Hi Guys

What is this and what does it do?



What is this and what does it do?

 

At a guess I would say either a fuel pressure or temperature sender.

 

Yeah that's what I was thinking but I'd like to know which one

 

It is the sensor for fuel rail temperature. I believe that if the fuel is hot (eg after a stop with a hot engine) it somehow helps starting by doing something or other to the fuel system. Cannot remember exactly quite what (age and senility)! I have a feeling mine has a vac tube on it (cannot see on your photo), and if that gizmo opens when hot, it raises fuel pressure or something like that by affecting the fuel pressure regulator's vacuum signal from the manifold.

 

The vacuum version dumps vacuum to the pressure regulator, increasing fuel pressure.

The electric version is wired between the TPS and the Air Temp Switch. I don't know the operating principle but I'll wager it's explained in Kirby's book somewhere

Cheers
DD

 

Top stuff Doug. Here it is:
“Asa result of the deletion of the fuel cooler, it is necessary to introduce aredesigned hot start system. The thermalvacuum valve fitted to pre-92 MY vehicles is replaced by the followingcomponents:
1.Fuel rail temperature switch. The electricallyoperated switch enables the fuel rail pressure to be increased when the fueltemperature exceeds 70 degrees Centigrade at hot starting.
2.45 second timer module. The timer limitsthe time for which the increased fuel pressure is applied.
3. Solenoidvacuum valve. This controls the vacuumsignal driven by the fuel temperature switch and the 45 second timer.
4.Vacuum delay valve. This controls theway the extra pressure is applied at hot starting to give a decayed reductionin fuel pressure over 45 seconds, after which the pressure is switched tonormal.”

 

Ahhh. Interesting, thanks.

That sounds like the later version of the electric fuel temp switch.

The earlier version is much simpler. According to diagrams it looks like it is normally closed. When 'open' (when hot, presumably) it breaks the 5v supply to the air temp sensor. That's it; no timers or solenoids, etc. This would richen the mixture slightly.

Judging from wire colors I'd say OB is the later type as the early type I'm referring to has two yellow/pink wires.

Cheers
DD

 

Hi Guys

Cheers!

Greg@Doug@Steve

Since I appear to have the later version. I'm now wondering what would happen if you didn't try and start the Car within 45 Seconds after you turned

the Ignition Switch on, or would it be better to wait until the 45 Seconds has run out and expired

As I have had a couple of embarrassing occasions on a red hot day, when She wouldn't Start on the button (although she usually does)

So I've left it for 10 minutes before trying again and She fired up first time, where I am suspecting that Hot fuel might be to blame

Although I'm sure that you guys would know that better than me, as well as any 'Hack' to get around that

 

What happens is that the fuel evaporates in the rail, so that when you come to start there is no liquid there, or not enough to pressure up immediately. The hot start thingy will not do much good if the rail is really empty. The hack I use is:
Turn the ignition on until you hear the pump stop, repeat about 10 times and the rail will be full.


I once had a Range Rover that was a devil under these circumstances. I fitted an old fashioned starter button type switch under the dash which would run the pump as long as it was pushed. This filled the rail in about 10 seconds and off she went.
Greg

 

Hi Greg

Cheers!

A bit more to add to my knowledge Bank of all things XJS and that was a really useful tip, as there's not much worse than having a Crowd of people standing round

Waiting for the mighty V12 to roar into life, only to find She doesn't want to play

 

OB, that is, the rail temperature switch, as has been pointed out. I haven't got my 89- book at the moment BUT I think it was related to the ignition system. The fuel lressure regulator themal switch is on the other side of the rail (B bank). That is also still there.

 

So, if the fuel evaporates, where does it go?
It's a closed, pressurised system?
Or is 'evaporate' just a misnomer for boiling?

 

Interesting question Steve, maybe the Vapor forces the fuel back in the Tank where it came from.

 

Only a guess Steve; but I have always assumed that as the temp rises the pressure in the rail does too. So fuel is pushed out of the B bank FPR. Then the pressure lowers and so the remaining rail-fuel vaporises. So no more liquid fuel in the rail, or not enough.

 

Ok, that makes sense.
Some kid on the radio the other day asked where does a candle go when it burns?

 

The fuel doesn't go anywhere. It just vaporizes. Now it's vapor. Vapor under pressure from the liquid fuel around it. A bubble. Vapor does not transfer heat very effectively so the vapor doesn't quickly cool and recondense to liquid.

Recently I was testing injectors and pulled B1 to confirm no leak under pressure. Twisted turnstile and got a good signal on B1. But it wouldn't fine fuel out.

Long story short, I was running out of fuel and had picked up a bubble of air in the fuel line. Nose slightly up it was trapped in the hose to B1. I fired it repeatedly and got no fuel. Puzzled, I poked the pintle valve in with my fingernail. Psssssssssssssssssss... then fuel. Fired fuel on twisting the turnstile.

It could take a lot of injector cycles to vent a vapor bubble.

Sometimes they can escape the fuel rail and be returned to the tank. Cycling the fuel pump multiple times may purge the rail as well as cooling it.

 

Why not wire the fuel pump @ the relay to run all the time instead of cycling it? That way it eliminates that 45 sec stuff & turning the key on & off 10 times in order to fill the rail with gas. It runs all the time when the engine is running anyway. Do other cars have this fuel pump on off thing with the key ?
It seems that hot start/stalling problem for me started with the 5sp conversion & compounded things with the aluminum flywheel. The Florida heat didn't help matters.
After replacing all the usual parts (regs, sensors,etc) I replaced & rerouted the 26 year old stock fuel lines away from all heat sources using -6 ss braided line & an fittings. Installed a FP gauge @ the fuel exit port & installed a adjustable FP regulator, eliminating the stock FPR's. I'm running around 43lbs FP , so far with no adverse plug, gas mileage problems & no hot/stalling restart problems."Just a though",maybe the stock regulators pressure isn't enough.

 

It does run all the time, BUT a safety feature in ALL FI systems is that if the engine stops, the fuel injection pump stops shortly afterwards. In a crash you will be very grateful! The problem is that the pump, ignition on, will stop after a few seconds if the engine is not turning; again as a safety feature.
I think that a bit of minor inconvenience caused by this feature is not too high a price to pay for a great deal of safety. And as well, it is easy to rig up a push button to cycle the pump if the inconvenience is really too much!

 

Although I'm not an expert, I seem to remember other stuff that Greg and Grant have told me about this in the past.

A Relay is like a low amp Switch that Activates a circuit that uses higher amps

And so without a Relay you would have a high Amp circuit in the Car, which you need to try and avoid to minimize the risk of

something like an electrical fire starting inside the Car

Also if you wired the Fuel Pump, direct to the Ignition Switch it 'May' put an extra load on that Switch which 'Could' burn out the

Contacts of that Switch over time

And as Greg has already pointed out, in the event of an Accident a relay is a Major Safety Factor, as if the Car turned over or the

Engine Stalled with a Ruptured Fuel Line

Then it would be well and truly 'Game Over'

As when using a Relay, the Onboard Computer Wizardry won't send another Signal to the Fuel Pump, unless it knows that the Engine is running

That's not to say that you couldn't install an emergency manual override switch but in the event that your Car refuses to Start on the Button 99% of the time

Then it would seem that there are components that may either need to be serviced or replaced

 

No modern or modern-ish car (that I'm aware of) is wired so the pump simply runs whenever the key is 'on'....for safety reasons as mentioned. Bad idea!

Many cars have some sort of 'prime' function to operate the pump for a few seconds only, as do our V12s.

As for the cycling the key ten times, I meant to chime in on that earlier.

I really don't think 10 times should be necessary unless, perhaps, you're dealing with a totally dry and un-primed system. After that, once or twice should be enough to pressurize the rail. If not, I'd suspect a drain-back problem or weak fuel pump.

On my old Series III XJ6, which uses the same pump, I did some pressure testing years ago. The pressure would bleed to zero over night but the pump would restore full 36psi to the fuel rail in literally 2-3 seconds. It was almost instantaneous.


Cheers
DD