Airwatch

I'm having a tricky problem with my Golf3 TD Variant, AAZ engine, manufactured in 1998, with a Bosch VEP4 pump. The ESP (Electronic Stability Program) was leaking somewhere, and after a night with temperatures below -8°C, the car wouldn't start. I had to crank it endlessly to bleed the system. The check valve installed before the diesel filter (monopole filter head with screw-on filter) only provided slight improvement.
Since I was planning to convert to a PÖL system anyway, I re-sealed the ESP. A new starter motor was needed (it wasn't an original Bosch part, but a cheap imitation that, surprisingly, continued to turn the engine even in a severely damaged state, until more than half of the commutator was burned away).

The check valve came loose again (likely due to PÖL), a new gasket was installed for the "Knackfrosch" (likely a specific component), and all clamps in the intake manifold and return line were tightened. Relatively new injectors with a pressure of 185 bar were installed, and the injection start was set to 0.85mm.
After that, it ran smoothly again and started on the first try, even with a completely cold engine block.

Last night, during the first really cold frost (-10 to -12°C), the old problem reappeared. It starts immediately, runs for about 15 seconds, and then shuts off. After that, it cranks, but not for as long as before. It's also clear that air has gotten into the fuel lines, because it runs roughly on only two cylinders for a while before it starts running properly. In the colder nights before (down to about -8°C), there were no problems except for a couple of times when there was a brief drop in power right after starting the engine after a cold night. The pump seems to be tight, and there's no visible leakage anywhere. I suspect the seal in the fuel filter housing, because everything else seems to be okay (except I haven't checked the fuel line to the tank yet). The problem only occurs when the box is exposed to temperatures below -10°C for longer than 12 hours. Otherwise, it starts up like a charm.

Do you have any ideas on how I could locate the source of the problem?

Quote:

It starts immediately, runs for about 15 seconds, and then turns off.

It seems like it's running out of air.
As long as the fuel is cold, the "knackfrosch" (a type of fuel pump) circulates everything in a nearly closed loop. You will probably need to drive for a longer period of time, i.e., until the thermostat valve opens, to get the air out of the system.
Otherwise, I would look for leaks in the system between the filter's suction side and the return line to the tank.
Can you see any bubbles in the hose between the filter and the pump, or is it completely empty?
hg
Herbert.

Apply grease to the suspected areas, including the "Knackfrosch" (likely a specific part), during installation.