Replaced the fuel lift pump on my Cummins ISL

Tom/Barb wrote:
I'm curious, to have air drawn into the system there must be a vacuum or a lower pressure than atmospheric, to cause this there must be a restriction on the suction side of the pump.

Have you checked the tank venting?

this could cause your 30% low pressure readings of the lift pump.

Good question and having had a truck with dual tanks where the one with the gauge got plugged it's something I considered. It took me two tries on this truck to figure out that the other tank which fuel was pulled from, was empty. After sitting for several hours it had enough fuel from the plugged tank to let you go a mile before quitting.

The third step in the Cummins trouble shooting guide was to measure the amount of vacuum on the fuel line before the pump (10 inch of MG). This was a test that would take more than $5 of parts from a hardware store.

This problem started in the first mile after the previous day of 400 miles with many long pulls without a single hiccup. There was about 30 gallons (out of 120) in the tank and even if it was plugged it would take more than a mile to create enough vacuum to affect the pressure.

The key was after putting the clear tubing in the line you could see the bubbles. I do not like that taking a fuel line off that was tight and putting it back on stopped the bubbles and it's the reason I left the clear tubing inline. I had to unhook from the CG and put everything away so I could put it through four hard (100% power) accelerations with the last one up a fairly long hill so twice as long as the first three. I did not want to wait until tomorrow to see if I had full power.

Wish me luck as hopefully there WILL NOT be more to this story.