Fuel vapor lock

(popop) #1

Hello everyone,
I believe my MK IV 3.5ltr is suffering from of vapor lock. Shutting off the engine after casual driving on a Summer Day, it becomes difficult to re-start or not Start at all.
When it does re-start, it would sputter at 1500 rpm. Temp gauge remained under 80c. I checked the fuel filtres, fuel Pumps, fuel bowls etc…no abnomalities noted. Has anyone experienced similar problems, and if so, what preventive actions resolved it ? Any suggestions ?
Thank you,

(Graham Jordan) #2

Yes. Exactly that in my Mk V 3 1/2.
Without resorting to installing a fuel return line back to the tank which some here have done,to allow cooler fuel to be available, I used exhaust wrap around both the exhaust manifolds and downpipes right down under the car and changed the aluminium spacers between the head and the carbs with a Bakelite type of non heat transferring material.
The exhaust wrap alone reduced my issue but I did experience the same problem once more, I believe due to bad fuel (possibly E10 in the fuel), so at that point replaced the spacers.
Haven’t had the problem again.
An electric cooling fan could also likely be a remedy.
Regards, Graham.

(Rob Reilly) #3

Yes, insulating spacers. They are sold for MGTF, MGA and TR2 by Moss Motors and others.

On XK120 and Mark VII, Jaguar themselves added 1/4" thick (6mm) spacers around late '52 after having problems with hot starting.
I added them on my '51 XK120 and immediately saw cooler carbs and easier hot starting. There was nothing but a gasket there before so I had to make longer studs and have 2 gaskets on each.

(Ed Nantes) #4

Except MGs have 1 1/4" carbs and Jags have 1 /2" so I don’t think th e spacers will fit. The Xks have 1/3/4" and 4 mounting studs The capillary return system is simple, Chrysler used in in Aust on Valiants And easier, less expenise and less visually intrusive that fans and exhaust wrapping. Depending how many and where the petrol pumps are , it only has to bleed back to between the tank and the first pump.

(Rob Reilly) #5

have 1-1/2" carbs.
Here is one on my Mark V.

1 Like
(popop) #6

Hello everyone,

Thank you all for sharing your knowledge and expériences. Insulating blocks, if available, and fuel return lines makes sense to help rectify the possible cause. The engine does Start, once it has cooled down a bit. I haven’t yet checked the fuel pump psi. Also, would more powerful pumps help resolve fuel locks ?

(Andrew Waugh) #7

No. If you increase the pump pressure then it will push fuel past the float chamber valve, resulting in flooding, and a potential fire.

Plumb a gauge in just before the carbs, then check to see if you’re getting ~3-4 psi. If there’s no pressure during the hot-no-start, then your pumps need looking at. If there is pressure then it’s probably a heat soak problem - putting some insulators between the carbs and the manifold will resolve the heat soak.

(Peter Scott) #8

I’m no expert on this topic. The only times I have experienced it, it has been when the car has been running in hot weather and I have returned to it after stopping for a short while. The fuel pump rattled away clearly indicating that the vapourisation was in the feed pipe to the pump and sure enough cloaking it in a wet rag to cool the pipe sorted it.


(popop) #9

Andrew & Peter,
Good cautionary advice. During my car’s failure to start, I did not recall hearing the
Pumps clickings. In the interim, using a cool/wet rag is a good idea. Will keep everyone informed on (if) successful solution. It may take awhile, waiting for the summer heat to return to New England.

(Rob Reilly) #10

Heat has to come from somewhere. Hint, its the engine.
Heat transfers through metal parts, especially aluminum, brass and copper, much easier than through plastic and rubber parts. That’s why the best solution is to keep the carbs cooler with insulators.
I don’t have any running experience with the insulators in my Mark V yet, but will report when I do, hopefully this summer.

(Ed Nantes) #11

Rob remember your SS is a 2 1/2 litre and so doesn’t have spacers on the carbs, putting them on wil stop the starting carb pipe from connecting.

(popop) #12


Question regarding the capillairy fuel return: Does that weaken fuel pressure coming from the first pump ?

(Rob Reilly) #13

You don’t actually care about fuel pressure, you care about flow volume, measured in gallons per minute or liters per minute or whatever.

I was going to raise the question, what about the 2.5s, is there another common car that used the 1-3/8" carbs and had insulators? And the 1.5s, I don’t know what size carb they had.

And that is right, that in all cases the starting carb pipe has to be reworked to fit up. I took the opportunity to put a check valve in mine in case of backfire.

(Ed Nantes) #14

Reworking the starting carb pipe is a difficult job, the spacers need to be the original thickness. It seems a hard solution to what is easily fixed. And it goes directly to the problem, too much pressure in the line twixt pumps and carb needles, rather than a round about solution.

(Peter Scott) #15

I think the 1.5 litre cars use a 1.5" SU. As to spacers I would agree with Ed that altering the starting carb pipe would be a very difficult problem.


(Ed Nantes) #16

1 3/8" carbs are like rocking horse droppings.

In the circumstance being discussed, pressure is the issue. While running the carbs are fed relatively cool fuel from the tank. When stopped, the fuel in the pipework between the float bowl needles and the SU pump[s] is trapped. The SUs have a one way valve. So that intially cool fuel starts to warm up from the ambient temp under the bonnet… and expands. And the only place t expand is to push past the float bowl top needles and so flood the carbies.
The capilliary allows this excess pressure to bleed of back to the tank side of the petrol pump[s]

(Andrew Waugh) #17

I wasn’t really paying attention when I replied to this - I saw “SU”, and “pressure”, without noticing that it was in #pre-xk. My bad.

(popop) #18

To note: when attempting to re-start, after engine shut down, no pumps clicking, nor fuel vapors which would indicate signs of flooding, even after pushing down the fuel bowl ticklers. Dry. I mentioned summer period, because the problem appeared only during hot weather +80F.

(Roger McWilliams) #19

In addition to fuel-supply possibilities, hot car restart problems may come from electrical causes.

Distributor caps with screws attaching the coil and spark plug wires can have heat-related failure. The screw which holds the high voltage coil wire in the cap is the one hidden by the carbon bush on which the rotor spins while in contact with the bush. The screw pierces the high voltage coil wire to make electrical contact with the wire center conductor. Sometimes the screw contact with the wire center conductor may opened up from thermal expansion when the distributor cap is hot. When that contact is opened up, there is no coil voltage distributed to the spark plugs and the car will not run until the distributor cap cools enough for the screw to be close enough to the wire center conductor element to share coil voltage with plugs.

A second failure mode in distributor caps is “tracking”, where the coil high voltage pushes current along the interior of the distributor cap to ground rather than through the spark plug lines. This problem may manifest when a cap is heated above normal operating range. This problem is more likely to occur on reproduction caps which do not use original specification materials.

A third failure mode may be the distributor rotor. Rotors may have designs and materials which allow the coil voltage to flow, but not to the spark wire. Again, this problem can be temperature dependent and a hot rotor condition brings it about while a cooler rotor temperature gives normal operation.

(popop) #20

Your points of possible electrical causes are interesting. I admit that during difficult starting events, I did not check electricals, due to inconvenient locations at the time.
Intermitent electrical failures are difficult to pin down when everyting is working. I considered a hot coil. On the LHD, the coil is mounted on the steering tube, with adequate air flow all around, but still a hot location. If the problem persist after installing spacers and a capillarry fuel return, It will most likely be intermitent electricals.

Thanks to everyone for interesting Analysis and potential remedies.