[Ron Nielson]

From here: Chevrolet-GMC Diesel Diagnostics

No Start or Hard Start

1. Excessive fuel restriction, check or change fuel filter
2. Use vacuum gauge, check the suction, you should have no more than 5 inches Hg at WOT (wide open throttle) or 7-8 inches Hg under load. If you have too much suction, after changing the fuel filter, the restriction could also be the sock in the tank. Too little vacuum (less than 2 inches Hg) means that it could be sucking air.
3. Check for air in fuel system, install clears lines before and after the filter housing to check for air in the lines.
4. Confirm actual versus desired rail pressure, even under crank no start conditions
5. If the above check good, then it comes down the following.
   a. fuel injectors (see injectors for more diagnostic information) Buy 6.6 Duramax Fuel Injectors
   b. high pressure injection pump – CP3 Buy Bosch CP3 Injection Pump for Duramax
   c. Fuel pressure regulator, check to make sure it is not stuck. Buy 6.6 Fuel Pressure Regulator
   d. fuel pressure relief valve (high pressure limit valve), check to make sure it is not leaking into the return system when rail pressure is 160 MPa. Buy Duramax 6.6 Relief Valve
6. Before condemning the high pressure pump you need to make sure there are no high pressure fuel leaks. Use the EN-47589 (same block-off caps as #9011 SPX Miller tools) cap set to cap off the injector rail to isolate the injectors from the injector return system.

 

[Ron Nielson]

The problem, it seems to me, could be the Fuel Pressure Regulator (FPR), or could be a bad new fuel (rail) pressure sensor, or bad ECM , maybe other things.

You could disconnect the FPR and see if the fuel pressure goes really high at idle, 23K 24K or so, whatever LB7's produce. Then re-attach it and see if it comes back to commanded pressure. Watch your FRPS and see if it looks like is responding to what you are doing to FPR, detaching, attaching, etc. And then look at the attachment and do what testing you can do to figure out what's happening.

You could call Lincoln Diesel Specialties and see what they suggest, or maybe Oregon Fuel injection. I think that both of these places will give you their best advice.

 

[Ron Nielson]

Rail Pressure is getting up around 18,000 while cranking when 6,000 is desired.

The truck should never have 18K of rail pressure while cranking.