenced" from that voltage.

So if you have .97 volts at key on, then the part throttle reference voltage will be .97 volts + the setting in the tune, which means you could have .98 volts in order to trigger part throttle mode. If you lower the tps voltage to .87 then it will be .87 volts + the setting in the tune, which could mean .88 volts.

Fraser

I got ya. Ill have to make a trip back to the dyno so the guy can look at it. Im not happy with the idle anyway. He has it set around 700rpm.

thanks