cyclone3565

My first post. I am sorry we need help.
We have a 96 Chevy Tahoe with a 350. It has been running absolutely great, getting fantastic mileage with it 105,000 miles on it. About two weeks ago, we noticed that the idle seems to have increased about 200rpm and when you take it from Drive into Neutral or Park there is a momentary jump almost to 1,100 before settling back down around 800-900. It used to be around 6 and 7. Also the mileage seems to have decreased substantially now being in the low teens. While sitting at a stop light the engine, due to its higher idle is surging. Any help would be greatly appreciated.

John86

it might be your idle air controller

cyclone3565

Thank you for your reply. What is an idle air controller and is that something I could replace myself, i.e. where is it located

John86

The VCM controls the idle speed with an Idle Air Control (IAC) valve to a calculated desired RPM based on the sensor inputs and the actual engine RPM. The VCM uses 4 circuits to move an Idle Air Control (IAC) valve, allowing varying amounts of air flow into the intake manifold. In order to increase the idle RPM, the VCM moves the IAC valve out, allowing more air to bypass the throttle plate. In order to decrease the RPM, the VCM will move the IAC valve in, reducing the amount of air bypassing the throttle plate. A scan tool reads the VCM commands to the IAC valve in counts. The higher the counts, the more air that is allowed to bypass the throttle plate (higher idle). The lower the counts, the less air that is allowed to bypass the throttle plates (lower idle). This is a type B DTC.

John86

yes u should be able to change it yourself. i have a diagram of it but it won't let me copy and paste it to this space. it's located on the front of ur engine next to ur throttle position sensor