I have a GMC 2500HD w/ a 6.0 Motor w/ 130,000 miles on it. Resently the RPM's began to flutter while it idled. Being that I hadn't had a tune up yet I changed the spark plugs, Wires, fuel filter, and PCV Valve. Now it s running terrible. Shutting off a stop lights and even when pulling into the drive way if the Rev's get to low. I have been told it could be anything from the gapping in the plugs to the fuel pump! I was even told by someone that waiting so long for the tune up may have damaged the engine beyond repair? Any thoughs would be great! Thank You.

 

it runs fine on the highway?..just idles too low?..\

a couple things...gm has a software update for low idles on those years...a dealer must do it and cl;ean the throttle at the same time.
Of course,it could have a bad idle motor as well