[diesel_john]

attach a high impedance volt meter between the output signal at the IAT and a good ground and watch it in real time as you drive. it should not change fast, if it does also check the input volts to the IAT, should be constant 5vdc, if it changes problem is the input or your ground, if constant input, then problem is the IAT.

i have never used a scan gage, but think there would be a delay between it and the engine.



is that as clear as mud?
many times the prob. is self inflicted, corrosion, oil, bent pins, broken wires, bad ground somewhere, replacement sensor out of spec., high amp. radio messes with everything else electronic, someone else 'worked' on it etc...