Re: 1992 XJS 5.3L V12
A reliable shop (and Jag-lover member) confirmed that the fuel rail thermo sensor switch is faulty: No Ohm reading at all.
A) I know the part is No Longer Available so, I am up the Sherbourne without a pole.
B) Hard starting when hot may be a problem.
That said, I want to do right to the engine and not cause any harm.
Should I take any measures to ensure no harm is caused to the engine ?
The archives mention a manual switch under the dash to by-pass the switch, I believe. Other than that, are there other work arounds ? I am not Bosch F.I. system smart ( This is a Bosch system, isn’t it ?) so, are there similar thermo switches used in other vehicles equipped with this type of Bosch F.I. system ?
Assume I know nothing yet want to rise to the occasion.
Greater Metro Chicago, Illinois; U.S.A.
No ohm value would seem correct for a thermo"swtich" that’s cold. An open circuit would mean that it’s downstream vac switch (interruptor I suspect) is not energized meaning full vacuum would be available at the FPRegulator lowering fuel pressure. (Higher fuel pressure is given when the fuel rail is hot helping overcome any vapor issues). I supposed the mechanism could work in the opposite way, but it would seem silly to designed a system that required a vac switch to be powered all the time until a high temp was reached.
There are, doubtlessly, thermal switches available that will fit there… the question will be finding out what their “specs” are so that they work properly at a desirable temperature.
You could ignore the issue, or switch it over to a direct thermal vacuum switch bypassing the electrical one as well.
By 1992, the fuel rail thermal vacuum switch (EAC5086), used on some earlier models, had been replaced with an electrically operated switch. There was a single FPR on the front end of the A bank rail, with no vacuum port for a hose connection to connect to a vacuum switch. The switch itself (DAC3474) had no ohm reading. Rather it operated by use of a bimetallic strip that was a make/break contact. There was a conflagration of parts associated with the full operation of DAC3474, but that is not what Richard is asking about here. SD Faircloth
I forget the MM size that fits the bung on the fuel rail, but given your site lists the vac switch operating around 156F, I’d assume one would look for a normally open fan switch that operates around the same temp. The FACET catalog: https://catalogo.facet.eu/EN# (looking in the “THERMOSWITCH FOR ELECTRIC FANS” group) gives a number of good candidates.
You’re thinking not unlike what I was beginning to ponder. For one, it is a Bosh FI system. Reasonable to speculate there may be makes other than Jaguar using such a thermo switch. And, the V12 does use several temp thermo switches - the fan switch among them - so, one of them could work. I’m no engineer but, I am supposing the temperature operating range does not need to precisely match the OEM fuel rail thermo switch. (I expect there are many variables in an engine compartment so some components need a “wide sway”.)
I’ll look at that FACET catalogue later today. Thanks.
Greater metro Chicago, Illinois; U.S.A.
Rephrasing and correcting one of my questions that has become sidelined - Will any harm be done to the engine because of a faulty Fuel Rail Thermo-switch (Correction: DAC10335 according to the Barratt parts catalogue) ?
Greater metro Chicago, Illinois; U.S.A.
Remember, the fuel rail switch is an add-on. It’s not directly tied to the EFI system in any computational or sensory way. It only affects the operation of the fuel pressure regulator, so yes, turning the vacuum on or off at the outgoing FPR to lower or raise fuel pressure is entirely situation dependent - climate - heat soak, etc. No harm at all can be done to the engine even with it disabled. The only possibility is of hard starting due to over-heat of the fuel rail resulting in vaporization, or if one wanted to bump fuel pressure on a timer for enhanced cold starting (richer fueling before the O2 sensors heat up and kick in).
That fuel rail switch is up for debate if it really helps. All it does is increase fuel pressure a few psi until new cooler fuel fills the rail. So instead of let’s say 8 seconds without it, maybe now it only takes 5 seconds? It also increases the rpm a bit with richer fuel.
Mine didn’t work when i got car. I bought a working used one, didn’t notice any difference on hot starts. Took a few infrared measurements after shutting down, my fuel rail never even got hot enough to trigger it.