308 - fuel pressure drop while sitting

(Robert Laughton) #1

Hi folks,

I had both pumps replaced on the VDP SC a while back.

Now it seems that fuel pressure is lost after sitting, as it cranks too long to fire.

I think there are one way valves in the pump assembies, but I’d rather not pull the tank again. Can I install an in-line one way valve somewhere in the line? I recall something like this from another forum somewhere.



(Art Ford) #2

You could have a stuck open injector and leaking at that end. This is the nightmare of injector manufacturers because it can cause a hydraulic lock. I’ve put two fuel pumps in my XJ8 (and 3 alternators). When I had an XJ12 in daily use I put a fuel pressure gauge in the console and a push button switch going direct to the fuel pump. Made for quick starts.

(DavetheLimey) #3

Wouldn’t see why not. This is from Mcmaster Carr, P/n 7768K52. I bought it for the same reason you have for my V12, but wound up replacing the fuel pump instead.!

Are BOTH pumps leaking down? I’d question that.

(motorcarman) #4

The fuel pressure will naturally drop after time. The trick is to keep the pressure up until the engine heat drops.
The pressure should NOT drop rapidly at ‘shut-down’.

There are several places for the fuel to leak.

Back through the fuel pump(s). (faulty check valve(s).
Faulty Fuel Pressure Regulator.
Faulty (dribbly) injector(s).
External fuel leak.

If the fuel pump(s) in the tank leak, I would install a check valve in the hose IN THE TANK past the pump(s), before the steel connector at the tank outlet.

You can try to find another spot along the fuel line but you will need to cut a factory line somewhere. If you cut a steel line, make sure you put a ‘barb’ or a ‘bulge’ so the rubber line will not become disconnected with fuel pressure.

Good quality hose clamps will help.