Hello, My Idle has been going from 400 rpm to 800 every now and then at a stop. It used to be 400 all the time. Any ideas?

Thanks Eric

just as a guess... maybe a fuel filter problem. or IP metering valve?

400 RPMs sounds really low. 80's spec at warm idle was 650RPMs and 800RPMs cold. I was told 90's warm idle was 700RPMs.

is the engine stalling out when it drops that low?

Aire

A Slight Air Leak In Your Fule System Could Cause A Varyation In Rpms. But I Would Check Out Your Fast Idle Solinoid, Mt Truck Did The Same Thing As You Decribe, So If You Notice Your Idleing High Just Grab The Adjust Knob On The Solinoid If You Cant Move It Back And Forth - Its Energized And It Should Not Be If The Truck Is A Running Temp. I Just Ran A Toggle Switch To Mine, Problem Solved

What rpm is the engine supposed to idle at?

850 RPM cold, 650 RPM warm.