Air in Fuel Injection

[Paveltolz]

Stock fuel caps are designed to have a slight vacuum.

Correct. GM designed the fuel system so the tank maintains a low pressure state. The low pressure is created by the LP pulling fuel and then pushing it to the injection pump. That low pressure assists in pulling the return fuel back to the tank.

FWIW, I recently learned that the IP’s internal housing pressure at the return output is + 7psi which is why one needs to have an unrestricted return line (no kinks, bends) as a full restriction will eventually start things going south in the IP.

I don’t have the clear line so I can’t observe what happens at engine shut down to confirm whether or not if that +7 psi along with the tank’s low pressure empty the return line when the engine is off. What may look like fuel returning to the IP (doubtful) could very well be fuel getting pulled to the tank.

To me, gotta be pre-injection pump, very slight, and yet, obviously enough to cause some issues until fuel can fill the voids/displace the air.

 

[Will L.]

That is not air in the system. That is fuel draining out of the line.

 

[Paveltolz]

That is not air in the system. That is fuel draining out of the line.

If you’re referring to the clear line void of fuel, correct.

 

[Will L.]

Yes I am. Problem is the ip is supposed to be the high point other than the filter bleed. And if the filter is sealed properly- the ip return could drain out either way.

I’m wondering about easier ways to determine which half. Cut the clear line in half, connect with double barb fitting. Then when it is shut iff for the night, pinch both sides, disconnect them from each other and leave both ends capped.
See which side drains. If both do- there is a problem both ways.