Motor Idle on Catepillar 330HP

Air on the suction side of the lift pump will cause erratic idle.

If you have a Caterpillar secondary fuel filter/housing with manual primer pump, next time this happens, turn off the engine. Go back and turn the manual primer pump kural nut counter-clockwise to open. Pump it. If easy to pump (air is compressible), you likely have air in the system. If hard to pump, all is well (fuel is NOT compressible). Be sure to turn the knural nut clockwise to lock.

If not Caterpillar secondary fuel filter/manual primer pump, do you have a clear bowl primary filter? Look for air in it, indicating a problem between tank and primary filter.

The dash tach might not be calibrated all that well at idle RPMS. I'd start it, or have someone else start it, and then go outside to the rear and use your ears to hear if it's running rough.

You said the tach will move from 500 to 800 rpms - does it then stay at 800 or go up and down?

My C7, 350 HP, same engine, doesn't go up and down, stays about 700 rpms.

Bill