I have a 1999 Dodge cummins 5.9 24V
Truck starts but has not throttle until the fuel return line is pinched.
Once it is pinched off it seems to run fine good throttle response.
codes are 0251, 0252, 1688
Please help:

 

Probably a bad overflow check valve....the excess fuel return line has a check valve to allow some pressure to build up. If your OCV has failed open, then it just bleeds by very quickl. Bad for your Injection Pump once running if the pressure at the IP is below 10 PSI. Yes - it might run, but it can quickly kill the IP.

If you don't have a fuel pressure gauge measuring input pressure to the IP, get one - ASAP.

 

I agree with what vineyardm said. You'll also want to upgrade your lift pump pretty soon too.