12 valve won't keep fuel pressure

My 12 valve will not keep fuel pressure I have checked the return line bolt I have checked the shut-off solenoid I've checked all the lines for air leaks I've done everything I even bypass the mechanical lift pump and put on an airdog I have done everything I can possibly think of and the truck will not run it gets up to 50 psi on fuel pressure but once you stop turning the engine over it instantly drops back to zero I'm at a loss and really need some help if anyone has any kind of tips or know anything I can do I would really appreciate it I use this truck for my little mechanic business I'm not really big on 12 valves but I know the basics of a diesel engine and it's got me at a loss please help

 

There is nothing in the system to hold fuel pressure when the engine is not running with a stock lift pump. The Airdog, as you have discovered is a poor substitute for the mechanical pump. A couple questions. Where are you measuring the fuel pressure at? The most common/logical place is the inlet to the p-pump. Which overflow valve are you using and how long has it been installed? Is the fuel shutoff solenoid working properly?

 

Never heard of a vehicle that holds fuel pressure with the engine OFF.
Do you have problems down the road? Starting problems? Not enough pressure with the engine running at WOT?

 

I'm with GAmes, it sounds like the fuel shutoff solenoid. With the key on pop the hood and lift the linkage from the bottom and it should hold in the up position by a magnet. Truck should start while it is up. I'm assuming it is a crank no start situation?

 

How did you check the shut off solenoid? Are you sure it is working? It takes two people, one to hit start and one to watch it. It has two circuits, one to pull it up (start) and one to hold it (run).

50 psi is way too much fuel pressure. You will blow seals/gaskets on the low pressure side of the injector pump.

 

Not on the P7100.

 

Okay, I stand corrected. The input shaft seal on the VE has low pressure fuel behind it. After I think about it, the P7100 has just oil I think. They kinda look like a really small Cummins engine

 

I cheaking the pressure on the return line to the filter housing

 

Yes sir I had a friend turning the starter over an everything is working properly

 

As others have stated, you will only have fuel pressure when the engine is turning over. Will the truck not start or are you having other issues?

 

So you have a no start condition. When/how did it occur? Driving down the road and quit or just a no start one morning? Any hiccups before it happened?

 

It started in idled for about 15 minutes I backed out of the driveway and went to go put my foot on the gas got to about 5 mph and just instantly shut off hasn't started since

 

It just won't start sir it'll crank over and crank over it just won't fire

 

I have no clue how that could possibly give you an accurate fuel pressure reading. Then again, I don't know how an Airdog is plumbed. Somewhere between the fuel filter and the input to the injection pump is where you need to check fuel pressure.

You didn't answer about the OFV.

Just as a FYI, to check the FSS only requires one person. Attempt a start, leave the key on and check to see of the FSS arm is up. If it is not you push it up. Does it stay or drop back down when released? If it drops back down check the #9 fuse. If it doesn't go up when you try to start the engine it is the relay or the fusible link.

 

A warning, you can burn up the solenoid with continued long cranking. The pull-up side is designed for momentary operation with a quick start. If you need to crank it a long time, unplug the solenoid and tie it up.

Has there been any work done to it lately? Have you cracked open one of the injector lines to see if your getting fuel to the injectors? It’s really a simple system. You need to verify you’re getting fuel to the injection pump, then that you’re getting fuel to the injectors, and report back.

 

To start even 15psi would be good, all it needs is flow into the pump chambers.
Most likely you have air in the system, like Scott says.

 

He had it running and it died. That isn't an air in the system symptom. It is the symptom of running out of fuel.

 

Could be pulling air in through a leak in a connection or cracked fuel line. Sound like all lines need to be checked also.

 

"Air in the lines" gets more accusations than it deserves. It is more than likely different with computer controlled fuel systems but once a 12 valve gets started it will run until it is out of fuel, even if the is air being sucked into the lines. Running out of fuel could be a faulty fuel shutoff solenoid, or the rarity of the shutoff lever shearing the woodruff key that positions it to the shaft. To complicate things the OP has wasted his money on an Airdog which could mean all my experience means nothing.

 

Gary's correct. My questions were pointing at making sure he has fuel to the IP and the injectors. If he's not getting fuel to the IP, he has a lift pump of pickup issue. If he has fuel to the IP and not the injectors, his shut-off lever isn't getting pulled up. If he's getting fuel to the injectors, he has a timing issue. Pretty simple system.

The fellow I sold my truck to is having a no-start problem. He's called and texted me NUMEROUS times. The truck has been sitting a few months and then was vandalized. He's working alone and can't grasp how to pull the shut-off lever up and tie it off. He couldn't find the primer button on the lift pump. He has "friends that know about diesels" telling him his in-tank pump is bad....