Hi Trev,

In the FI system that "priming" buzz is just a byproduct of the first k-jet relay design that sensed ignition pulses to safely allow fuel delivery only when the motor is cranking or running.

There's no practical purpose to it, but a huge mythology has arisen, especially since Bosch "brought it back" after two generations of LH. We tell each other the lack of that one second buzz keeps our cars from starting well. They didn't buzz key-on when k-jet switched pumps with the airflow sensor, and they didn't "prime" throughout LH2.0 and LH2.2. Some k-jet relays do not prime (different internals) and some LH2.4 ECUs don't do it (different eprom).

There's a telling graph in Probst's FI book, on page 12 of chapter 5. It shows the curve time vs system pressure in the k-jet cars with and without the accumulator. It shows what I know from experience, watching a gauge on the fuel pressure. The pumps create full system pressure in 100 milliseconds. When they have to fill an empty accumulator, it takes 800 milliseconds. In LH, without any storage except what is behind the fuel pressure regulator, the 100 millisecond number applies. No priming needed. And at this paragraph is where my experience ends and the theorizing begins.

There's no damage that will occur deadheading a lift pump (centrifugal tank pump), which is the only type you'd dare run up against a carb's float valve. I just predicted the pressure resulting (without flow equivalent to the pump's normal rate) will be unstable over temperature. I could be all wet on that. Undoubtedly someone will pipe up and say it works fine. Also, I can't predict what sort of extra pressure you'd need to prevent an existing vapor lock problem, and whether that would tend to force or impede the float valve operation. I am pretty sure the float valve does not like serious pressure.

I like the tee idea. You could check the pressure and make adjustments by how restrictive the return line is. But all in all, this is just armchair musing on my part, all the while understanding how you can only turn over a mechanical pump's diaphragm so many times (if you even can with a volvo's) and that a rebuild kit might be tough to find. On the other hand, our FI lift pumps are pretty crappy for longevity (back from postulating to experience).


--
Art Benstein near Baltimore

The short fortune-teller who escaped from prison was a small medium at large.