LMM no start after head gaskets

LBZ_Baker98

Member
Jul 8, 2016
205
2
18
Texas
Have a LMM we just finished head gaskets on. I don't know if I am just tired and over looking something or what. Truck will crank but won't start. only codes are IAT and Coolant temp sensor. Truck shows 5700ish psi of desired rail while cranking and only 71psi actual. Actual PSI does not fluctuate at all, stays steady at 71PSI. I unplugged the FPR thinking desired would shoot up to 26,500 PSI and it didn't. Rail pressure stayed the same on desired and actual even with FPR unplugged. I ran a hose from a fresh can of diesel straight to the line going to the CP3 to rule out any issues with the filter head or with the lines going in to the fuel tank. Still no start. Any ideas are greatly appreciated!
 

2004LB7

Well-known member
Dec 15, 2010
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Norcal
Disconnecting the fpr will not change the desired. If the engine was running and cp3 was making pressure as it should and you unplugged the fpr then actual will jump to max.

If you unplug the fuel pressure sensor then you should see the actual jump to max right away running or not. Desired is not something you can really effect from unplugged stuff.

First you need to determine that you are priming properly. Unhooking the hose to the cp3 to see if you have fuel is good and all but doesn't tell you if the primer bulb is provided good pressure and no air is getting in. Does the bulb pump up hard after pumping for a bit? Does it immediately get soft when you crank it over?

Have you cracked open one of the injector lines and tried to get fuel out through priming and cranking a few times?

How much return are you getting from the cp3 during cranking?
 

LBZ_Baker98

Member
Jul 8, 2016
205
2
18
Texas
Disconnecting the fpr will not change the desired. If the engine was running and cp3 was making pressure as it should and you unplugged the fpr then actual will jump to max.

If you unplug the fuel pressure sensor then you should see the actual jump to max right away running or not. Desired is not something you can really effect from unplugged stuff.

First you need to determine that you are priming properly. Unhooking the hose to the cp3 to see if you have fuel is good and all but doesn't tell you if the primer bulb is provided good pressure and no air is getting in. Does the bulb pump up hard after pumping for a bit? Does it immediately get soft when you crank it over?

Have you cracked open one of the injector lines and tried to get fuel out through priming and cranking a few times?

How much return are you getting from the cp3 during cranking?

Primer bulb gets hard after about 4 or 5 pumps. Goes soft after cranking. I ran a line straight to metal line that goes to cp3 from a can of fresh diesel and truck still wouldn't start.
 

2004LB7

Well-known member
Dec 15, 2010
4,417
575
113
Norcal
Sometimes you need to provide pressure on the fuel to get the pump to prime. The gear pump on the back of the CP3 doesn't pump air very well so you may need to pressurize the fuel. Gravity feeding a dry pump may not be enough. Another help is relieving the pressure on the output of the pump so it can prime easier.

Try putting the fuel system back together then loosening one of the injector lines and priming and cranking until you get some fuel out of the injector line. Then tighten it back up and try again. If you can't get it after doing this a few times, double check that you put the fuel lines back on the CP3 correctly and measure the output on the cp3 return while cranking after properly priming it
 

LBZ_Baker98

Member
Jul 8, 2016
205
2
18
Texas
Sometimes you need to provide pressure on the fuel to get the pump to prime. The gear pump on the back of the CP3 doesn't pump air very well so you may need to pressurize the fuel. Gravity feeding a dry pump may not be enough. Another help is relieving the pressure on the output of the pump so it can prime easier.

Try putting the fuel system back together then loosening one of the injector lines and priming and cranking until you get some fuel out of the injector line. Then tighten it back up and try again. If you can't get it after doing this a few times, double check that you put the fuel lines back on the CP3 correctly and measure the output on the cp3 return while cranking after properly priming it
Okay, I got it figured out. I'm almost to embarrassed to say what it was but let's just say 1 loose fuel line will cause it not to build rail (obviously) lol. Like I said must have just been tired and frustrated or I am an idiot. None the less, thank you for the advice and all your time! Much appreciated!