no fast idle on cold engine

[stangbangerrick]

Had my 95 gt 331 stroker dyno tuned last May and it ran just great (SCT chip) but about a month ago I noticed that it did not have the fast idle after startup on the cold engine. Put on a new IAC no change. Check all the other sensors and they are ok. Has anyone ever had this happen with the SCT chip?

Tomorrow I will pull out the chip to see if it will fast idle on cold engine start up.

 

[HISSIN50]

Is the wiring to the IAC ok?

 

[stangbangerrick]

Is the wiring to the IAC ok?

Yes the wiring is ok, with key on the engine not running its getting 12V and when the car is running its getting 14.3V.

 

[HISSIN50]

The IAC uses PWM - the power supply is constant and the ground is what is modulated.

 

[stangbangerrick]

The IAC uses PWM - the power supply is constant and the ground is what is modulated.

Are you saying that I should check all my ground wires?

 

[bayellowstang]

I would check the ECT too. When mine went bad it started thinking the coolant was 170 when it was only 50 degrees outside. I can datalog my coolant and air temps with the tweecer that's how I knew it was bad.

The big one by the thermostat is for the computer the one on the other side is for the gauge.

 

[HISSIN50]

Are you saying that I should check all my ground wires?

You can't do much unless you have an oscilloscope.

The idea of checking ECT calibration is a good one. Also pull codes.

 

[stangbangerrick]

I would check the ECT too. When mine went bad it started thinking the coolant was 170 when it was only 50 degrees outside. I can datalog my coolant and air temps with the tweecer that's how I knew it was bad.

The big one by the thermostat is for the computer the one on the other side is for the gauge.

Changed the ECT and still no fast idle on cold engine after start up. even the old one was reading 24 ohms at room temp on cold engine. Checked the new ECT 24 ohms at room temp and after engine was at operating temp it had 2.5ohms which is within limits stated in the repair manual.

 

[HISSIN50]

You're scale in the readings is in K-Ohms, correct?

 

[bayellowstang]

Did you try pulling the chip yet?

 

[stangbangerrick]

Did you try pulling the chip yet?

I pulled the chip and still no change in the idle on cold start up, so I put the chip back in. Pull the plug to the IAC and hooked up a 9 volt battery with some tiny jumper wires and started the car and it had a fast idle of 1400 rpm. With the IAC off the T/B and the plug connected you can turn the key to on and can see the valve open and then it will close within a second.