The truck just had a new lift pump (airtex style) and injection pump installed when brought to me, I found the airtex pump to be bleeding fuel back to the tank and causing a hard start hot every time!, I replaced the airtex pump with a delphi style pump (oem) and solved the issue, or so i thought, two weeks later they bring the truck back to me and it does the same thing but only when it wants to, like driving to store and cutting off, going in getting a drink, come back out, hard start, but if you get it hot, cut the key off and crank right back up no problems, the fuel pressure is draining back to the tank obviously! does anyone know if the return check valve can cause fuel drain back to tank? any help would be greatly appreciated.

 

Sounds like an injection pump or as you said loss of prime, these trucks rarely suffer from loss of prime issues. Even less likely with intank pumps. Whats the fuel pressure at now, how many mL in 10 seconds, and with initial lift pump start up is there any air in the fuel ? A failed overflow would cause a low fuel pressure issue over a loss of prime

 

You talking about something other than your 96, obviously?

No it will not drain back because of the return on injection pump if that's what you're wondering. Do you have a fuel pressure gauge? Possible your getting air in the lines somewhere, maybe?

 

With the truck idling no load the pressure is about 12 psi, and wot no load about 7, i havent checked the flow yet, and this isnt an in tank pump, this one still has the block mounted pump, i just changed it from the cheap airtex style to the delphi oem style. How do i check to see if it is the overflow causing drainback other than replacing it?

 

You could remove it, and visually inspect it. Unless you can find a way to regulate 18psi towards it, it is a visual inspection or replacement part. If memory serves me correctly, it is less then $30.

 

I seriously doubt its the overflow. The more I read about your situation the more I feel confident you have a vp issue. To verify you could do a bucket test using clear tubing, and command the pump on with a scan tool or bump the starter, and view the first 30 seconds of fuel flow. We use a bit more advanced method but this would be a simple way to check without a expensive gauge set.

 

Agreed. Classic symptom of a cracked diaphragm is hard start when hot. You can try, and probably waste the money on a overflow valve, but I bet your issue is much larger.

 

It's would be easier to buy an overflow than confirm with the test I explained, that's for sure.
Our gauges have multiple inlets and a way to regulate the the output to the bucket. So with a gauge showing filter head pressure,(eliminating the line between the filter and vp, using the gauge manifold as fuel line) one at the vp, we verify we have equal pressure on both, lock the bucket line down and watch the pressure drop. With the bucket line being higher than the vp, and the filter side locked down, any overflow leak will be indicated in the level in the bucket line lowering. You effectively created a water (fuel in this case) level.
One more edit and I'm done I swear .
The erratic no start and as Mr Williams said the no start when hot is The true giveaway. The chances of intermittently having a loss of prime issue is extremely unlikely.

 

I thought the same myself when they initially brought the truck to me, but they had just had the vp and the lift pump replaced and this hard start hot issue didnt start until after the new pumps were installed, bad pump from reman possibly?

 

I've got a shop pump for that, eliminate the entire system and determine if you can recreate the issue again.