It could leak slightly which could be enough. It won't leak much since the pump bolts right up to the block. They could ALMOST be made without the backing plate.

 

I purchased the 340 also after reading up on it it appeared to make sense.

 

Not if your phasers are Dorman.......if they are Motorcraft, then yes, it could be the pump. I have read a ton about oil pumps. The whole "back-plate" flex theory is mute. It cant flex as Stephen said, its mounted to the engine block. Go with Melling or OEM.

 

The tensioners have a small bluish gasket that gets blown out and reduces oil pressure. I would not use oem plastic tensioners use the old style metal ratcheting type .I failed to do that but if I go back in I will . They don't have a gasket and will maintain some pressure even when oil pressure drops for a second .
There are no gaskets in oil pump at all, none on mounting either .

 

Backing plate flex is bogus on the 5.4. Just a marketing ploy for others to get you to buy their pump.

I didn't see as I was glancing through but if you mentioned ot already, I'm sorry... Did you clean the pickup tube/screen when you did the timing job?

 

UPDATE:
Drained ALL the oil into a clean pan and investigated it, no metal material so I feel pretty good that the few seconds it ran with out oil did not kill it. Also pulled the driver side valve cover and investigated lobes (look to have normal wear). Pulled VCT solenoid (newer model) and didn't see any debry in the little mesh screens it has.
Going to keep tearing down in to it and investigating as I go.

@nrivera04 Yes I did clean the pickup tube, removed a couple pieces of plastic. Not sure how long they had been there so this also leads me to think premature oil pump failure.

@redfishtd That is a good call out on the tensioners. What do you mean by "old style metal ratcheting type"?

 

I just had my timing done by a trusted mechanic with all OEM parts and Im pretty sure the tensioners were metal not plastic. I'll ask him to find out for sure.

 

F6AZ-6L266-CA and F6AZ-6L266-DA...Tasca lists these as a direct fit for the 5.4 in this generation of truck. There's plenty of discussion across the web concerning the use of these and the benefits to them...

 

It was messy but I felt that was the only way to help the initial start up,I also filled pump up before mounting. I felt that the oil above the oil pump would help it . There is no reasonable way to prime the system . I debated filling pump with grease or vasoline . I tried to turn motor over 5 or 6 different times to prime it ,that won't do it, not enough rotation .
But I had hand oiled cams and lash adjusters.
I'm sure others have worried about this too.

 

You can unplug the crank sensor then crank the engine, or just hold the gas pedal to the floor while cranking, both won't let the engine fire and will prime the system. Sure, you're rotating components without lube, but without the stress/strain of combustion. I also agree with filling the pump prior to install. You could fill the oil filter too...