Lb7 losing prime

Hello everyone im new to this forum. I have an 04 gmc lb7 which i bought new it has 260,000 on it. New bosch injectors around 20,000 miles ago. Truck was running fine up till about 3 weeks ago i drove it to work no problem went to start to leave and it turned over really slow . Turned ignition off tried again started right up. No issues for 2 weeks parked it at my house it sat for a couple of days and now it wont start at all. I have serious amount of air in system replaced fuel filter housing new filter new wit sensor. No visible fuel leaks. No fuel in oil. I ran fuel line directly into filter housing from fuel can to eliminate everything back to tank and still getting air and no start. Primer pumps up firm soon as it turns over its soft again. I dont have a scan tool so i guess im asking which way to go now. Thanks for any help.

Welcome aboard!

A scan tool won't tell you anything you don't already know. External fuel leaks require the old fashioned approach. Unlike modern technicians, you're gonna get dirty.

The fuel system, from the tank to the high pressure pump, is entirely suction. The OEM system has no inline lift pump. If you have a small fuel system leak between the tank and pump, it isn't likely you'll see it under normal conditions. Any leak or weakness in the system is very easily exploited. A failed fuel cap vent will also exaggerate any leak condition, as will any fuel line restriction for the same reason. A common leak location is the coupling and line at the top of the tank (of course, the least convenient). If the condition is persistent and serious, and you can't locate the problem easily, just get the bed off so you can get a good look and access to everything (it's much easier than it sounds).

Pumping the filter primer only pressurizes the fuel between the filter and pump. Once the engine is cranked, even if it doesn't start, that pressure is very quickly reduced to suction again. The fuel system is very tolerant of air intrusion, but the pump is not self-priming. Once the prime is lost, you have to re-prime, as you know. Simply pumping the primer usually does that, and it will run with no further issue until the next time. If air intrusion is causing a running engine to stall, the leak isn't small.

Locating an external leak isn't difficult. Since you're replacing the fuel filler cap anyway (step one), modify the old cap with a tire valve stem, then use shop air to apply about 5 PSI. Any significant leaks will show themselves very quickly.

Thanks for the reply wouldn't running fuel line directly from inlet side of filter housing and into a fuel can of diesel eliminate any leak from tank to filter?

It wouldn't eliminate it (having more than one leak wouldn't be surprising, and they add up), but would indicate a leak between your fuel can and the pump if the condition continued. Was your fuel can above or below the filter? How are you determining the amount of air? A leak between the filter and pump may cause loss of prime, but wouldn't cause air in the filter unless the fuel was being pulled back to the tank when not running. You need to pressurize the system and find the leak. If it's sucking in a lot of air, it will leak out at least some fuel under pressure.