My previous Bosch FI pump would always maintain pressure after the engine is shut off; there is a fuel pressure gauge in line.
I just installed a new Bosch pump; now the pressure falls back to zero after turning the ignition off
Should the circuit maintainin pressure ?

Did you install the check valve? Some pumps may not have them. I am guessing that is why you have no residual pressure.

The rollers in the original pump serve as a check valve. As does the valve part of the fuel pressure regulator. (The reg actually is a spring-loaded valve.) You either have something plumbed strangely, or there is a fault in the pump that is letting fuel back out past the rollers.

--DD

No check valve. Only replaced the pump with the same model #
So, you are saying I should still get a pressure reading after stopping ?

What year is your car, and what pump Jim?

Its a Bosch 69469, same as my original. The car is a ‘75 L-Jet. Car starts right up, haven’t driven it yet. The new pump was made in Ceckislovakia (so) and not Germany like the original.

should hold some residual pressure, Jim, are you positive that you do not have a leak somewhere?

No leaks; the previous pump would slowly dissipate the pressure after shutdown.
I’ve inquired with the vendor. Will post their response.

Yes, you should absolutely have residual pressure after the pump stops.

Hard to hook up a two-port pump wrong, so there is a leak somewhere. Could be an injector sticking open, could be a leak out into the air (which would be pretty detectable by your nose), could be through the fuel pressure regulator back into the tank, could be back through the pump into the tank.

--DD

so I would then change the pump.

In my bug I use a Ljet FP and have a big pressure gauge, it takes several hours to bleed down.

Just got off the phone with a very knowledgeable Bosch Automotive Tech person; he reviewed the specs for the pump before returning my call and stated the pumps have check valves and pressure should gradually dissipate after shut down.

Update: I removed the new Bosch pump and put the spare back in. Same thing happens with the old one too.
Is is possible the pressure regulator has failed ? I never heard of one not working.
There is no small of gas leaking out of the system, and the pressure goes right up to 30 when the engines running.

Any ideas ?

How do you bench test the regulator ?

924,944 and lots of Audis, VW have a check valve you can install inline with the pump outlet. On Ebay for Bosch one for under $22.

I had a similar problem and isolated the culprit this way.

Attach a pressure gauge.
Activate the pump to pressurize system.
Have a pair of hemostats positioned at various point on fuel line.
Deactivate the pump and immediately clamp the hose with the hemostats.
Repeat the above for each of the following positions for the hemostats:

Return line from regulator to eliminate regulator as the cause
Output of pump to eliminate back flow through the pump
Each injector to eliminate leaking injector

My culprit turned out to be a leaking injector

Thanks ! I will try that... I have an on-line pressure gauge already. Did the leaking injector cause the pressure to instantly fall to zero when the power was removed ?

It took about 10 seconds to get to zero.

using a Bosch 044 pump, it came with a check valve that I left off when I installed it.

Was there a reason I should have left it on?

Kind of late now since I would have to remove the tank to access it.