1999 2000 ME7 fuel system pressure measurements

[abscate]

Placeholder for measurement thread

Put the AKN fuel gauge on the schraeder valve, keyed on.got 52 psi but pressure dropped after the ETM selftest of 8 seconds

Seems like pump only works for this period if Car isn’t started, makes sense

In 10 minutes , pressure bled down to 30 psi, but I had a drop forming at valve every 2 minutes.

Disconnected gauge, cycled key again

Had to find the right pressure on the valve to seat with no leak, then measured the 52psi, and 40 psi after 20 minutes bleed down.

Fuel pump and relay all original on this car, 19 years 200k miles

[Roger_850T]

Good stuff!

Adding some additional data to this useful thread:

The main reason you have a check valve and want to keep the fuel pressure up is due to hot start issues. In a fuel injection system, in the summer when it can get really hot, and you park your hot car in the sun, the fuel in the fuel supply lines can get very hot, and flash into vapor, causing vapor lock. This is a no-run until the car cools down kind of situation. Car builders (Volvo, and their vendor Bosch) addressed this by adding a check valve to the fuel system. This keeps the pressure up (and thus above the point where it would flash into a vapor) for long enough to get past the peak temperature times under the hood. This is why we have a check valve.

For a cold start scenario, they don't care if the system has bled down. They cycle the pump briefly, which (in a properly sealed system) should be more than enough to provide pressure at the fuel injector rail for normal operation.

Thus, it doesn't matter much that it's bled pressure when cold.

For reference, my '02 V70 T5 holds 55 psi, and bleeds down to ~30psig overnight. My '95 854T is about the same.

Roger