|The MGA With An Attitude
FUEL PRESSURE DECAY -- FP-110
On March 11, 2012, Dave Headrick in Lexington, SC, USA, wrote:
"It has seemed to me that my fuel pump (new SU replacement installed during restoration) has been cycling excessively every since I put the car on the road. So I decided to rig up a pressure gauge and measure max pressure and pressure decay once the switch was turned off. (I only had a 30 PSI gauge so the following numbers are not precise).
Full pressure = 4 psi
3 psi after 30 sec.,
2 psi after 54 sec.,
zero after 1 min and 40 seconds.
Also with the switch left on and the engine not running the pump cycles every 12 seconds.
Is this normal"?
The short answer is yes.
The all metal check valves are seldom perfectly leak proof. One click every 12 seconds at rest sounds acceptable to me. If your numbers were accurate, you lose 1 psi in the first 30 seconds after switch off. The 12 second leak down time then represents roughly 0.4 psi drop before it clicks and restores pressure to 4 psi. This pressure range for switching is perfectly normal. The pump clicks multiple times per second to supply full flow at high engine speed and full throttle, so one extra click every 12 seconds is of no consequence. If it would click once per second at rest I would be concerned enough to try to fix it, mostly because it might otherwise get worse from there.