P0507 cant get idle down

cameronc · April 22, 2016

1997 c1500 5.0 vortec 5 speed. I just got the truck on the road. I bought it needing some work and got it all lined out except for the idle. It starts and idles normal but when I drive it the rpms doesn't drop between shifts. And if I'm rolling down the road it stays around 2500rpm. I can put the truck in neutral while rolling and it will stay at 2500 at least but I can come to a stop and as soon as the wheels stop rolling the idle will come back down to normal??? What does moving have to do with the idle? I've replaced all o2 sensors, new injector, fuel pump, new distributor, plugs, wires, iac valve, pcv valve and new hoses. I can't find any vacuum leaks anywhere. I have soaked the intake down trying to find an intake leak with no success several times and I can't feel it pulling any vacuum through the valve covers either.

cameronc · April 29, 2016

Well I'm not sure what exactly fixed it but swapped throttle body assembly out today and it's back to normal so either the tb itself was bad or the iacs I tried were junk. Either way I'm back in the saddle, now get a new converter soon and get rid of that annoying light. Thanks guys.

cameronc · April 22, 2016

Yes I have replaced iac and cleaned throttle body also. It throws a p0507 code and I don't think it's normal because when it's in gear I have to ride the breaks to keep it from speeding up

Doug_Scott · April 22, 2016

Don't know if that is "normal" or not, but do know that at one point in the emissions control history they did strange things with manual transmission vehicles in order to get them to pass. Holding the engine speed up during shifts, and disabling engine braking were a couple of them. The fact that once the vehicle stops moving it returns to normal idle speed leads me to think this is part of the design, as strange as it sounds. Don't know if the 2500rpm is the spec though. I would expect it to return to idle once clutch is disengaged for a set amount of time while vehicle is moving.

Is it setting any codes? Idle speed is controlled by the IAC stepper motor. Try cleaning the IAC ports and stepper motor.