But it’s not a T. It only has a line from the fuel pump and a line to the fuel rail, the latter being the pressure relief line.
I replaced both FPRs two years ago. The B bank cost wasn’t too bad for a quality Bosch part ($95). But you’re right, the A bank Bosch was quite expensive ($120). Since it wasn’t as important, I bought a Beck Arnley, for about $52.
There have been tons of discussions if the A bank FPR is needed or not, and nobody has a definitive answer. I left mine, as it must do something, and ‘balances’ the symmetrical look of the V12 
It isn’t needed to make the the engine run well. I feel safe in saying that’s a definitive conclusion.
The next question would be “What, then, Is it needed for?”. According to Bywater it was actually used as a noise suppression device. This is a plausible explanation not only because of the source but because many manufacturers have used some sort of device…most commonly called “dampers”… to ensure no fuel system knocking or hammering or gurgling or whatever. Leave it to Jaguar to be different.
Interestingly, tho, I don’t recall anyone complaining about new noises after ditching the inlet regulator.
The “why?” of Jaguar design features and engineering can sometimes be baffling. I long ago gave up trying to understand the rationale every decision they made
Cheers
DD
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Jaguar has a definitive answer. Eliminated the A bank FPR in later models.
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OK. So it really doesn’t help anything.
Unless it’s leaking or not working right, can it ‘hurt’ anything being hooked up?
Stop and think for a minute, Jaguar did some strange stuff but they wouldn’t design a fuel system that uses a part that they have to pay a vendor for that serves no purpose. I agree with Greg’s first assertion that it maintains the outlet pressure of the fuel pump.
Fuel pressure in the rail is regulated to 36 psi with engine off (no vacuum on left regulator). Inlet line on right regulator has some heat from engine; if held at 45psi+ it does not vaporize. But if doesn’t hold, fuel vaporizes-
Try to start car, fuel in rail has vaporized and fuel in line from pump is also vaporized, so it tales a while for liquid fuel to reach injectors…
My opinion is that both regulators and inlet check valve need to work.
Nope.
I happily ran my XJS for a long time with both regulators. When the inlet regulator packed in, I ditched it entirely…and drove the a couple years afterward, happily.
On my present V12 I ditched the inlet regulator right from the get go, a few years ago.
My gut tells me that neither your hot-soak problem nor mine has anything to due with the inlet regulator. Sometimes my gut is right. Sometimes not
If you reinstall yours and the problem goes away I’ll do the same and we’ll see what happens !
Cheers
DD
By the way…
How hot is ‘hot’?
Do you mean simply fully warmed up and 30 minutes of driving under mild conditions? Or do you mean 4 hours of 70 mph driving across the desert to Las Vegas in August?
If the former, I rather doubt there would be enough heat to cause a problem.
My own hot soak problem, which developed a few months ago, occurs under mild conditions, as I described.
Cheers
DD
Yes, mild conditions, just a good 20-30 minutes driving. Coolant is at 190F at shutdown.
I still have inlet FPR, will just leave it hooked up.
Will try putting back check valve.
Did another test today, drove 20 mins, shutdown for 45mins, turned key three times each time waiting for 3 secs of prime. It started rough again and idled a bit rough. So…
It happened to me twice as I said in all the years I own the car, but both in very similar extreme conditions.
On a very hot summer day, after half an hour or more driving and then the car sat idling for quite a while, trying to start it after 45mins or so.
Maybe a possible remedy would be to hook a fuel relay bypass switch so one can manually run the pump for 30sec or so, the couple os seconds the the pump is running by cycling the ignition key are clearly not enough.
I thought of the same, but shouldn’t be necessary. My ‘88 does it consistently after fully warmed up and parked for more than 20 minutes- even when the ambient temps are 60F. Have yet to tee in a pressure gauge on the line from the pump, but suspect that the fuel pump check valve has failed. My rail pressure holds and shows 36PSI when the problem is occurring, but I am convinced it is mostly vapor. Cycling the key 3 times doesn’t seem to correct it either, takes 4 or more with plenty of gurgling noises from the fuel tank return.
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Well, I’m not 100% sure, but it looks like Walbro pumps, or at least the one I bought, do NOT have a check valve built in. So even three turns of the ignition may not do enough if it’s just compressing fuel and vapor inside the fuel rail, like Robert says. I hope to re-install check valve today or tomorrow, and will report back.
Some of you, depending on the age of your fuel pump, could be having check valve issues?
Here’s a photo of the check valve I have. Quality made of brass. $15 Not worth messing around with plastic.
Anxious to hear the results.
This is very curious to me.
Until a few months ago I never had any such issue with either of my V12s, even if conditions were extreme.
Why now?
FWIW, my fuel pump is about 5 years old.
Cheers
DD
I would think that a failed check valve would make it similarly reluctant to start when cold. The difficulty with hot starts is likely something else. Do you guys have anything installed in that boss on the left side of the fuel rail?
Yes, a thermo vacuum switch that cuts vacuum to the left FPR on mine and it works; doesn’t do anything when starting because there is little vacuum anyway. No excessive crank cold.
Hmmm. The whole point of that thing is to help with hot starts. The earlier version was electrical, basically disconnected the nearby air temp sensor, hence making the EFI think the engine was at cryogenic temps and hence called for massive fuelling. And I guess it would work when starting. The later vacuum version you have cuts vacuum to left FPR, which presumably also richens mixture, but you’re right it won’t make any difference when starting, only with how it runs once it’s started.
Someone needs to try manually disconnecting the air temp sensor and see how the engine starts hot.
I had bypassed it at one point and didn’t notice a difference. The other thing that occurred to me is the rubber injector rings; when I replaced the fuel hoses I also replaced the tip seals, but did not replace the larger body insulators. I wonder if the hardened rubber would transfer too much heat? Can’t see how it would…
Was curious, and found this…
“Gasoline has an initial boiling point at atmospheric pressure of about 35 °C (95 °F)”
So if fuel rail is not holding pressure because faulty or no check valve, i guess it could easily start to vaporize?
Fyi, i always turn key, and wait 3 secs for pump to stop priming.
Day 1 with check valve back in
Cold morning start in one crank vs three cranks.
Tried one hot start 40 mins later parked in garage. Started right up, didnt run perfect but it idled very close to normal. Within 10 secs, its perfectly smooth.
Tried second hot start 30-40 mins later parked in sun. Slightly difficult start, but it idled very close to normal like other hot start.
Ill keep testing, but i think having a check valve makes a huge difference. Obviously my Walbro pump doesn’t have one built in.
Maybe that fuel temp vacuum valve will fine tune the last 10%.
Before w/o check valve, i thought it might stall, had to wait 30 secs while giving it some gas. Now w/ check valve, i just notice a slight roughness and its gone in 10 secs.
FWIW, i have no AC, so no fuel cooler. Not sure how much that would raise gasoline temp with a full tank.
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FWIW, the 6.0L V12 set up does NOT have fuel cooler by design. Apparently this is not the critical part of the system.
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