1987 R1500 Tbi 5.7

[murdrd87]

Question guys? I have a 1987 R1500 tbi 5.7, I have a holley intake and throttle body on the motor, everything else is stock. I have replaced all the sensors on the truck. I eliminated the egr valve over 8 years ago when i bought the truck because it did not affect the emissions, I pass emissions everytime no problem. What would cause the truck to idle low when warmed up, if I come to a redlight or stopsign and am in Drive it wants to idle down real low, put it up in Park and idle is ok. Fuel Pressure is set to 12psi and timing is correct. I know holley throttle bodies are crap, hadn't been able to get a stock one back on. Also, is that intake and throttle body to big for that motor. Any help is appreciated. Throttle position sensor is set at .65 volts at idle

 

[chengny]

From your description, I would start off with inspecting/testing the P-N switch. GM explains it's function differently than the way I like to think of it. To me; if the transmission gear selector is moved to any position other than P or N while at idle, the ECM opens the IAC valve slightly more than when in P or N - to compensate for the static drive train load. GM says basically the same thing but kind of backwards.

Anyway, here is the diagnostic chart/test procedure;

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