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MG TD TF 1500 - vapor lock workaround

Hi all, at the most recent GOF in San Diego, the subject of vapor lock came up during a carb tech session. It was suggested that to work around this annoying problem was to install a solid state or equivelent type fuel pump near the tank to supply cooler gasoline to the fuel pump under the bonnet. Will this really work? I already know that choking the carbs will help but have never really found the culprit. Thanks
Keith Yarbrough(TD 1275)

I'm not sure if I understand what you're suggesting, but installing a second pump at the tank won't do any good. It's the same fuel regardless of where the pump is. Even with only the stock pump, it still pulls cool fuel from the tank. It's just that it gets hot when it reaches the engine.

Vapor lock can occur at the pump, where bubbles get trapped in the valves, or at the carbs, where bubbles get trapped just about anywhere. When you get large, single vapor bubbles in the fuel lines, the fuel will usually move past them, but not always, so even large bubbles in the lines can be a problem. Modern fuel foams like ginger ale when it's warm. I learned this when I put a transparent fuel line on my Sprite once just to see what was happening. It was impressive.

If you're suggesting locating the pump at the tank instead of under the hood, that solves the problem of vapor lock at the pump but not elsewhere.

Modern cars have a fuel pump at the tank, usually inside it, and a return line. The fuel pressure is regulated by returning fuel to the tank, so it doesn't sit under the hood and boil. I wouldn't want to have to engineer something like that for a TD, but that's what works. Short of that, a heat shield and a fan that moves more air might be the best you can do.
S Maas

Modern fuel evaporates at 70 degrees celcius, and under hood/bonnet temperatures in our cars are often twice that, so it's a challenge !.

What Jerry was talking about at the tech session was the addition of a Facet Pump at the rear of the car. This becomes the primary pump, while the SU pump sits idle due to positive pressure upstream. If the Facet pump fails, the SU will seamlessly take over.

The idea behind this is to prevent vapor lock within the fuel pump in the engine bay, which is more of a problem with today's fuels than yesteryear. You may have heard the story about the woman who kept her car going by sticking halved grapefruits on the pump. She would pull over every 50 miles and change the grapefruit just as vapor lock would start to set in again.

Several members of the Vintage MG Club of Southern California (the club Jerry is also in) have installed the rear-mounted pumps with great success. Personally I have never suffered vapor lock in my car, even in 110+F temperatures, with only the stock setup. I can't explain why some have it and some don't. Worst I ever get is a bit of stumbling after re-starting on a very hot day, which clears itself up in about 10-15 seconds.
Steve S

I have both an old SU type and a new Facet under the tank in back, WHICH HELPS A GREAT DEAL, but without a return, they can still vaporlock. The old pump has barely 1 psi, but the Facet has around 2psi, so when the old pump can't get the job done, I switch over to the backup Facet.

My wife cruises above 80mph in her supercharged TD and the fuel line at the firewall can run 150F+. When the air fuel ratio meter disappears completely lean and the engine starts to stumble at speed on a really hot day, it is time to switch on the Facet. There have been a few occasions when the Facet has been taxed to the limit and the engine has stumbled a bit... time to slow down to the speed limit for a while!

I recently measured the temp at the normal float bowls location (ours has single carb on blower) after shutdown, and it got up to 255F, enough to vaporize what is in the bowls and in the fuel lines, too.

With the fuel pump mounted up in back on the firewall, the vaporizing gas pushes the fuel back to the tank. It cannot pump up vapor and refill the carbs with liquid gasoline. That's why they often refuse to restart after shut down for a matter of minutes.

Interestingly enough, when we picked up our first TD, the previous owner DID indeed have a return. He used siamesed banjo fittings on both carbs and had a return line with tiny restriction hole attached to the front carb. He was indeed ahead of his time! Since I have a pump attached to each of the two fittings on the bottom of the tank, I didn't bother with a return, but maybe next year I will.

I'm sure the facet pump did help a great deal if your SU was delivering less than 1 psi! Should be around 1.5, and it will handle more.
Steve S

I'm with Steve S. I've had the fortune to have never been subject to vapor lock (knock on tub wood). Bud
Bud Krueger

Could we connect the overflow pipes to a return line to the gas tank, remove the float and just let the SU pump run continuously keeping fresh cooler fuel in the bowls?
J K Chapin

No...If you did you would be pushing the fuel past the jet and flooding the engine constantly.
MG LaVerne

oh well. i figured there had to be a reason. jud
J K Chapin

"install a solid state or equivelent type fuel pump near the tank" has nothing to do with VL.
(Unless your front pump is bad.)

I certainly would not look at this for a cure.
My TF has always (since I purchased it anyway) had a rear mounted SS faucet type pump.
It had VL problems when I first got it.

Again "IMHO" in order of importance:
Clean & properly adjusted carbs
Proper carb to manifold spacers
Clean fuel filters
Heat shielding "metal" fuel lines (not rubber) at carbs
Exhaust manifold treated on "inside" to allow hot gas to leave quicker (Jet-Hot or other, but treated on inside is the ticket.)

If all this is in order I would be surprised if you had VL.

As a post note:
Although "heat-shields" were a common accessory sold for these cars, I am not a believer. I base this on the fact that mine actually got worse when I tried this as a quick fix before attending to the above.

My name is David, and I have been "VL Free" for over 13 years!

David Sheward 55 TF1500 # 7427

I drive a 1250 TF every day of the year in Las Vegas where it is very hot. With a good heat shield I still experienced vapor lock, especially in the summer. The day I switched to a facet pump at the tank, about three years ago, vapor lock stopped. Completely.
P. Hejmanowski

Fuel boils at some temperature when heated. This boil temperature is higher when the fuel is under pressure and lower when the pressure in lowered, like on the suction or inlet side of the pump. Putting the pump inside or close to the tank puts the entire outout fuel line under pressure making vaporizing the fuel in this line less likely. The more pressure the better. Our td's have the pump high up, requiring sucking fuel uphill, with a long suction line and a screen in the tank and in a hot spot. Not good!

Once the fuel gets into the float bowl, which is vented, it is at atmospheric pressure and will boil or vaporize if it is hot enough but there will be more fuel coming in if the pump is doing it's job.
Here's where choking will help. A rich mixture will cause cooling and things will get better after the engine starts.
Note: winter blends of fuel boil at a lower temperature so make sure you get fresh summer blend fuel before you drive your TD in a summer parade on a 100 degree day!
cj schmit

I'll amend my answer a little. If the problem is vapor bubbles in the fuel pump, putting the pump back where the fuel is cooler might well help. But the fuel at the engine compartment will be the same temperature, regardless of the location of the pump, so if the problem is in the carbs or elsewhere in the engine compartment, it won't help.

The fuel flow rate is determined by the demand by the engine, not the pump. You won't get more cool fuel by using a different pump or location.
S Maas

OK, now I'm pretty sure I'm going to really step in it or at least show beyond any doubt my ignorance. To start, liquids are pretty non-compressible whereas gases (vapors)comnpress like a spring. Secondly, a liquid can lift the float in the float bowl whereas (I think) a vapor can't perform this feat. It would seem, therefore, that if the liquid in the bowl turned to vapor the float would drop allowing the pump, if liquid fuel is reaching it, to pump more gasoline into the bowls to be sucked up by the carbs. In addition, the venturi effect in the carbs should result in cooler fuel/air at the downstream side but I haven't thought the effect, if any, of this through yet.

Anyway, if any of the forgoing analysis is correct, it would seem that vapor lock must occur before the pump since, it seems to me that vaporizing after the pump would be self correcting.

I'm sure there's something obviously wrong with this but that's the difference between empiracism and analysis.

J K Chapin

Jud, I am not going to dispute anything you have posted,..but the forward mounted S.U. Delivers fuel fine in temps up to 105F. I cannot speak to temps beyond that range. The vapor lock occurs downstream from the firewall pump..if it was upstream you would hear a rapid clicking of the pump as it would be drawing vapor and would be unable to deliver pressure. The issue with the cars in temps under 105F is under hood heat soaking after a run with a brief heat shield will help that.
Seems to be a lot of chatter and discussion of mods for an issue that only requires a pull of the choke, but that is just my opinion. Regards, tom
tm peterson

I can report they work well up to 116F on 2-lane low desert highways.
Steve Simmons

I believe that it's Dave DuBois who has expressed an opinion that the culprit is in the passage at the base of the float bowl to the banjo at the base of the carburetor body. Since this is beyond the closed float bowl needle it's beyond the control of the fuel pump.
Please forgive me, Dave, if I've misinterpreted your opinion. Bud
Bud Krueger

Bud - You have expressed my theory just fine, but I have to think that there is more to the story than my theory, particularly in those areas with summer temperatures being way higher than here in the Puget Sound region of Washington state (where we really start bitching when the temperatures exceed the low 80s). First of all, I believe (but have not confirmed) that there is no longer a winter grade and summer grade fuel being formulated. The reason that I believe this is that all of the present day autos all use fuel injection rather than carburetors. Pumps for fuel injection systems deliver the fuel under high pressure. While I don't know what the pressure from fuel injection pumps are today (our 1980 Audi pumped the fuel at a pressure of 80 psi), but I believe that it is around 30 - 40psi. At these pressures, the fuel doesn't need any special formulation to deal with seasonal temperatures and I suspect that today's fuel formulations are on the side of easy vaporization.

I started having a problem with vapor lock shortly after restoring our TD, around 1984/85. At that time I believe that there was probably still a seasonal formulation of fuel. I believe this because the very first warm days, I would experience vapor lock in the carburetors (as Bud stated in the channel between the float bowl and the jets) after the car had been driven enough to get the engine compartment good and hot, then shut down for a short period of time - just enough to cause the engine to stumble when first restarted. This was definitely a heat soak situation. The problem would then go away after the fuels formulated for summer weather came to our local fuel dispensers. Fast forward to 2014 and every time we take out TD out (nearly daily) in summer temperatures, we have the same heat soak induced vapor lock in the carburetors.

The problem of heat soak inducement of vapor lock in the carburetors cannot be cured with a higher pressure fuel pump for the simple reason that the higher pressure only affects the fuel on the down stream side of the float bowls. The vaporized fuel in the channel from the float bowl to the jets cause a back pressure on the fuel in the float bowls, that prevent any (cooler) fuel that the pump might try to push past the closed needle valves. The work around for this is to pull the choke to disrupt the bubble of vapor and start pulling in cooler fuel. This particular problem can be cured by removing the side panels of the bonnet, so the engine compartment can get better ventilation (I remember as a kid in the 50s in Southern California, seeing TCs and TDs running around with those panels removed during the summer months).

Now the theories that the vapor lock is occurring in the fuel line and the pump in the engine compartment, this is more of the classical causes of vapor lock and why an electric fuel pump was the answer in the very early days before fuel injection. The pump sitting high in the engine compartment is not an ideal place (or even a good place) for a fuel pump to be. I don't remember (and am too lazy to go back and find it) who stated that a pump is this position only serves to exacerbate the problem. This is true because the pump is pulling the fuel up from the tank, which results in the fuel being under a vacuum, making it that much easier for the fuel to flash to a vapor. Once there is vapor in the line, the pump just moves this vapor bubble back and forth, especially if the vapor bubble occurs inside the pump. When one thinks about this a bit, not too many of the later TFs and later MGs with the pump in the rear of the car are experiencing the problem.

Circulating the fuel constantly is one way to alleviate the problem, although I am not sure that this would improve the longevity of a pulse type pump such as the SUs and Facets. Putting a centrifugal pump in with a pressure reducer would probably be a better solution. Beyond that, one can only try to ventilate the engine compartment better, by removing the side panels of the bonnet or some other means of getting more air through the engine compartment, or perhaps a thermostatically controlled electric fan that would continue circulating the air when the car is stopped. The MGA people have found that a 3" bilge blower greatly improves the situation. Cheers - Dave
D W DuBois

Knowing there is a lot of concern on carburetor heat and trying to alleviate the problem, I have also given thought about the fuel line where it passes the muffler, as in the photo. In stop and go traffic on asphalt in 90+ degree weather, the heat build up in this area on the fuel line has to to contribute to the problem, as the fuel is fairly hot when it reaches the carburetors. I'm thinking of insulating the fuel line here with fuel line insulation from a speed shop. I did this on our MGB where the fuel line goes behind the engine and exhaust manifold. It solved a vapor lock problem on it and has never come back! PJ

Paul S Jennings

I'm just going to pull out the choke and move forward to enjoy the ride!!!!!!!!!!!!!!!

Steve Wincze

Steve - "I'm just going to pull out the choke and move forward to enjoy the ride!!!!!!!!!!!!!!!"

Two thumbs up! :-))
D W DuBois

Interesting thread and comments. One of my other cars is a 1990 Jaguar XJ6 which has a 70-80 lb pressure Bosch pump located near the tank. I have twice experienced fuel pump cavitation in ambients over 90F. When this occurs the fuel rail pressure drops and the car immediately looses power and dies. There is no choke to pull out as a work around! The reason being is that only a small amount of the fuel sent to the engine is actually consumed, the excess being returned to the tank. Over time, say more than a few hours of driving, the entire tank load of fuel is heated by the high engine compartment temperatures so that it is so hot it vaporizes as the pump sucks it out of the tank. The problem can be minimized if the tank is relatively full and I am told by Jaguar forum posters that leaving the fuel cap loose is a help. I only wonder if the new 10% ethanol content fuels exacerbate the issue. Jaguar's factory solution, back in the day, was to modify the car with a second inline pump, a solution I find overly complex and I'm not sure why, if one pump cavitates a second one would not do the same.

With a T series I wonder if wrapping the carb fuel bowls in foam pipe insulation might help. It certainly would be easy to try and reversible.
John Quilter (TD8986)

Since we have a good thread going on this ...I have said this before but will do it again to have it here.
Dad used to wrap his TF's carbs with dry ice and burlap bags to drive in the 4th of July parade to avoid VL. Also put pans of it in the foot wells to keep cool.
I have cooled mine down with those cold snap packs from drug store and it works.
David Sheward 55 TF1500 # 7427

As we are concerned with our engines stalling or refusing to start after a hot shut down, concern should also be applied to storage over the winter months or long periods of storage with Ethanol impregnated gas in the tanks. After a period of time the gas will separate causing a water problem in the tank. I use Marine grade StaBil in my cars, which helps alleviate this problem, to a point! It's expensive, but it has worked for me. The Marine grade StaBil tames the Ethanol to a point it helps prevent eating of rubber components in the system. Boaters use it to prevent the destruction of fiberglass fuel tanks, which it will do among other things. Non Ethanol gas is very hard to find around here. PJ
Paul S Jennings

This thread was discussed between 02/09/2014 and 08/09/2014

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