The summer has passed, and the problem has been completely gone since march. Now its back again, with dropping temperatures. I still havent changed the HP pump or pressure regulator,... Any news in this thread? Its so frustrating...
Puzzle-solving mode engaged! ;-)
Are you still running without the return tee that recirculates warm fuel? Presumably that valve would now be wide open in cold temperatures?
I *did* reread this thread over a coffee (it would appear I need to get out more?) and am still left wondering where the air could be getting drawn into the rail?
You reported a rail pressure of 5 bar for the first 10+ seconds of cranking, then a jump to 340 bar and engine start. This is certainly consistent with air in the HP pump and/or fuel rail.
You reported fuel being syphoned towards the LP inlet when you opened the filter, that stopped when you plugged the hole with your finger. That tells us the return path is clear, and sucking on the whole system. This is a condition that can suck air into the system.
The problem went into remission with warm weather, so it’s unlikely to be the return fuel draining back to the tank, with an air bubble working its way forward through the return lines.
So it sounds like the air is entering at one of the upper engine components, under vacuum created by the supply and return fuel sitting in the hoses.
The system is not weeping fuel when running, so the seals are all holding against their normal operating pressure. You’re not pulling air into the clear lines when running, so supply hoses are tight against vacuum.
Seals can be tight against pressure but pass air in under vacuum. For example, air can enter the ABS solenoids if you pull too hard using a vacuum pump to flush the brake lines.
Seals can also get stiff in cold weather and not do their job as effectively (as in the failure of Challenger’s o-rings).
The HP Pump has O-rings on the LP supply and return line, and a shaft seal behind the eccentric that seals the shaft against the LP fuel supply pressure (not Item #2 below... that seals crankcase oil). The shaft seal may be getting stiff in the cold, or one of the plastic fittings could be shrinking in the cold and reducing o-ring pressure at the joint.
Any air entering the HP pump’s LP gallery through that shaft seal or an o-ring could be drawn up into the fuel rail by the vacuum on the return line, passing the various ball valves in the normal direction of flow. The HP Pump also has a LP relief valve and return line connection, but the path up through the Rail has less resistance, and any air would want to rise up to the outlet port.
Unfortunately this is a hard theory to test... You could remove the HP fuel line between the pump and rail and apply a slight vacuum to the HP pump outlet port and see if you pull air through?
You could also block the rail inlet and see if it still fills with air, which would point to the issue being downstream of the HP pump.
If you can pull air from the HP pump, then smearing grease on the o-rings is a common method to improve their seal, and you might identify (cure?) the problem if you can no longer pull air from the outlet port.
Smearing some grease over the back of the shaft seal would help it seal out air and help with your diagnosis, but I suspect that the seal is not accessible, even with the HP pump off the engine? You could cap the inlet and return ports of the HP pump, then apply a stronger vacuum to the HP pump’s outlet port and see if you can pull air out if it, which would directly implicate the HP pump, and likely the shaft seal (though you wouldn’t have confirmed the seal as the fault with the pump)
I remain very curious about your problem, and the eventual diagnosis.
Good luck,
-dave