I can’t see how a change from a recirculating loop to a fixed pressure system with a variable pump will have an effect on the fuel vapour recovery. Is there a different positive/negative pressure in the tank that needs to be taken into consideration?
I was about to say the same.
I would even say that with a returnless system it should be even worse, as the fuel is not circulating inside the fuel rail wouldn’t any vapour stay trapped in there ? With the conventional FPR arrangement fuel is circulating regardless if the engine is running or not, so if you run the pump for a few seconds any vapour would flush from the rail as new cool fuel arrives, no?
I think I will tee a fuel pressure gauge into the supply line to see what happens to the fuel pressure there during hot soak. As I stated, the pressure in the rail holds to regulator pressure. I would assume this should hold at 50-60 psi if the pump check valve is good; if the right regulator has failed, I should see rail pressure, correct?
I would think the same, but there seems to be enough anecdotal evidence of vapour locks + hot restarts that doubts that theory, unless there is some other issue at work.
Then it is not a “returnless” system.
Yes in that there is no return from the rail.
I am making an educated guess here.(smile).
In our conventional system, heated (by the engine) excess fuel is returned to the fuel tank via the return line. This introduces heat back to the fuel tank which increases the tank pressure. This in turn causes fuel vapors to escape the tank when the cap is opened. It was not unusual to physically feel this excess fuel pressure when the gas cap was first opened.
With the new “On Demand” system the pump is located in the tank along with the pressure regulator. This variable pump will increase or decrease fuel pressure based on various engine sensors. Most importantly there is no heated fuel being returned to the tank. The cooler fuel allows pressure in the fuel tank to be controlled and fuel vapors minimized or eliminated when the gas cap is opened for refueling
I think that is a good analysis. Of course, if the fuel venting system worked efficiently then there wouldn’t be a pressure build up in the tank. But you would still be creating fuel vapour unnecessarily compared to a return less system. I like the idea, but my concern would be the extra complexity of an electronically controlled system - 'cos they never go wrong in automotive applications 
On the other hand my 1979 pre HE coupe and 1988 convertible never have hot start issues, and this is in a country with very hot summers. There is nothing out of the ordinary with these two cars, or at least not originally. The coupe later had an HE engine and aftermarket EFI system fitted, and still no hot start problems.
Robert, I suspect your hot start issues are not simply due to vapour lock. The symptoms you describe are more complicated than the usual complaint the engine takes a lot of cranking to get going.
I know this vapour lock answer crops up on the forum from time to time, yet I do not recall any test or evidence that shows it to be the real reason for hard starting. Jaguar fitted a switch to the fuel rail to overcome the problem of vapour lock, but that might have been to head off criticism that the V12 was prone to vapour lock, not a scientifically evaluated solution.
I do not know why some V12s are hard to start after a bit of heat soak.
I have raised another odd V12 characteristic on this forum, with no answer.
Both my cars take a fair bit of cranking to start if they have not been used for 3 or 4 weeks plus. I always charge the battery overnight first, no shortage of cranking power. No shortage of fuel or fuel pressure, well primed before cranking. It will take typically 3 or 4 four second cranks. They never fail to start, and once started run perfectly.
As long as the V12 has not been left unused more than 1 week, bioth cars will start with a single 2 or 3 second crank.
There is no obvious reason for this characteristic, it makes no sense.
One other thing: I never put the cars away unless thre engine has had at least 15 minutes running time to get them up to temperature.
More to come on this topic; I have some theories to test, but a failed alternator has interrupted my efforts.
Any possibility the problem is with the ignition rather than the fuel rail? Some who have experienced intermittent problems with the ign amp have tried cooling it, or just setting ice cubes on top of it, and it starts working again. The AB14 amp stays very cool while the engine is running because it’s bolted to an aluminum intake manifold with fresh air and evaporating fuel inside. Once the car is shut off, though, it’s liable to get quite hot simply due to heat rising within the engine compartment.
So, next time the car is uppity during a hot start, put an ice cube on the AB14 amp. See if it makes any difference at all.
I mentioned before that I tried that with no change; if you keep your foot in it a little, you can keep it running some of the time. No exhaust backfires, so it doesn’t seem ignition. I am going to try to measure the fuel pump outlet pressure and the rail pressure and compare- if the fuel pump side of the right regulator is low, it would indicate a pump check valve issue. Otherwise maybe a right regulator…
Richard Dowling indicates he has never experienced an issue; in Australia- maybe it is related to the E15 we have in the US. I may try a tank of non-E15, there is a station that sells it here for $$$$$.
Interesting update; replaced the alternator with a Land Rover 100amp Chinese knockoff (outlined in another post)- now charges at 13.8v at idle, even with a/c on. Fuel pump is quieter, and a significant improvement in the hot starting. No stalling, just a brief low idle…even though ambient temperature is higher around here.
Probably not related but true.
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Evidently related and true- new alternator, not a peep from the fuel pump in 500 miles driving. Weather has been as hot or hotter. Hot soak restarts are back to acceptable; idle flares up on start, almost dies, then rough for maybe 10 seconds until I suspect vapor is purged from rail.
No other changes, same fuel (never got around to trying the non-E15).
Mine does exact same thing, takes about 10 seconds. Dont think its vapor, its 170F fuel (i measured). In 10 secs, its down to 120F.
I was recently having very bad hot starts lasting a good 30 seconds, which I think was fuel vapor, put back fuel check valve and that helped. My fuel pump doesn’t have one built in.
Lemme get this straight: You’re saying an alternator upgrade providing better charging helps with the issue of hot starts? And the fuel pump runs quieter?
Odd but true, at least based on recent experience. With the Lucas rebuilt alternator that I installed when I first got the car, charging always seemed marginal, especially with lights and a/c on; was worse when I upgraded the condenser fan to a SPAL. The fuel pump was audible at times and the hot soak starts got worse, prompting this thread. I didn’t connect the fuel pump noise to the electrical load.
Upgraded the alternator with a 100A, now maintains 13.7 volts with everything on, at idle. Zero fuel pump noise…and hot soak starts have become acceptable.
Maybe the higher system voltage during a hot restart purges the hot fuel and vapor quicker? Perhaps with the old alternator the hot start RPM was too low to commence charging.
I think it’s been established that this car runs like $%%^& if the system voltage drops off, even just a few volts. So perhaps that’s the issue behind the hot start problem – although you’d think it’d cause a cold start problem as well. Perhaps during a cold start it’s trying to enrich it so much that the issue of low system voltage doesn’t factor in. During a hot start the car is trying to run as though it’s fully warmed up but the voltage is dragged down and the EFI system starts faltering.
Interesting theory. Now I’m wondering if anyone has noted any change in hot start performance after they replaced the battery.
I’m also not sure how the fuel pump could get quieter. I can easily see how it might run faster, but how does that result in quieter? Unless running slow brings it into a resonance regime where it buzzes the trunk floor or something. Since it needs to be moving the same amount of fuel, presumably when running slower it’s more loaded up, but I dunno how that would make it noisy either.
Returning fuel to the tank does indeed lead to a temperature increase in the tank, but this doesn’t cause any running issues.
What it will do, if there is a swirl pot placed into the circuit, is kill the round Bosch (Lucas 2FP?) fuel pump as the only thing stopping the high pressure pump from overheating is the cooling effect of the fuel it pumps.
My car’s original twin propane and petrol setup ran beautifully, but LPG (propane) being half of the price of petrol meant I simply stopped running on petrol. The petrol side of the fueling had the original twin SU lift pump filling a swirl pot. This then fed a Lucas/Bosch XJS fuel pump. The return came back to the swirl pot and the excess (from a totally full swirl pot) then returned to the tank. When running on LPG, the petrol circulated and hot petrol accumulated in the swirl pot (as no petrol was ever used). There was never any problem switching over and running on petrol during a drive, but after six months, the high pressure fuel pump just died. After a second (brand new) pump died, the penny dropped and the swirl pot was deleted in favour of an in-tank pump directly. This now mixes the returning (hot) fuel with all of the petrol in the tank, so the average fuel temperature is lower. The problem was solved. Normal users don’t have this problem because “half of the fuel” is always used, so is replenished with cold(er) fuel - but this wasn’t the case on my engine.
As the car never had a problem running on petrol, this also points to hot starting issues as not being directly fuel temperature related, but being tuning issues because the coolant and air temperature are fooled by heat soak. If you take a look at the Megasquirt forums regarding hot start issues, they are always solved by altering the fuel and spark settings during the first 120 or so seconds of running. Running an old ECU like a 16CU means you have no access to datalogging so cannot see what the sensors are returning to the ECU and how it is misfueling the engine during that time. (You also have no control over spark advance.)
Go and read up about hot start issues on the Megasquirt forums.
kind regards
Marek