XJS V-12 Fuel Temperature Switches

This document is not complete. It represents some of my notes on this topic, and is in dire need of editing to make it more digestable.

My 1985 XJS (project car) has a broken electrical switch. If you look at the part still on the fuel rail here, and the shot of the vacuum one below, you'll note that they look quite similar. At first, I believed that this was merely a broken vacuum switch, and that the vacuum line had been re-routed from the source at the front of the B-bank intake manifold to the fuel pressure regulator. After it was brought to my attention that someone's 1986 had an electrical switch here, I looked around, and discovered that there was indeed a pair of electrical connectors at the Air/Coolant temp sensor harness which were plugged directly to each other (pictured on the right, above). These wires are not present on my 1988 XJSC.

Mike Morrin, of the Jag-Lovers lists posted a photograph of the (intact) electrical switch on his 1986 XJSC.

My 1988 XJSC has a vacuum switch. I'm not sure whether these were present on 1987 MY or not, but comments on the Jag-Lovers XJS list indicate that the vacuum switch is present on later (Marelli) cars as well. This is Jaguar part number DAC 3474.

The Jaguar ROM doesn't cover this well at all - from what I can tell from the diagrams, it doesn't appear that pre-HE's were fitted with anything like either of these at all. Perhaps because the pre-HE's ran at lower compressions, there wasn't quite as much heat soak.

p86A-71, Component Location - Engine 5.3 HE in my edition of the Jaguar Service manual shows this as an electrical switch. In the entire fuel section, there appears to be no mention of the fuel rail temperature switch - electrical or vacuum.

In 1990 (and later?) XJS', the electrical switch reappears on the fuel rail, located on the A-bank, but is now wired into the A/C circuit. Al Askevold posted two photographs of his fuel rail showing the presence of BOTH the electrical switch and the vacuum switch. This switch is shown under 1990 MY on in Supplement C of the Jaguar Service manual (this not being part of the documentation I currently possess).

Note that this is specifically 1990 and later -- Wayne Estrada confirmed that his 1989 Marelli only has the vacuum type switch.

Unlike the vacuum switch, the electrical switch does not have anything to do with the fuel pressure regulators.

On the 1990 and later Marelli, it is wired to the A/C compressor relay, so that it can engage the compressor to circulate the A/C refrigerant, thus improving fuel cooling -- a process which takes place on the fuel AFTER it has left the fuel rail, and helps to cool down the fuel returning to the fuel tank, where, if it is too warm, it will give off polluting fuel vapours. Additionally, if the fuel is warm on returning to the engine later, it will require less engine heat (from that not-so-cool-running V-12) in order to near vaporisation again. I haven't seen the circuit diagram, but unless they're doing something really funky, this means that the switch is a normally open (NO) switch - contacts close when it reaches its switching temperature.

The way I'd think this works is that if the fuel temperatures are sensed as being higher than the switch threshold, it engages the A/C compressor even if the A/C is off, so that the A/C refrigerant cycles around it's circuit, so that there is more effective cooling at the fuel cooler. I had wondered about this on even my 1988 -- if the A/C circuit isn't on, then the fuel cooler can't be doing much of anything, since the refrigerant isn't being pumped around the system. Perhaps up until the Marelli, Jaguar simply assumed that drivers would actually be running their A/C when it was warm out, but in 1990, decided that perhaps they should make sure.

This brings up a decent point: if your A/C has problems, it'd probably be a good idea to locate your compressor relay and PULL IT OUT, so that (on those model years so equipped), the fuel temperature switch (electrical type) won't engage the compressor when you'd rather it didn't. Obviously, fixing your compressor or A/C circuit would be in order, but if you don't want it coming on, this would be a good way to ensure that it doesn't.

On the HE's prior to the introduction of the vacuum switch, the electric Fuel Temperature switch is wired in series with the Air Temperature sensor. If the Fuel Temp switch is open (hot), the ECU will NOT get an Air Temp reading, and must switch to a map or other default value (as per previous contributions by Roger Bywater, this can be taken to a 10% enrichment given the ECUs on which this switch would be found, if I've interpreted correctly). Below 68'C/154'F, the switch should be CLOSED, and above 75'C/167'F, it is OPEN. If it is failing (or you're "simplifying" your collection of underhood components), you'd want to CLOSE the circuit to which the sensor plugs into, or your economy will suffer. In fact, if your economy has recently begun to suffer, you might check this switch.

In consideration of these design changes, I suspect that the simple A/C cooling apparatus was Jaguar's original attempt to resolve vapour lock, and then they wired the electrical switch in series with the Air Temp sensor, but that later, it was determined that you could still suffer hot start problems due to heat soak after shutdown, so they replaced the electric switch with a vacuum switch to maintain elevated fuel pressures after startup, which would do a better job of addressing the vapour lock problem. With the 1990 MY, they elected to introduce an electric switch - in addition to the vaccum switch - in order to ensure that fuel was cooled on return to the tank. This of course is entirely conjecture on my part.

Theory of operation for the vaccum type switch: when the fuel temperature is high enough, the vacuum switch interrupts the vacuum to the B-bank fuel pressure regulator, causing the fuel pressure to be raised (the B-bank fuel pressure regulator is what bleeds the fuel from the fuel rail, through the fuel cooler assembly, and back to the return line to the fuel tank). At some temperature (which varies wildly with the brand and grade of fuel used), fuel begins to vaporise, and by increasing the rail pressure, the circuit can prevent the expansion of those vapours, and/or help force them out of the system by "pushing them to the nearest exit".

It is somewhat ironic that the vacuum switch is present to assist with problems with hot starts (due to fuel vaporisation causing vapor lock), yet no engine vacuum is present until the engine is running - until that point, the initial fuel rail pressure will be higher than under normal running because the pressure regulators will not have any vacuum with with to operate (this happening even without the presence of this switch). I would guess that the vacuum switch will ensure that the pressure is maintained at the higher level until the fuel is cool enough that the switch is no longer actuated, lest you start the car, and have it die almost as soon as vacuum appears at the regulators.

While I haven't confirmed it myself, Wayne Estrada indicates that the sensor (at least the vacuum variety) which appears on his car "does not contact the fuel directly", from which I would take it that it is possible to unthread this sensor and replace it without concern about fuel spillage.

Here's the route the fuel takes: the fuel comes from the tank, picked up by the pump and forced through the filter, delivered to the engine on the A bank side (RH, from driver's perspective), is sent through the regulator (EAC 6314,, at least on HE's) on that side (which may be acting as a pulse-inhibitor - I understand that several people have scrapped them to simplify the plumbing - has anyone checked the pressure characteristics of this regulator in comparison to the one on the B bank?) and into the fuel rail, around and out the B-bank side, through the regulator there (EAC 4864, again, at least on the HE's), through a pipe that feeds UNDER the intake manifolds (EAC 9986 - IMO, a stoopid design - why intentionally subject the returning fuel to heat when you're just about to cool it?), and into the fuel cooler assembly, coming out of there near the front of the B-bank intake manifold, then dropping down and passing through the firewall to return to the tank.

By having the temperature switch on the exit side of the fuel rail, they're reasonably assured that the fuel will be warmer than the opposite side, having been in the system at least a small amount of time longer than the fuel entering from the fuel tank (basically, the exit side is the better side to put it on if you want it to trigger on the hotter temperatures, versus failing to trigger because it is sensing a temperature from the cooler side). The temperature switch (vacuum type), when actuated, should cause the return fuel pressure regulator to not bleed off as fast, causing the pressure to rise in the fuel rail (hopefully condensing any bubbles of vapour and forcing them through the injectors - or better, back through to the return line).

WHY they don't cool the fuel on the feed side is a bit puzzling though - seems like it'd have a greater positive effect on vapour lock that way.

As an aside, and a real shot in the dark, but considering as plumbed, the B bank is the side more likely to suffer localised vapour lock (fuel in that side has been in the fuel system a touch longer than the A-bank), I wonder if the presence of the PCV valve on the B-bank was intentional (as versus putting it on the A-bank side), since it can (in only a very minute way) contribute to enrichment, due to the re-introduction of HCs from the crankcase...

Reid Vapour Pressure (RVP) varies dramatically with different fuels, but for the most part we can say it is under 10psi (possibly MARKEDLY below this - the higher pressures come from more volatile fuels). RVP is the amount of pressure that fuel will create when heated from a circa-0'C state to 38'C (so actual _operating_ pressure within a car will be different - for one, the fuel rail may be much hotter than 38'C, and the source fuel isn't nearly as cold).

I do not know what the switching temperature of the fuel rail temperature switch is, but my guess is that it is probably right around the point at which the vapor pressure of a "typical" fuel would be approaching the regulated pressure in the line (i.e. the vapors are beginning to expand to the point at which they would become a problem).

(Side note: because LPG conversions require only low pressures (like about 6 psi or so) to maintain a liquid state, AND because when they're injected, you should WANT them to be gasseous (I'd think), LPG conversions are unlikely to suffer vapor lock problems.)

Personally, and without sufficient engineering background, I would think that you could reduce the vapour lock problem by increasing fuel circulation: by having the feed-side regulator trying to maintain a set pressure (that which the rail is supposed to be at, or perhaps slightly higher), and having the return regulator bleeding it off trying to maintain a lower pressure (though not dramatically lower), you'd be promoting flow - rather than letting the fuel sit around in the fuel rail getting warmer.

This is probably pretty close to how the system is working normally (I have no info on the internal operational specs of the two pressure regulators - but they are different part numbers where they could have managed with one if they were in fact spec'd the same) - the feed side letting just enough new fuel in to satisfy consumption and to boost the pressure a touch, and the return side constantly bleeding off that additional pressure to maintain balance.

It's all very complicated at times: lower pressure affords the vapors a better opportunity to expand, yet, a DROPPING pressure correlates to a lowering temperature. Of course, the fuel system isn't running at pressures high enough to affect a significant thermal change, but if you've ever used a can of compressed air to spray out the dust bunnies from your computer, you've probably noted how the can gets COLD as you discharge the gas (which is reducing the pressure).

There was mention that the fuel cooler may have been in order to reduce vaporisation in the return line due to some design of the fuel sump, where the fuel may being dumped near the pump pickup, perhaps to produce a venturi effect (for what result, I have no idea). I have't ripped into my sump yet (let sleeping dogs lie - I have no fuel problems yet, and don't feel like developing them either). Gut feeling is that the pump should do it's job without such an external "inducer", AND the pump has to start pumping in order for the pressure to build up to feed the inducer anyway, so what's the point of it?

There are three fuel rail pressure values: 42, 36, and 28 psi. If you turn the ignition to RUN, but don't actually crank the engine, you should have about 42 psi in the line (the regulators are getting no vacuum). 36 psi is what the fuel rail would be pressurized to if you applied vacuum (say, with a vacuum pump) to the B-bank pressure regulator, when the engine is off, but the ignition (and therefore, fuel pump) is in RUN. 28 psi is what you should have in "typical" running condition if you hooked up a fuel pressure gauge to a running system (with fuel being purged from the fuel rail through the injectors). The running fuel pressure (28 psi) should climb (approx 6 psi I would think) when the vacuum is removed from the B-side regulator while the engine is running, but I don't have measured values for this. I hope to check fuel rail pressures on my 1988 XJSC in the near future.

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