Low idle in drive when stopped?

[sschaefer3]

My normal idol is 650. In park and neutral, a steady 650. If you shift into drive or reverse and just sit there and hold the brake, it will go down to 550, 525. Does not die just slows down. High idol works, it comes on then the truck is cold and goes off when warmed up. This is after I have been driving it a bit. This just started happening last night and it's doing it still this morning. It was fine before. I have a tiny tach so I know the RPMS.

Is there some vacuum line that kicks the idol up when you stop in drive? Maybe it's shot or rotted?

 

[m38inmaine]

Vacuum only there for shifting, has nothing to do with engine rpm.

 

[Barrman]

If it just started after not doing it before. I would suggest a fuel filter change and a look at the lift pump.

 

[sschaefer3]

Well I figured it out. I have an aftermarket throttle cable and it must have stretched very slightly. That day I did drive from Phoenix to Flagstaff on the I17 and it is all up mountain grades. I had it floored a lot, but never over 3200 rpm. I bent the bracket upward that the cable connects to at the IP 1mm at the most and it's back to normal. I wish you could still get a factory GM throttle cable.

 

[Skinny]

Sounds like your idle speed is being set because the throttle cable won't fully retract. There should be a throttle stop screw which sets idle speed. At full rest position (idling), the cable should have a bit of slack in it.

My M1031 used to idle so low that the Gen lights would glow. I bumped it up slightly higher than stock to eliminate that and help with power steering assist since I have 37's. That extra 100 or 200 rpms helps just enough with both problems.