I just did an experiment this morning, on how much air the CP3 has to deal with under "Normal" conditions.
My CRD Libby has been on a lift since September, and the fuel in the tank was put there in Redding CA in mid-august, during 100F+ weather, so it's KNOWN summer fuel mix, and right here right now its 20f and dropping. Expected to hit +5F tonight. Not fuel I want to deal with when starting this thing.
I borrowed a Bosch fuel pump from an old Ford 6.9 diesel. (Good fuel pulling ability) I unplugged the fuel line to the CP3, and clamped that line to the inlet of the Bosch, and ran an outlet line to fill a couple of Jerry cans that were available. With this setup, the Bosch would pull the fuel from the tank all the way through the system exactly as the CP3 would, and I could see how much air is introduced in a system that had no known problems with the fuel system.
I powered the Bosch first to 6v 2a setting on the commercial charger, so it was turning very slowly, perhaps 2gph estimate, and the fuel was drizzling, but clean and clear, then switched to 12v 2a, the fuel was flowing slowly from the line, rather than drizzling but still clean and clear.
Then I switched to 12v 10a setting for normal operation, and the fuel was flowing at more like 15gph, but was suddenly so full of tiny air bubbles that it looked more like milk than fuel. Not just bubbles on top like one sees when filling at a station, but bubbles clear through. I ran some onto a sheet of plastic that was nearby, and it took several seconds for the bubbles to come to the top, combine, and begin to dissipate.
This system was functioning without problem when the vehicle was running, so it's likely this is normal for a vehicle with 100kish miles on it. There are so many possible points of entry, from the filter head, the priming pump, the connections in the lines to the tank, even though there is no evidence of fuel leakage in any of those places, when under vacuum, air can still get in.
With this evidence I can see where under this condition, the priming pump at the filter head really is a necessary feature when there is no in-tank pump. With the cp3 pulling aerated fuel through the system, and the system is shut off, the air in the fuel in the filter rises to the top. if sufficient, when this larger amount of air hits the supply pump in the cp3, it loses it's ability to draw fuel through. we have to do it by hand with the priming plunger at the filter head.
I guess one moral of this story is IF you choose to use a lift pump that is NOT located in the tank, get it as close to the tank as possible, and take every measure you can to seal the connections / lines between the tank and the pump against air entry due to vacuum.
With the 20second supply the pump puts out when you turn the key on, it would move any air in the fuel filter past the cp3 supply pump and enable the vehicle to start and run normally.
|
