There is a check valve on the fuel pump itself that traps initial starting fuel charge for days , by cycling the key you are overcoming this

in the normal starting sequence once the key is in the on position the fuel pump will turn on for 4 seconds then turn off by design for a refresh fuel charge in the line

without replacing the pump some have put a new external to the tank check valve of a certain size unknown to me

Your experimenting and definition of the problem has led to your solution

 

 

Hello and thanks a lot to both of you DON B and LADY P ! If you ever come to portugal give me a call and you will be received in a x300 at the Lisbon airport!

i will follow your tips and start with the basics, try the FPR and replace if necessary, if it still keeps with the syptoms i will place a check valve and your instructions in that mega hiper great website!

I cannot thank you guys enough

 

The easier try is the FPR as they don't last forever and cheaply avail though the local parts store with the rubber diaphragm over 22 + years

The fuel pressure should be 43 PSI at idle and goes up a couple of PSI at throttle up

 

 

 

Don B's suggestion of being just after the tank prevents air from gurgling back into that long line which necessitates a couple of fuel charging cycles to rectify for a successful engine start

But I have seen white check valve in the engine compartment of late year X300s

 

If your issue is the check valve that is part of the pump assembly then you will need to put your additional check valve as near to the original as possible as @Lady P and @Don B suggested. Putting a valve on the return line would not stop fuel trickling back in to the tank through the pump so your problem would be the same.

If it was the FPR that was not sealing off pressure then putting a check valve on the return line would work. But it would be better to replace the FPR as it's easy to do and relatively cheap.

 

Some Jaguars have a check valve in the fuel return line at the inlet of the tank, and I think the X300 is one of them but don't have time to check the diagram. Ideally the check valve in the fuel feed pipe should be as close as possible to the outlet of the pump or tank, but I have experimented with putting a check valve in the engine bay just before the fuel rail and could not discern much if any difference in performance, so changing the FPR and adding a check valve anywhere in the fuel feed line will usually make a big difference overall. I know Brendan didn't mean to imply that a check valve could replace the FPR since the two operate in different ways to achieve different results.

Cheers,

Don

 

Hi Don

No I didn't mean to imply that.

I may have misunderstood how the fuel lines work. This is what I think.

There is a check valve on the fuel feed at the pump. When the engine is not running (i.e. no vacuum) the FPR closes and acts like a check valve preventing fuel in the rail returning to the tank. So we have the pump check valve at the feed end, the FPR at the return end and all injectors off so the pressure in the line is maintained. Is that not how it works?

So what I meant in my previous post was that if the FPR was not fully closing when the engine was stopped and hence leaking pressure, then putting a check valve on the return line would fix that only. It would still require the FPR to be doing it's job when the engine is running.

 

 

Hi Brendan,

This is a great discussion so I thought it would be worth adding some info from the Jagaur documentation. The FPR never "fully closes," even when the engine is shut off. The diaphragm spring is calibrated at the factory at atmospheric pressure to release pressures above 44 psi/3.0 bar, so even when the engine is shut off, excess fuel pressure can be relieved. With the engine running, vacuum from the intake manifold references the regulator to manifold absolute pressure and "assists" the fuel in lifting the FPR diaphragm to reduce strain on the fuel pump. I guess it's similar in principle to a vacuum-operated power assist brake servo. Here's the explanation from the XJ40 Engine Management System manual (the X300 uses the same FPR):




There are at least two check valves (non-return valves or "one-way" valves) in the system. One at the fuel pump outlet, and one at the fuel tank inlet. The one at the pump outlet works with the FPR to maintain correct pressure in the fuel feed line between the pump and fuel rail. The one at the inlet to the fuel tank prevents fuel back-drain and directs returned fuel into the surge pot to keep the fuel pump primed, even under low-fuel conditions. Here's some info from the X300 Technical Introduction Manual:








Note that both check valves work in the same direction, viewing the path of the fuel as a circuit or loop. The valve at the pump allows fuel to travel toward the fuel rail but prevents it from re-entering the tank. The valve at the inlet of the fuel tank allows fuel into the tank, but prevents it from draining back into the line, either from gravity or positive pressure in the tank. If the tank inlet check valve was oriented the opposite direction, as a backup to the FPR, no fuel could flow in the system, since the check valve resistance to reverse flow is many times the system maximum pressure. Check valves open at very low pressures, typically just 1 psi or so, to offer as little resistance as possible to the fuel pump. But the maximum reverse pressure resistance is very high, typically 2,000 to 3,000 psi.

Cheers,

Don

 

Hi Don

That is really great information. Thanks.

 

 

The non return valve may not be present on the 2 different systems on the fuel tank / lines / EVAP for the X300 production years

See page 60

http://www.jagrepair.com/images/Trai...20-%202000.pdf

The way you can tell is the single square vapor canister on the left side only or the dual with one on the left and right just fwd of the rear wheels

 

Hi Paul,

The fuel pump check valve is built in to its outlet per the documentation I posted earlier. On the X300s on which I've changed fuel pumps, the original pumps were made by Denso. Walbro was the OEM for the earlier XJ40 and XJ-S cars, but I don't believe those pumps have a built-in check valve, since on those earlier cars there were check valves either in the outlets of the plastic pump canisters/modules, the evaporative flange on top of the tank, or in the fuel feed line just ahead of the fuel filter. You might do some research on that Walbro pump you identified to see if you can confirm that it has a built-in check valve, but personally I wouldn't replace the pump just yet.

Probably the easiest next step for you to try is to add a new inline check valve somewhere in a flexible section of the fuel feed line somewhere between the pump and the fuel rail. Ideally the check valve would be as close to the tank/pump as possible, but I've experimented with a check valve close to the fuel rail and it performed nearly as well as the near-tank location.

I have photos and descriptions in the albums at the link in my signature, but the large images are still down and only the thumbnails are currently available.

McMaster-Carr (mcmaster.com) carries inexpensive check valves with fluoroelastomer seals suitable for gasoline. Off the top of my head, I think I used a 1/4 inch I.D. check valve with 5/16 inch O.D. hose barbs to fit the 8 mm I.D. of the flexible fuel line.

Cheers,

Don

 

Paul,

Just to clarify, what you need is a check valve between the fuel pump and fuel rail, not between the FPR and fuel tank. Before you install the check valve, confirm its proper orientation so that it allows fuel to travel from the pump to the rail but not from the rail to the pump. The valves I've purchased from McMaster-Carr are marked with an arrow indicating the direction of flow.

Cheers,

Don

 

 

found this check valve, https://www.ebay.com/itm/Sytec-8mm-F...BB5:rk:15:pf:0

ti says "viton seals" i cant find nothing that says fluoroelastomer !!