Hot start issue in Ireland

Hi guys,
First of all my thanks for the help jag lovers has given me in the past…without it my xjs would be undrivable. Had a stripped plug thread and jag lovers search showedup a “back tap” what a wonderfull invention !! So plug hole sorted and new seals on the injectors, new plugs and leads and off we go in the pouring rain here in Donegal in Ireland. Runs perfectly but after a good run I leave it for 20mins and I have a hot start issue. Eventually catches but has never happened before. So, reading the topics I look at the fuel rail… It has a brown sencsor on the left hand side. This has one exit and has to my knowledge never been conected. I did not know what it was until I read the posts. So there is only one take off from the inlet manifold on that side and it is taken up by the fuel pressure regulator. I put in a T piece and connect to the brown sensor on the fuel rail…same problem. not to be defeated I look at Nigel Tholeys book on the xjs. I see a brown sensor with TWO take offs but cannot work out where they go. So digging through my spares I come up with said sensor…with two take offs. So does anyone know where they go ?? Any help would be gratefuly recieved. Now I am off to get soaked on a motorbike in this God forsaken country. Ireland is very green…there is a reason for that.!1 many thanks. Frank xjs 1989 marelli ignition.

It sounds like you’re talking about the fuel temp sensor. It should have two nipples: one to a source of intake vacuum and the other to the LH fuel pressure regulator.

When working correctly the function is to dump the vacuum signal to the pressure regulator when the fuel reaches xxx-temperature. This vacuum dump results in increased fuel pressure to clear the fuel rail of vapor lock or fuel percolation.

Many have tossed the switch into the bin with no ill results. Unless you’re in an unusually hot situation I somewhat doubt that you’re suffering a vapor lock/percolation scenario.

However, do make sure the regulator is getting a vacuum signal. If not, the increased fuel pressure might be causing an over fueling problem…which a hot engine might not like

Another consideration is injectors dripping after shutdown…likewise causing an over-fueling situation

Others will chime in with other ideas


If you are asking about the EAC5086 Thermal Vacuum Valve (also called the Fuel Temperature Sensor) shown in the first picture, the forward facing port should have a vacuum hose leading to a nipple on the left intake manifold and the port facing the left fender should have a vacuum hose leading to the nipple on the left hand fuel pressure regulator as shown in the second picture.

I just took both pictures of the engine in my wife’s 1990 XJ-S convertible(5.3L.V12 with Marelli ignition).
The EAC5086 was also used on the EFI equipped Series III XJ6s and XJ12s. We used to have two XJ6s, three XJ6 parts cars plus a few spare XJ6 engines. So I had a bunch of the EAC5086s (10?) at one point, tested them all, and about half of them were inoperative. I recommend that you test any EAC5086s that you get to make sure that they work properly otherwise they will not help with hot starts.


Hi Doug,
Thank you for the reply, I have a hose going from the fuel regulator to the inlet manifold so I think thats ok. Could i have damaged the injectors while removing the fuel rail ? I put new seals on them and it seems to run great…until the hot start. will see what I can do to check the injectors.
Thanks again,

Hi Paul,
Thanks for the reply and the pics. My fuel regulator has only one hose going off to the inlet manifold on the left hand side, so I am going to copy what I see in the picture tomorrow and we shall see ! ! Again many thanks for the help.

Hi Frank,

This week I just finished replacing my auxiliary fan with a new 2000 cfpm fan. It completely solved my hard start after shutting the engine off while doing errands and having to restart within after an hour or so. I live at around 3500 ft elevation and the temperature has been 105+ deg F constantly. Prior to that I also replaced all radiator caps with 16 lbs back pressure. One was blowing out coolant upon engine shut-down.

The engine was getting rather hot with any kind of a run except a long down hill’er. Then after shut off the auxiliary fan was not pushing enough air to cool the radiator down and the engine actually continued to heat up just sitting parked. At first I thought it was the battery being weak. One does not immediately get a good temp reading upon cranking the engine to start nor look at the temp once the engine has started and of course if it didn’t start, there is no indication of engine temp at all.

Because I knew the engine was getting hot and the auxiliary fan was obviously lacking in the cool down period after parking and shut-off, I tackled the fan first and that has solved the 'Hot Start Issue’ completely.

I may be missing something, but I had the same issue with my 86 XJ6 and 91XJS. Turned out there was an check valve by the fuel pump that was defective, and caused the fuel to flow back in the tank (due to the fuel in the rail evaporating due to the heat (vapor lock). You can test this (sometimes it works) by turning the key to the start position and then off several times and then try starting it.

Hi Paul,
Just to clarify, (before I break something, as I am good at that ) the nipple facing the left fender goes to the inlet manifold. The picture shows a white piece. Is this a T piece and if so where else does it go to ? The fuel regulator on mine goes straight to the inlet manifold. so tomorrow I will give it a go .I have no way of checking the vaccum valve . so its trial and error ! And yes, I am a begginer at all this stuff. !
Thanks again.

88 XJS. I tested my hot start valve and it didn’t work. So i removed it and hooked vacuum line straight to FPR. I’ve also removed my a/c fuel cooler. I am also running 180F thermostats, the coolant temps have never got past 195F.

I live in a similar climate to Ireland (Seattle), and have never had a hot start problem, in fact, it starts better when parked hot than if it’s cooled off.

I would look at electrical. When a bad GM ignition module gets hot, it can cause a hard start. Over time they go bad, or the ‘cooling grease’ doesn’t work as well. Only $40 (only get AC Delco/GM!). A bit of work first time trying, but not too bad to replace.

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Do you own a copy of the Jaguar Parts Catalogue for the XJ-S? If not, you should seriously consider getting one. I got one for my wife’s 1990 XJ-S convertible when I purchased the car in 2005 and still use it regularly. It is worth it’s weight in gold for sorting out the parts I need, what the part numbers are and where to purchase them. It is very helpful in answering simple mysteries like this.

The port on the Thermal Vacuum Valve is a different diameter than the nipple on the intake manifold port, so one vacuum hose will not fit both ports properly. Jaguar used a EAC9286 “Reducer” in the hose to adapt to the two hose diameters. You don’t need to purchase the part from Jaguar (although they might have them available since they used them on several models), you can probably find a similar part in any auto parts store. I suppose that some might say, “just force the hose onto the nipples”, but that is not my way especially since the correct part is readily available and I prefer to do it the right way.




The thermal vacuum valve has two ports (nipples). One port connects to the vacuum port on the B bank fuel pressure regulator. The other port (per OEM fitment) connects to one of the nipples on the front end of the B bank inlet manifold. It does not matter which port on the thermal valve you connect the hoses. It is not a one way valve. ****If you do not have an operational thermal valve, you can connect a vacuum hose from the B bank FPR vacuum port directly to the nipple on the front of the B bank intake manifold. When operating correctly, the valve is suppose to be open at temperatures less than 155F, allowing vacuum from the intake manifold to work on the FPR diaphragm. Above 155F, the valve should close, blocking vacuum to the FPR diaphragm and increasing fuel pressure to the rail by several pounds. There is a page on my web site with a bit more info but thats’ most of it. I have a valve if you would like to contact me off list. SD Faircloth

Over the years I have connected a fuel pressure gauge to the EFI fuel rails in our four XJ6s, XJ12, and XJ-S for a variety of reasons to study their fuel pressure under various conditions. On some projects I left the fuel pressure gauge connected for days at a time and taped the gauge to my front windshield so I could observe the readings as I drove the car around my neighborhood sorting fuel issues out (much to the amusement of our neighbors). One thing that I learned was that fuel pressure in all cases bled off after the engines were turned off. Some bled off quickly, while others took an hour or more. I am sure that there are multiple reasons for that like leaky fuel injectors, aging fuel pumps, etc. I agree with you that charging the fuel rail with pressurized fuel, especially on hot days with a hot engine, should minimize vapor lock especially if the engine is started right after charging the fuel rail. I don’t believe I have ever suffered from hot start vapor lock problems over the past 20 years in any of our Jaguars although I have charged the fuel rails as you suggest on especially hot days with a known hot engine. But I have tested and made sure that all the Thermal Vacuum Valves worked properly by testing them as SD Faircloth suggested to make sure that the higher fuel pressure was available to avoid vapor lock. As I previously mentioned, about half of the roughly ten Thermal Vacuum Valves that I have tested failed the test and would not have increased fuel pressure as they were designed to. So I suspect that a large number of XJ6s and XJ-Ss out there have failed Thermo Vacuum Valves and are more prone to hot start problems now than when they were new and the Thermal Vacuum Valves worked as designed…


Thanks to all that have replied, and to paul for the pics (thats a clean car Paul !) I had an idea of testing the valve by closing off the vacum from the inlet manifold when the car was having problems from a hot start. Worked first time. So problem solved…I need a new valve. I got a parts manual and tested the old valve as described by SD Faircloth. it opened at 90c…As Paul said, a lot of valves do not work… My thanks to all for your help guys. I must have saved 10 times my donation by visiting this site. Regards to all. Frank

Thanks. Here is a picture of the full engine bay in my wife’s 1990 XJ-S convertible. I restored the engine bay a few years ago after it started to look a bit shabby and after the bare metal repaint and new convertible top that I installed with the help of a friend. It now looks much like it did when new, 30 years and 132K miles ago.

We got it in 2005 at about 65K miles and I have done most of the work on it since then with a lot of help from my Jag-Lovers friends.
If you are looking for a good used EAC5086 Thermal Vacuum Valve you should try David Boger at in North Carolina, USA. David is a Jaguar Enthusiast and Jag-Lovers member who has a side business of buying and breaking old Jaguars and then selling off the good used parts. David is a great guy who really knows his Jaguar parts. I have purchased several parts from him over the years and I have always been pleased with the parts. I wouldn’t be surprised if he had some known good and tested EAC5086s available on his website (strong hint).
I have no affiliation with David Boger or other than as a satisfied customer.


Thank you Paul I will be in touch with David. Love that engine photo. Its not an engine, it is a piece of art ! As regards the hot starting issue, when the engine was up to normal temperature with the aux fan going, I would switch off and it would start no problem in 5mins…or 10mins.or 15mins. But at 20mins the fuel rail was at 75c and it would start but was not happy for a good while. After 25mins it would be perfect…go figure. Removing the pipe from the fuel regulator to the manifold or blocking it off meant it would start during its 10min window of a hissy fit. So new valve and all will be well.

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Please write back after installing a known good EAC5086 to confirm that this fixes your problem. This adds to our collective knowledge.
There are those who jump at the chance to modify the original design at the drop of a hat. I believe that the Jaguar engineers knew what they were doing and in most cases I just try to keep our Jaguars looking and working as advertised. I have made modifications to things like the dash lights, interior lights, convertible rear windows, adding a ski slope with cup holders and adding an auxiliary cooling fan advisory light to increase reliability or usability.


The thermal vacuum valve was arguably a good idea poorly executed. Works great until the valve fails or gets busted by a gawker leaning over the engine compartment, and replacement thermal vacuum valves are harder to come by every day. It doesn’t help that the port in the fuel rail seems to be an oddball thread.

Before Jaguar came up with the thermal vacuum valve, they had a thermal switch in the same location. The intention was the same, help with hot starts, but it worked differently: It disconnected the air temp sensor in the LH air filter housing, thereby making the ECU think it was 400 degrees below zero out and it needed to pour major fuel in to get it started. I dunno why Jaguar eventually superceded this idea in favor of the thermal vacuum valve – which likewise richens the mixture, although in that case by boosting the fuel pressure – but one can presume the thermal switch works better than nothing. And if neither item is available from Jaguar, it’s probably easier to find a generic thermal switch that can be rigged to work. It might not even have to go into that existing port on the fuel rail; perhaps it can be just strapped to the side of the rail and wrapped with some insulation so it sees fuel rail temp.

One other idea: Once a thermal switch is wired up, perhaps it isn’t ideal to simply disconnect the air temp sensor with it. Perhaps it’d be a better idea for the switch to substitute a fixed resistor, perhaps telling the ECU that it’s 200 degrees below zero rather than 400. Once a switch is installed, one could tinker with resistance values and see which one works best.

Finally, one more radical idea: Rather than interrupting the leads to the air temp sensor, one could interrupt the leads to the coolant sensor. Disconnect that sucker and things get very rich in a hurry. Might be too much, so again perhaps cutting in a resistor so it only richens a bit rather than a ton.

Will do Paul. Looking for one right now !

Thanks for the ideas Kirbert, I will certainly give them some thought. Electrics are a bit of a dark art to me but it could be time to learn ! Temp in fuel rail was 75c after heat soak of 20mins and the valve did not close so as you say might be worthwhile trying something else if I cant get a new valve.

Thanks to all for the help , problem solved! As Paul said, half of the EAC5086s are probably dead, So working on that average I bought two used ones…one of them worked ! I tested both and one was dead and the other fine. I must have started the car 20 times when at its "hissy fit time " and it started every time. Thanks to all for your help. Now all I need is some sunshine in this sponge of a country !

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