LT1 Idling problem (to low)

[drexel_j]

I have a 1995 LT1 with a Painless wiring harness for a 92-93 Corvette I am having problems with the idling; when I start it up it will go to about 1200 RPM and then go to around 900 to a 1000 which is fine as I have a Crane 227 cam as my only modification. The problem is when I am driving and I let off the gas suddenly (it is a 5 speed) it will go to around 200 to 500 RPM and die, it starts and runs fine. I had FIS burn the chip and this is the last problem I have in getting the car going properly. Does anyone know it it is the computer or the chip that determines what the idle should be. I think they set the idle in the chip to low when they programmed it. I have reset the IAC and have tried it from another LT1 that worked properly and it did the same thing. Any sugestions?

[wheelman]

Do you have a manual or automatic?

If the tranny is not a 4L60E, T56 or T5 with a VSS, did you wire up a vehicle speed sensor (VSS)?

 

If the VSS is not connected to the PCM then the computer always thinks the car is going 0 MPH and tries to keep the car at idle by adjusting the IAC. When you are opening the throttle the IAC is not controlling anything but the PCM closes it down in an attempt to bring the engine to idle. When you suddenly let off the gas the IAC is closed and the engine gets no air and dies.

 

One way to fix this is to disable the IAC and use the idle screw to set the idle.

Another is to adjust how you drive to compensate.

The correct way to fix it is to hook up a VSS so the PCM knows how fast the car is moving. This also helps with control of the cooling fan(s).

 

I ran into this same problem with the T5 I've got, it has a mechanical speedo output and no VSS. I hooked one up inline with the cable going to the speedometer.

 

Wheelman