It would be at the fuel head inlet. That causes the fuel head to hold vacuum and release air bubbles from the fuel. The air accumulate at the top near the check valve unable to escape until bleed off. Some diesel fuel don't have much air in it so no problem. When I'm on the west coast I never have a problem, here on the east coast I have problems so I added a lift pump to prevent the fuel head from going under vacuum. Without the check valve the fuel in the lines will drain back to the tank causing starting problems.

Is there a check valve in the Liberty CRD's fuel suction line, perhaps like the "drainback check valve" shown here: http://www.glacierdieselpower.com/image ... xposed.pdf

Wobbly.

On another thread it was determined the CP3 pumps have "feature" where they bleed the air themselves before starting. It was in a thread by DRHRV as I recall who's injector pump died after he updated to a CAT filter. It has been found with the 05 and 06 KJ CRD's they relocated the fuel filter head higher then previous models and that seems to be when air trapped in the filter heads became a problem. Air problems have been traced to every part of the fuel system from the push on connections on the tank to a loose hose on the fuel line at the CP3 The hand prime pump has a valve in it. Before adding a pump I just put a plastic tube on the bleed screw and pumped till no air bubbles came out without closing the bleed screw each stroke. I've had a lift pump for about 9 months and have not had a drivability problem since. Air problems showed up the first day I bought it less then a mile from the dealership. Early VW's had a check valve in the tank and it caused cold weather problems and the cure was to remove them.

Joe

So if the check valve in the filter head leaks wouldn't fuel empty from the line from the filter to the IP and exit the pump through this valve? Air from the tank would flow back to the filter.

Wobbly

As I see it there would have to be a air leak somewhere above the check valve to allow a leaky check valve to let the fuel run back into the tank.

But there does not have to be a air leak at all. Just air seperating from hot fuel then collecting in the hightest place in the fuel system, the filter head. It's been talked about before that with the high pressure of the common rail system he temperature of the fuel returning to the tank is hot and the air seperates. Also the fuel returned to the tank is dumped back into the basket in the tank where the pickup is. It's also been determined that at a certain fuel temperatue the engine power is cut back. It's a vicious circle

I see the problem thusly:

1. The engine won't start because no fuel is being injected.
2. No fuel is injected because rail pressure is too low.
3. Rail pressure is too low because there's no fuel is in the high pressure pump.
4. No fuel is in the high pressure pump because no fuel is in the low pressure pump.
5. No fuel is in the low pressure pump because no fuel is in the piping from the fuel filter to the low pressure pump.
6. The fuel that was in the piping from the filter to the low pressure pump travelled back to the tank because of gravity.
7. Air travelled from the tank to replace the volume of fuel that travelled to the tank. Some of this air ends up in the fuel filter.
8. With enough battery capacity, and assuming that the starter didn't burn up, the low pressure pump would eventually pull fuel from the tank and the engine would start. It's a positive displacement pump and as such requires no priming.
9. Air in the fuel filter is a symptom, not a cause of non-starting.
10. Besides the proven mehods of using in-tank or inline pumps to get fuel to the low pressure pump, it might be possible eliminate the extra pump by looping the return line to the tank so that it has a high point above the fuel filter elevation. This would prevent fuel from draining from the line from the fuel filter to the low pressure pump and back into the tank when the engine is stopped.

The loop won't fix the performance issues though. As you note, an extra pump solves those problems.


Wobbly