Need help...
 

Okay i bought a 05 cummins last night and the truck has 210k milea and its bone stock engine. When its in park and you rev it up the RPM's go down very slowly so it sounds like when you rev it, it's whining out. Anyone know what this is or could be?? The oil doesn't have diesel in it.

How high are you revving it (1k,2k,3k)? It's not going to drop instantly since there is 20k+psi of rail pressure - does it get stuck or steadily continue to drop? Make sure you are in neutral revving if it's an auto :fly:

I can rev it feom 1k to red line and its just a constant slow drop of rpms

As long as it stays dropping consistanlty then I wouldn't worry about it much- like I said before with all that pressure built it won't drop instantly. There are YouTube videos on injector hang where it'll get stuck above idle which is an issue but doesn't sound like your situation- those injectors been replaced yet?

Up to 150k we dont know from 150k to 210k no they havn't so im sure the injectors have 210k on them

Not sure your plans with the truck but probably want to start saving for some! Mine had little/no blow by at 217xxx but new sticks really woke it up!