Hot Engine Not Catching - Do I understand correctly?

Ahoy !
1991 XJS Convert.; V12, 5.3L
I’ve been reading the numerous related posts here in the V12 forum.
Please correct me -
Appears to be not an uncommon problem
Problem is: Actual Loss of fuel pressure in the fuel rails or, False message of pressure loss is being sent.
Most likely reason: Actual loss of fuel pressure in the rails caused be Faulty Fuel Pressure Regulators (FPRs)
Next likely reason: Faulty vacuum switch in the vacuum line from the FPR on the fuel rails (Two vacuum switches because there are two FRPs?)
Third and least likely reason: Faulty Coolant Temperature
Sensors (CTS), one on the left thermostat housing (for the EFI ECU) and
one on the right thermostat housing (for the Marelli ignition ECU).
Both should be in the range of 2500 ohm @ 20C and 250 ohm @ 90C.
[Assumption: Fuel pump is working properly as engine starts, runs and shuts down just fine otherwise, both local and highway. Ditto for coolant system: Coolant temps in range.]
Typical situation as described to me ('tis my brother’s car) -
Summer day, two - four hours highway drive > Stop for gasoline > Cranks, no catch > Open hood & allow to cool several/five minutes > starts right up and off one goes as if nothing happened.

Thank you.

Richard Cielec
Greater metro Chicago, Illinois; U.S.A.

It’s the famous Vapor Lock Richard…
Lots of info in the archives.

Thank you.
Yes, the famous Vapor Lock, a popular, though not loved, topic.
I am hoping some of the Mentors or some Veterans of the Vapor Lock wars could confirm my understanding of the archive discussion.
I should have mentioned, also, that I understand this is an inherent design flaw having no permanent fix; that is: No problem when the car was new; however, there are several parts that, when worn & faulty cause the vapor lock so, the “fix” is a part by part change-out, whichever fails first.

Best Regards,
Richard Cielec
Greater metro Chicago, Illinois; U.S.A.

Many of us have experienced the V12 Vapor Lock, and tried to resolve it, but with little success. We have a 1990 XJ-S convertible (5.3L V12 w/ Marelli ignition) and a 1990 V12 Vanden Plas (5.3L V12 w/ Lucas ignition). The V12 VdP never has a problem with vapor lock, but the XJ-S convertible does. I have tried swapping some components between the cars in order to identify the culprit, but I haven’t been successful yet. From anecdotal evidence in the archives it does not appear that the type of ignition system, Lucas or Marelli makes a difference. I recently removed an old fuel pump in our XJ-S convertible due to another problem and hoping that a new fuel pump would help, but it didn’t. The fuel injectors in both cars were professionally cleaned and serviced by the same shop and that made no difference. For now I am living with it, and driving the V12 VdP more when it is hot out.


I thought I had vapor lock starting issues, but for me it ended up being bad Coolant Temp Sensor and the actual plug for it. Very important for starting it seems. Cheap and easy to replace, worth eliminating as the culprit.


Thank you for mentioning that. I may swap the EFI Coolant Temperature Sensors (CTS) (EAC3927) between my two V12 cars to see if the vapor lock hot start problem migrates with the CTS swap. It is the same part in both cars, and I have some spare ones just in case I discover something interesting along the way.



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It also turned out, my plug was not gripping onto the CTS prongs perfectly tight. I cleaned off corrosion, but I had original 34 year old plug. Pretty easy to get new plug and wire it in.

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It’s not so much that parts have become worn or faulty. The real issue IMHO, is that the fuel composition has changed over the years. The current fuel composition is not one that these cars were ever designed for.
As cars get older they seem to take on personalities of their own. It seems that there can be two identical cars where one suffers from vapor lock and the other does not. Ed Sowel had a 1976 XJS and cured his vapor lock by primarily installing a later fuel rail. I had a 1978 XJS (essentially the same car and electronics) and cured my vapor lock problems by fabricating a new fuel rail. In both instances the issue was the Pre-HE fuel rails that became heat collectors and provided alternate paths for the fuel thus bypassing entire sets of injectors.
The later cars suffering from vapor lock face additional challenges, because Jaguar supposedly had fixed this issue. That said the diagnostic process is essentially the same.
Start by installing a fuel pressure gauge and do a leak down test on the fuel rail.
If OK move on the FPR vacuum assist increasing the fuel pressure as designed. Then to the sensor of the fuel rail. Report back after each test. The people of this list will help you sort it out.

G’day Gents,

Maybe completely unrelated but the later X305 V12 cars, with two fuel pumps, activate one of the fuel pumps for several seconds at the IGN 1 position and then the other pump runs when the engine starts.

Would the running of one fuel pump before the engine starts purge a vapor lock?

Having flown several light aircraft over the years most of them have a priming fuel pump which is activated before a start is attempted. Sometimes a cold engine will start without priming but a hot engine is much more problematic.

You could possibly rig a switch to activate the fuel pump before you attempt to start the car.


This would be a very interesting experiment Paul.

It has been already mentioned Jeff, and indeed very easy to implement. Either a manual switch or temperature triggered delay relay if one wants to go more high tech.
But no one has tried it… I don’t have a hot start problem (nock on wood) so there is not much of a point for me to try.

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I am curious as to what temp your cars are shutdown at, and what your ambient temp is.

My V12’s shutdown coolant is between 190F and 200F (I have lower temp aux fan sensor, so engine rarely sees temps above 200F). I have measured hot soak, and it will usually go up 10F within 20 minutes. My ambient temps are usually 60-75F here in Seattle.

So 200-210F may not be enough to vaporize fuel?

My shutdown temps are the same, 90° - 93°C, and it can get really hot here is Southern France.

Actually I lied, I had a hot starting problem once now that I think about it.
It was after the Control Technique (MOT), it was a very hot day and the guy had the car idling for about half an hour… guess he liked the sound of the V12… He turned the engine off and twenty minutes later when the paperwork was done and I wanted to leave, the car just refused to start. It did start eventually though.
I imagine that if I lived in a city like Athens, constantly getting stuck in traffic jams at 40°C summers, I would have that problem more often.

I do have a temperature sticker on the Vee, it definitely gets hotter than actual coolant, which would contribute the actual radiant heat to fuel rail.

I’ll check next time, but if I recall last time i did, engine shutoff was around 200F, 20 minutes later coolant peaked st 210F and engine block at Vee was around 220-230F?

Many years ago when I was experiencing hot start problems I was fortunate to have access to the Jaguar headquarters in New Jersey. Remember speaking to an engineer there that said heat soak on the V12 could approach or exceed 300 degrees. I assume this would be under extreme conditions.

I posted this article a short time ago, but it didn’t get much traction. It’s a long read, but for you guys looking for a scientific description of what is happening, you should read/copy this article. Unfortunately, the solutions used in the article are not available for the V12s. SD Faircloth

I am curious if the “Hot Start” issues are as prevalent on V12s that either don’t have or have removed the air pump.

I removed my air pump smog equip…the pump only operates when cold, right? But are those pipes still full of hot exhaust all the time?

Another good point, those of us who have blocked off the exhaust ports in intake manifold usually plug from top. Should we be plugging from bottom?

I removed the air pump many years ago on our 90 XJS as well as all the plumbing associated with it. I installed the idler wheel from Simply Performance.
It seems to me the air pump rails would transfer radiant heat from the exhaust ports to the air pump manifolds that sit very close to the fuel rails. My rails are gone but it would be interesting on a V12 that still has them installed to put a heat gun on them after running and shut down, and see how hot they are…I bet VERY hot.

I have no air pump gear of any kind. I have a hot start problem. In my specific situation, the car will fire instantly, but will then die unless you hold the throttle slightly open. At that point the idle will surge, badly, to like 1500 RPM, for probably a minute until it stabilizes.
Of course air temperatures in the high 90s or low 100s are not unusual at all here, (that’s called “spring”) and of course the problem gets worse on days like that.
It’s entirely possible that my car would not have a hot start problem in Seattle. Operating in air temperature that is a 20° hotter, e.g. Seattle v Phoenix, is certainly going to make a difference in how hot things get.

I am curious Bob if you ever pop open the bonnet after parking, and if that “venting” makes or doesn’t make any difference?

I frequently pop mine open when I park to vent out hot air. And always raise at home to vent. But not because of hot start issue…but to try to extend life of rubber lines, insulation, etc.