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MG TD TF 1500 - Is This Vapor Lock?

I guess I'm not so smug anymore. I used to read about you guys struggling with vapor lock and grinned "Glad I don't have that problem." Driving over to the Club meeting on Tuesday about 3 pm with temps at about 92+F, engine just purring, after about 30 miles at 45-50 mph, I came to a red light and started back up after about 30 seconds. I noticed the temp reading had climbed to about 200F (normally runs about 175F and drops to about 160F when cruising under light load) and then the blessed thing just almost shut down. I waved traffic around and sputtered along for a minute or so looking for a place to pull over and tap on the #1 carb float bowl (it stuck twice 2+ years ago when I first got the TD but hadn't shown any signs of misbehaving since). Just about when I was ready to give up and pull over the sputtering stopped and she was purring once again. Four or five miles to the next red light and the second verse (same as the first) started up. I was close enough to my sister's house (the girl the safari picture) to sputter on in and take a look.

I always thought that vapor lock was a carb phenomenon but I think this was pre-fuel pump. My TD has a glass-bowl fuel filter just before the fuel pump (see pic attached) and the fuel was just bubbling in it. I know there is some air trapped in it but I've never seen bubbling. I think the fuel line runs from the tank along beside the exhaust pipe up to the filter and that the fuel was just boiling. Could that be?

If so, would a simple solution be to re-route the fuel line away from the exhaust pipe? Has anyone run the fuel line in the engine compartment fhrough a finned aluminum heat exchanger tube to cast off some of the heat? Would such a tube gather heat instead of tossing it away?

I guess there weren't too many 92+F days in Jolly Old England.

Jud


J K Chapin

Bubbling before the fuel pump could also be an air leak.
Lew Palmer

A dead give away for vapor lock in the fuel line below the pump is the old fuel pump clattering away like mad. When the T cars stop after coming up to temp on a hot day, the bonnet becomes a real oven with heat pouring off the exhaust well above the boiling temp of gasoline. As the pump tries to suck it up, the boiling point drops even lower in a slight vacuum, and fuel can vaporize in the line. Don't you suppose you were watching gas boil in the filter?

As you were suggesting, it would be wise to reroute the fuel line on the other side of the engine. The idea of a finnned aluminum radiator would make the problem worse. What can help is a few feet of the plastic corrugated wiring loom slipped over the fuel line for a bit of insulation. Avoid high ethanol fuels and keeping the tank topped up will make it easier to draw fuel up to the pump. And make sure the spring loaded umbrella in the cap is clean to vent the tank.

I installed 2 pumps in the rear at the bottom of the fuel tank to push fuel under pressure, rather than draw it up by vacuum. One is an old SU type pump and the other is a new Facet cube (for backup and also a 3 quart fuel reservoir). I believe we've encountered vaporlock at highway speed (air fuel ratio meter drops to nothing) with the old pump under very hot conditions, as switching on the Facet restores power instantly.
JRN JIM

Lew, thanks. My first thought was an air leak but I figured that such a leak would cause a fuel leak when the car was sitting with the engine off (Where air can get in fuel can leak out?) and tested - not leaking fuel at all.

Jim, that's what I was afraid of with the heat exchanger tubing - it naturally transfers heat from the hotter to the cooler - in this case that would be from the engine compartment to the fuel line - not desirable.

I'm seriously looking at the Facet aux pump 'cause pushing sounds more reliable than sucking.

Thanks all.

Jud
J K Chapin

Lack of fuel leak does not prove the fuel line to be good on the suction side of the pump. I am not saying that is your problem just that it can not be overlooked. Air can gain enterance at vacuum through a hole the fuel will not flow at little pressure difference.
Don
D Hanna

i cannot say it couldn't happen, but i have never had vapor lock on a properly tuned XPAG that was running. only after a run..quick stop at the store back out in 15 minutes i have to use the choke and it fires right up.
2 summers ago the temp was 105F..i was in stop and go..mostly stop traffic for over 2 hours..no issues at all. and i run cheapest regular ethanol swill all the time. regards, tom
tm peterson

Jud - Don is correct, an air leak on the inlet side of the pump doesn't usually show up as a fuel leak when the pump is not pumping.

Tapping on the carburetor float bowl will not normally relieve a vapor bubble in the channel between the float bowl and the carburetor jet, which is where the vapor lock occurs. What happens is that a bubble of vapor forms in the above channel and puts enough back pressure on the fuel to push the float up, closing the needle valve. There are a couple of ways to clear the bubble. First is to push the tickler pin down as far as it will go. This will often times clear the bubble. The other way is to pull out the choke - all the way out and keep tickling the throttle. This will get fresh fuel into the mix and cool the carburetor down a bit.

Unfortunately, today's fuel is formulated for fuel injected engines and there is no need to keep it from vapor locking - you can't get vapor lock in a fuel system that maintains the fuel at 30 - 90 psi. If this starts to be a real problem, you can always do what T series drives did in California in the 50s - remove the side panels from the bonnet and get a leather strap to hold the top of the bonnet down.

Finally - one further piece of advice - loose that filter in front of the fuel pump. The SU fuel pump and carburetors are extremely tolerant fine debris. The filter screens in the tank, fuel pump and float bowls are in there to stop rocks and small birds only. A modern day, high efficiency filter will trap the fine debris (mostly fine rust particles) and clog up relatively quickly. When this happens on the inlet side of the pump, it causes the pump to stall in a current on condition and if power is left on for any period of time (such as while troubleshooting) the pump will overheat and be damaged. If you feel that you must have a filter in the line to the carburetors, put the filter on the outlet side of the pump, where is is not a danger to the pump. Cheers - Dave

D W DuBois

I'm with Tom and Lew- air leak and I also have only had vapor lock after shutdown hot soak. Pulling the choke out would make it better if some vapor lock. Make totally sure you diagnose this exactly before you re-plumb or change a bunch of stuff. George
George Butz

The T cars very susceptible to vaporlock are the pre-Mark ll TDs and earlier. The Mark ll and TF models were given a second fuel pump, but more importantly, I believe they all received the return line off the front carb double banjo fitting that the previous production didn't get. Our '50 TD came retrofit with the return, but Moss catalog doesn't show it and there's no info in the archives on this upgrade.

The return facilitates vaporlock in two ways, the fuel circulates past the carbs and back to the tank so the temp stays lower and the vapor has an escape route back towards the tank where it will condense back to liquid.

As some point out, pulling the choke out (enrichener down) may enhance flow enough to make some difference. When stranded, popping the bonnet to let heat escape helps immensely, and cold water poured on the fuel lines & carbs can get one back on the road.

If the fuel pump is running like mad, it is a sure sign it is trying to pump vapor and isn't primed. In this case, if water isn't handy, I've cracked the fuel line to the carbs and fired up the pump until it primes (wrapped rag/paper towel to catch fuel spray).

Keeping the tank topped up helps.

It seems like at least once every season, I've had to deal with someone's MG crapping out during one of our summer events.
JRN JIM

JRN/Jim -

I believe you are mistaken about the 2 fuel pumps on the TF and also the purpose of the additional fuel lines on the TD/C MKII TDs.

The extra pump was only fitted to the TD/C MKII - not the TFs. The extra fuel line fed the front carb and was not a return line of any type.

The extra pump was simply provided to address a possible fuel starvation at sustained high speed.

DLD

Totaly agree with DLD, about the fuel line routing,, don't reroute it till you find the problem,, I've also never heard of vapor lock happening to a running engine... only after it has been stopped for 10-20 minutes,,,

IMHO, Problem is something other than vapor lock,,,

SPW
Steve Wincze

Personally, I've never experienced vapor lock in Lazarus.
Only temperature related stumbling occurred in the first days after getting Lazarus on the road. I could drive a number of miles and stop for a bit. Would restart immediately with no trouble but wouldn't start if it sat for a few minutes until it got some cooling down. Barely made it back to the garage. Cured it by replacing the capacitor. Took the old into work at Polaroid's QA lab. Discovered a radical temperature coefficient. Heat soaking would cause the ignition to go to pot. I kept the bad one for years. Just threatening to install in someone wise guy's car. Bud
Bud Krueger

George is correct about the problem being brought on by heat soak, normally shortly after shut down. I did experience the problem one time going over a pass in Southern Oregon in mid summer. The car start missing just before we got to the summit. This brings up another possibility for Jud to check out. Insure that the grill slats are open sufficiently so that there is plenty of air going through the radiator and the engine compartment. The grill slats should be open enough that standing in front of the car, you have a clear view of the radiator. Cheers - Dave
D W DuBois

The grill slats are open. I'm extracting the fuel filter and thinking about an aux pump in the rear near the tank outlet. Moss wants $77+ for the Facet pump. I was at Adv Auto getting some fuel line and saw a 4 - 7 psi pump in a verycompact package for about $47. It's fully insulated with two leads so polarity shouldn't be an issue. What else should I consider? I would plan on having it on a switched circuit so it would so mostly off I hope.

Thanks for the heads up on the possibility of the problem being a temp sensitive capacitor. I'll get a new one and see if that helps. Someone here has mentioned that 99% of fuel problems are electrical.

Jud
J K Chapin

Jud, I'll let Dave DuBois give an intelligent answer, but I think that 4-7 psi is asking for problems. Also, do you know that the extra pump will not impede the pumping of the main SU? Bud
Bud Krueger

Thanks for getting me off that track. Those were the two questions I was trying to ask but couldn't seem to articulate. I think for $30 I'll stick with a known quantity and but the Facet - probably from Jeff at LBC.

Thanks again for getting me thinking straighter.

Jud
J K Chapin

Jud, Good source for Facet pumps is Aircraft Spruce aviation supplies. They have a full catalogue selection of all the psi ranges.
C.R. Tyrell

Jud - Bud is correct, 4 - 7 psi is way too much for any MG, let alone a TD. Here are some Facet P/N that will work in the TD: 40171, 40178, 40217, 40241. You may have to search for any of these Facet models, but I know that NAPA can get them for you. Aircraft Spruce http://www.aircraftspruce.com/catalog carries the Facet pumps, bub perhaps not these particular models as the pressure is below 4psi on all of them. Another source is Pegasus Auto Racing Supplies - they do carry a couple of low pressure (under 4psi) pumps. They also carry sound mounts for installing the Facet pump on to keep the noise from the pump from transmitting into the passenger compartment - they can be quite noisy and annoying. You can also get the exhaust sound mounts for the MG midget from Moss Motors.

If you install this, I would suggest that you put a single pole, double throw switch in the power line going to both the original pump and the one you install under the car, such that you can select the pump you want to use. Having both pumps pumping through one another could very well over whelm the needle valves in the float bowls.

In answer to Bud's question as to weather or not the pumps will pump through each other without a problem, the answer is yes. I have that Facet pump on our MGB and the SU and the Facet pumps pump through each other without any problems. Cheers - Dave



D W DuBois

I'm not a big advocate for adding complexity and mechanical modifications. I've never experienced a vapor lock, but for insurance I encased my fuel line from the bottom of the tub to the stock fuel pump in plastic hose slit down the middle. I think it serves to insulate the metal pipe from engine compartment heat and is not a mechanical complication and is totally reversible.
John Quilter (TD8986)

DLD,
Thanks for reminding me that the TF has the pump located lower, in the rear. Why do you suppose they moved it there?

As for the pair of pumps on the Mark ll, the idea that an extra 4 horsepower needed an extra pump was more a selling point. TFs went back to a single. The old electric pump can pass a cup of gas in about 15 seconds, and even a supercharged engine like ours can't burn through a tenth of that rate.

Our '50 TD came with 1 1/2" SUs with a double banjo fitting on the front carb with the line returning to the fuel tank. Made good sense to me and my assumption was the Mark ll incorporated that feature, as there hasn't been any discussions on the Mark ll fuel lines. I thought about adding a return, but each fitting under the tank feeds a separate pump and with two pumps under the tank pushing fuel, my thinking was vapor lock couldn't happen anymore. Wrong. Air/fuel ratio meter confirms when the single 1 1/2" carb goes dry. This is when I switch on the higher pressure Facet. By the way, the two parallel pumps feed through a Mr. Gasket adjustable pressure regulator to trim the pressure down.

I bought a handheld dual temp digital gauge that accomodates a pair of thermocouples. I plan to attach a thermocouple to the fuel line and another to sense various different regions of the engine compartment. Our passenger footwell gets scorching hot- instant burns on my feet when wearing sandals (and most of it is well insulated now). Over the winter, I welded a fitting onto the exhaust pipe, just under the flange and stuck a thermocouple in there. Exhaust temps can be up in the 1200F region.

Have to leave right now............... JIM
JRN JIM

Jim - Place one of the thermocouples on the arm from the float bowl to the carburetor jet. Actually, if you have an extra thermocouple in that place on both carbs. See what the temperature is on those two areas after the car has sat for 10 to 15 minutes after a good run in hot weather. I got our TD set up with a thermocouple on the rear carb right at the end of the hot season (short season in the Puget Sound area), but I did see a definite rise in the temperature in that arm (channel from the float bowl to the carburetor jet). I am going to rerun the experiment this summer if I get the chance.

By the way, If you want to see what the MGA people did for the same problem, there is an exceeding long discussion on the MGA BBS. You will need to search the archives to find it - it took place in 2012/2013 as I recall. Their solution to the problem was to get more air flowing through the engine compartment with the use of a 3" bilge blower. Cheers - Dave
D W DuBois


Dave,
Your discussion with the MGB crowd...
http://www.mgexp.com/phorum/read.php?1,2458389
Interesting and informative.
As pointed out, gas boiling temp is all over the map, from under 100F to over 400F, depending on grade, alcohol content, other additives, what time of year it is (winter gas boils much lower), how old, etc.
My wife's favorite toys are supercharged, so first off, I have only single carbs to play with, and secondly, the location is not as close to the exhaust as the typical dual carb XPAG. I stuck a thermocouple down the vent tube right down into the float bowl which will still be pretty close to the channel you're focused on. Should be interesting. However, the temperature that concerns me most is at the fuel line coming up the firewall. The other thermocouple is on a fitting at an elevation just above the starter.
There's no predictiing the next hot day when my wife will be blazing 80+ down the highway while yours truly studies gauges.
One of my intentions is to document temps all over under the bonnet and in the exhaust pipe this year and compare to the next engine with the roller lifter cam ground with minimal overlap for a supercharger, so 5 psi boost doesn't push air/fuel right out the exhaust.
Besides considering rerouting the fuel line to the drivers side (if I had 1 more length of 3/8" stainless tubing I would), I want to explore picking up the intake air from that side, if it is markedly cooler.
JRN JIM

This thread was discussed between 11/06/2014 and 14/06/2014

MG TD TF 1500 index

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