Hot start issue in Ireland

Great that you solved it! But i still don’t understand how your fuel rail fuel is getting so hot, that you need that device? You live in Ireland, not the Sahara.

I’ve removed mine, and i never have a difficult hot start.

IMO something else is going on -I’d make sure your engine isn’t running too hot or that your fuel pressure isn’t too low.

Hi Greg, fuel pressure is fine, fuel rail temp is 45c at shutdown but this rises to 80c
after 20mins of heat soak. Thats when the problem occurs. Leave it to 30mins and no problem. taking the tube off the FPR does the same thing. I am just glad to have solved it Could have been like that for years and if i never tried to start it in that time frame I would have never known !
Thanks again to all .
Frank

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Hmmm, i guess that explains it, I’ll have to check mine for comparison.

If mine is the same, i wonder since I’m running a fuel pump with much higher pressure than stock, that it flushes out all my hot fuel better?

Hi Greg and all, I was having the same problem when my aux fan went out and I upgraded it with a 2000 cpm fan. It was obvious that my aux fan was caput. It was barely spinning, the bearings were totally gone and the edge of the blades where where hitting the enclosure.

Now no problem because the new upgraded aux fan takes something like 10-15 minutes to cool the engine down upon shut down and now the engine spins freely upon hot re-start where before it would grind to a halt and kill the battery charge.

FWIW I tested today with a hotter day (82F). Letting car sit for 20-30 minutes, it started right up IF i waited the three secs for fuel pump prime. The one time I didn’t bother waiting for fuel pump prime, it had trouble starting. Interesting… perhaps my higher pressure pump clears the fuel rail in 3 seconds.

The regulators are going to control the pressure, not the pump. SD Faircloth

Yes, but if the regulator is trying to keep it at 40 psi, and the pump is putting out 60 psi, doesn’t the regulator have to bleed a bit of fuel through to keep the pressure from getting too high in the rail?

Greg,
The fuel pressure regulators are always bleeding some fuel through while the engine is running. It has been stated by others on Jag-Lovers that new OEM fuel pumps could put out 100 PSI or more during a dead head test. I have never confirmed this myself. But a marginal failing fuel pump would put out considerably less. I believe that there are several issues that effect hot start performance and that as cars age and components like fuel pumps, fuel injectors, fuel pressure regulators, thermal vacuum valves and more are subjected to years of use and engine bay temperatures that performance will degrade. That is why my usual approach to problems is to return the system to as close to what the OEM design was and proceed from there. In my opinion, and I know that others will disagree, new car OEM performance was very good indeed.
I am glad that a properly functioning Thermal Vacuum Valve fixed Frank’s hot start problem and since he was able to source a good used one so quickly that shows me that there is still a supply out there for those that want them. The Bosch/Lucas Thermal Vacuum Valves were used on the Series III XJ6 and XJ12 as well as the XJ-S and other makes/models , so a supply of used ones will contine as people part out those cars.

Paul

Well, now im starting to have hot start problems. Ha ha. After about 45-60 minutes, car starts up weakly (makes that pinging sound), and then roughly idles bouncing between 800 and 400rpm. Takes about 30 secs to smooth out back to normal.

What’s changed? During the winter, i changed my thermostats from 180F to 190F. Could that make a difference? I also decided to remove the fuel check valve that i had installed after the fuel pump (i dont believe my Walbro pump has one built in)

I will put back fuel check valve this weekend and see what happens.
I may also try buying the vacuum fuel rail temp valve for the FPR and see if that helps? As it seemed to help Frank (OP).

I sometimes have a “poor running after hot soak” problem as well. Haven’t done anything to address it…yet. My driving habits are such that it isn’t an everyday occurrence so I’ve sorta shrugged it off.

One thought is that dribbly injectors are over-fueling the cylinders. A cold engine wouldn’t object. A warm engine would. Dark exhaust plumes might be a clue here.

Cheers
DD

It has happened to me twice, and it was no fun, it took for ever to start again.
I would definitely put the check valve back in and get the vacuum fuel rail switch thingie.

The check valve is there to prevent bleeding through the fuel pump.
The FPR does nothing when there is no flow.
On the engine side it’s the injectors that hold the pressure.

That’s supposed to be the purpose of the vacuum fuel rail switch thingie.
It cuts vacuum from the FPR therefore rising the fuel pressure.
Apparently not very efficient though, especially considering that there is almost no vacuum when cranking…

it must have to do something. If a check valve prevents bleeding backwards through the fuel pump, then what prevents bleeding forwards past the B bank FPR?

The injectors Greg.
and some 20 characters…

Yes, the check valve and the injectors will keep fuel pressure in the rail. But it can still bleed out of the rail exit at B bank FPR? With no vacuum after shut down, the B bank FPR must be keeping fuel rail pressure at 40 psi?

Once the fuel pump stops the pressure can’t rise so the exit of the B bank FPR will be closed, in theory no bleeding.

I’d reinstall the check valve on GP’s. But you could cycle the key a few times to operate the pump. If the rail is empty, that’ll refill it.

Next time the engine misbehaves, turn it off and try this exercise. If it now runs well, there ya go !

Also try jabbing the throttle a couple times with the key ‘on’. This will operate the injectors and squirt some raw fuel …similar to the accelerator pump in an carburetor. If the engine now starts/runs well (or better) then it confirms lean fueling

Cheers
DD

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Correct. So i’m just trying to figure out what the A bank FPR does during shutdown.

So let’s assume you are idling before shutdown. The B bank FPR is keeping fuel rail pressurized to about 32 psi? The A bank FPR is keeping fuel pressure between pump and fuel rail at about 43 psi.

You shut down. The B bank FPR has no vacuum now, so I assume keeps things at 36 psi (maximum at 2.5 bar) in the fuel rail. The A bank FPR is keeping pressure between the pump and fuel rail at 43psi. IF i have no check valve, then I’ll have to assume the pressure between pump and A bank FPR looses pressure and goes to 0 psi. What happens to the pressure in the fuel rail? THe B bank FPR is still trying to hold 36 psi, but does the A bank FPR still do anything? Or because there’s no pressure at it, does the fuel rail now bleed back thru the A bank FPR? I assume it’s a two way valve? If we have 32psi on rail side of A bank FPR, and 0 psi on supply side of A bank FPR, does the fuel go backwards to equalize?

If A bank FPR holds, but the fuel starts vaporizing in the fuel rail because of shutdown heat, does that increase pressure, and thus the B bank FPR now lets off some of the vapor/fuel as it’s higher than 36 psi?

Very confusing for me with these two FPRs…

only tested once, an hour after shutdown when it would sometimes have a hot start, so not conclusive, but I cycled the key three times to operate the pump. Started up no problem and idled smooth. I’ll try again over the next couple days before I put back the check valve.

Imagine the FPR as being a T, A to the fuel pump, B to the rail and C to a pressure relief valve (fuel return - vacuum adjustable).
Regardless if there is flow or not the pressure relief valve will be closed under the FPR’s working pressure.
The passage between A and C never closes, only pressure between them is regulated.
So when the fuel pump stops:
If there is no check valve the system will bleed through the pump and the pressure will drop to 0 everywhere.
If there is a check valve (closed on both ends) the system will hold the pressure of the lower rating FPR.

Remove the A bank FPR from the equation, it does nothing.
When mine went cablui it got replaced with a piece of hose, and by the way it’s three times the price of the B bank FPR. This was back in 2006.