Vapor Lock Season - Cure

Its that time of year again, hot as blazes, long drives with a 20 minute stop somewhere for lunch or grocery store, and its a struggle to get the XK running smoothly before being able to pull out into traffic. It’ll start, but it coughs and sputters until you rev the engine several times and wait 5 - 8 minutes.
Search the archives, and 90% of the time, this inquiry topic gets derailed “must be a timing issue”,… “do you have the insulator blocks on your carbs”…which do not address the issue. And BTW, when insulator blocks were put on the Jaguars, that was before ethanol gas. One suggestion even said, “leave your gas lid open”… that does not help or address the issue of boiling gas in the fuel line, which creates an vapor pocket !
I have owned & driven my OTS for 43 years, but have only had to tolerate it since ethanol gas. The most agreed upon cure for carburetor cars is to install a simple Vapor Lock fuel filter that has 2 outlets, one of which you run a line back to the gas tank. It looks like this:
There are loads of vapor lock cure videos on YouTube, and all are essentially this same cure. Its not a problem with EFI cars because the pressure is so much greater than our little 1-10 psi pumps. Greater the pressure = higher the boiling point.
Every summer it is the same, but this year, with record breaking heat indexes throughout the US, its even more pronounced.
So, my question today is, how would be the best way to tie the return line into the gas tank ? I do not want to install a fitting into the gas tank, so perhaps there is a way to tie into the filler
hose ? Has anyone done this ? Thoughts ?
Thanks very much,

Hi Knight,

Hope you’re well.

Is there a specific place where the vapor lock occurs or starts? Is it the braided hose or perhaps the copper pipe that fills the two float chambers? I had the same yesterday (30 C or 86 F here) with “coughs and sputters” for about 15 seconds after which the engine picked-up again. I have the XK 150 AC fuel filter installed at the inner fender and thought that that would cure the vapor lock problem. But apparently not completely.


Bob K.

Hi Bob, Great to hear from you and I hope you are well. I’d only be guessing, but I suspect the engine bay. I’ve thought to try leaving my hood up, but every time I stop ?? :stuck_out_tongue_winking_eye: And I know it attracts attention that way. I drive, on average, twice a week, 40 or 60 miles through the mountains with friends. We’ve got a great group of vintage sports car guys, and we go to lunch, stop to pick up groceries, etc.
I remember a time long ago, on the Lovers list, where some guys were talking about installing computer fand, which draw very little, in the side of the engine compartment panels. I’m not anxious to do that, but it would probably work, if turned on when I stopped.
Thanks, K

I’m not being pedantic, but in order to solve a problem one must first determine exactly what the problem is and what is causing it.

I suggest that the problem is not specifically “vapor lock” as I understand it. Vapor lock usually occurs when a mechanical fuel pump which is bolted to the engine gets hot and the fuel inside it vaporizes. The pump can’t pump vapor so it gets “locked”, and it can take hours for the fuel to cool down enough to condense into something the pump can push against. An additional problem with that system is that since the pump us sucking fuel from the tank, the fuel line from the tank to the pump is under a slight vacuum, which lowers the boiling point of the fuel. If you ask any classic American car owner, he’ll tell you the cure for vapor lock is to install an electric fuel pump near the gas tank, which Jaguar has already done for us.

In my youth I was driving with my girlfriend in my MK2 on the interstate on a very hot day and I had just explained to her why there were several cars parked by the side of the road with their hoods up when my car lost power. I opened the bonnet and showed her how the fuel in the glass sediment bowl was actually boiling. Since carburetors are designed to handle liquid fuel, they were not able to deal with the vapor that they were being provided with. I poured some cool water on the filter, wedged the bonnet open against the second lock and proceeded on our way. Later I deleted the sediment bowl so the fuel spent less time in the engine bay and installed a filter in the line under the car.

As for the problem that we are experiencing in this case, I believe it is a result of heat soak. When the car is running the carbs are being continuously provided with a supply of cool fuel, which is being vaporized, which also cools the carbs down. When the engine is shut off for a while, everything under the bonnet reaches the same high temperature, whether or not you have insulators between the carbs and the engine. This causes the fuel in the carbs to boil and the vapor goes into the intake manifold and anywhere between the carbs and the air cleaner. Once the fuel has boiled away the carbs heat up to the same temperature as the engine, which can be 100°C. Now, when you turn on the key, fuel is fed to the hot carbs and instantly vaporizes, which combines with the other vapor to form more excessively rich mixture in the intake manifold and into the engine. The engine will need to be cranked until the excess fuel vapor can be flushed through the engine and the new fuel cools the carbs down enough so it stays liquid and they start functioning properly again. I find that holding the gas pedal to the floor when starting, as would be done if the engine is flooded, helps flush out the excess fuel with fresh air and makes starting easier.

You think you have problems? You should see how much under-bonnet heat a V-12 puts out! The S3 E-Type has a pressure regulator under the bonnet which circulates fuel back to the tank, so the fuel lines are cool. Unfortunately, that doesn’t keep four carburetors worth of fuel from boiling when the engine is shut off, so I deal with the same problem as you do. That’s also why, when you see E-Types parked, they often look like this:


I agree with Mike Eck. When I get “vapor lock” in my Mk2, the fuel in the carburetors is boiling.
It overflows the vent ports in the starting carburetor and makes my worry about fire. I have the insulator blocks, but on a 100 degree day after a 10 or 15 minutes parked, the carbs will absorb enough heat from the engine to boil the gas. I think the only way to minimize this is to have more effective insulator blocks. The OEM ones are not very good thermal insulation. I am sure there are materials now that would be much better. Also the 4 steel studs that go from the manifold to the carb flange will conduct heat right through the insulator. You can see, that on modern motorcycles with carburetors, that the carb is connected to the engine with a short section of rubber tube. There are no metallic connections. This may be beneficial for vibration isolation, but I am sure thermal transfer is also a consideration.

Mike and Kenneth,

Thanks for your contribution and it all makes sense. But (unfortunately) the solution is not clear.

I’m sure my (XK 150) AC fuel filter is reasonably far from the engine and I can see that the fuel is still “fluid”. See picture.

But if the carbs and more in particular the float bowls are the culprit, that will make it more complicated. I will have a look at the insulators between carbs and manifold. I guess Jaguar opted for a thermoset material as temperature resistant material, not for its thermal insulation properties.

A pressed phenolic material using phenolic resin filled with various types of fibers for strengthening purposes, has a thermal conductivity of about 0.94 W/mK (sorry: SI system). This is of course much, much better than Aluminium with 237 W/mK, but not as good as real insulating materials which are typical between 0.03 and 0.04 W/mK but lack the strength we need.

Bob K.

Well, you know, Bob,… I’m not sure of the cure either, :rofl::rofl: but I do know there are a lot of XK-ers inquiring about how to fix this, and the carburettor muscle car guys on YouTube seem to have it figured out with this method,… and I can let you know if it works, or not, in about 48 hours, if I can simply figure out a way to tie the return back to the gas tank without having to install a fitting into the tank… which was the original question to the list.
IMHO, its worth the $28 buck I have invested, and a couple of hours of easy work to try it.

There is a fitting I know exist, but cannot find online, that allows you to install a nipple fitting to the side of a hose. This would allow me to tap into the filler hose. My brother’s '37 BMW 328 had something similar in the radiator hose for e thermostat sensor fitting. The return will not be under any significant pressure. Its just a way to vent the line, release the vapor bubble, and allow our dinky 5psi pumps push gas.

Then again,… maybe this will not work if it is, in fact, a heat soak in the carburettor. One way to find out, and put this theory to bed. If you search the XK-Lovers archives, there are several inquiries about this malady,… with absolutely no solutions offered. Thanks

Bob, the insulators (partially) isolate the carbs from the engine heat when the engine is running, but the problem with heat soak is not affected by the insulators. After you shut off the engine, everything under the bonnet gets hot, so the carbs get hot because of the hot, stagnant air that surrounds the engine when the car is sitting still.

Knight, I believe there is an air vent at the top of the gas tank that connects to a tube under the fill door with a section of rubber hose. Perhaps you could tap into that for your return line.

However, if you REALLY wanted to do it properly you would drill holes in the sides of your float bowls just above the proper height of the fuel, connect those together with hose to the input of another fuel pump which would pump the excess fuel back to the gas tank. That would circulate fuel through the carbs themselves and keep them cool. An additional advantage would be that you would be able to eliminate your floats and needle valves because the scavenger pump would dictate the fuel level. Unfortunately that still doesn’t address the fuel boiling in the carbs due to heat soak after you shut off the ignition, but neither does your suggested recirculating fuel filter setup.

Thank you, Mike.

Hi all,
I recall Ed Nantes commenting about making a return line to the tank to cure vapour lock; here is the link to his posting

I experienced some symptoms of heat soak when hot starting on my recent drive around France in my 140, with temps reaching 28-30˚C and parking in full sunlight. On turning on the ignition, the electric fan would cut straight in and was left running for about 10sec. to move air rapidly through the engine bay), and the high torque starter motor turned the engine over quickly, so within around 3sec. of WOT the motor would catch and run, every time. All part of old car motoring…


Not sure how helpful this may be but I’m in Florida and running my XKs in 90+°F (32C) and do not experience any vapor lock even with stops. However, I run only non-ethanol gas which is readily available here. Not sure of the availability in other areas.

Alan, I should have said that I run everything on UK 97RON, which has no added ethanol (only what’s left in the washings of the tanker from the last load).

Hi, could you utilise the air vent line from the tank to the petrol filler box?
On my 150 S I had set up an electric fan which was wired direct through a thermo switch and it would bring the engine temp down to its normal cut off temp, the fan would run for a few minutes when engine heated up after stopping I had no problems with vapourisation but am also using unleaded 98 Ron in Aust. I also run two 12v batteries.