TR2/3/3A vapor lock?

I often find myself in the fast lane. I don't feel like I am a 'fast' driver, but I recently drove over 300 miles in 5 hours, including a short visit in the left of the fast lane, where I waited with the hood open. I was making pretty good time and trying to get through Dallas before the Friday afternoon rush hour. Without a clue, the normal traffic flow went from 75mph+ to 0, a few miles north of I45. I was lucky enough to be under a crossover (a perfect place for a shade tree mechanic). The temperature was lower than usual, due to a 'cold front' that came in overnight. Still, I watched my temperature gauge intently while I sat in the fast lane next to 5 or 6 other lanes, all completely stopped and waiting to move again. My temperature needle did not move away from the center point. I'd guess we were stopped for at least 10 minutes -long enough that I thought about shutting off the engine, but was afraid the delay would end, and restarting a hot engine would not be fast enough.

Just as abruptly as we stopped, the far right lane started moving and within seconds the cars to the right and then in front of me were moving quickly. I stuffed the car into 1st, and when I gave it gas, it died and refused to start. I jumped out and pushed it to the side. By the time I got next to the concrete barrier, the fast lane was moving at normal speeds. I popped the hood and saw the glass fuel filter completely empty. I thought 'maybe I ran out of gas', and went back to the car, turned on the key and rechecked the fuel level while remembering making a mental note that I had enough fuel to make it across Dallas. Returning to the engine bay, from a side view, I could see a little fuel at the bottom of the filter, bubbling and percolating.

I let it rest with the hood open for a couple minutes while I watched the passersby look at me as if I were the reason for their delay. I walked to the back of the car, pulled out a can of starting fluid, shot a quick squirt into both air filters, and put the can back in the trunk. I jumped in and the engine fired up like nothing had happened. With it idling normal, I got out and looked at the now full fuel filter while closing the hood and drove the rest of the way home without incident. -almost, but that's another story.

My question is: How do I determine where the fuel is becoming vapor locked? Is it before, after or at the pump? This is the second time I've had this happen. Both times involved a sudden stop on a very hot engine, but not overheated (as determined by the coolant temperature). Maybe I should just add a manual override for the electric fan and hope that will fix it? Suggestions and opinions are welcome as I'd rather not find myself in a more dangerous position.

I have toggle switch on the dash for an electric fan in front of the rad for times when I get stuck in traffic on hot days. I'm running a yellow TR6 fan in place of the stock 3 fan. I replaced the stock fuel pump with an electric one. I wonder if an electric pump would be less likely to be a victim of vapor lock?

Are you running the stock mechanical fuel pump? If so, the first thing I'd do is rebuild or replace it.
Tom

TomMull said:

Are you running the stock mechanical fuel pump? If so, the first thing I'd do is rebuild or replace it.
Tom

Click to expand...

I'm thinking the same thing. If the fuel was boiling after the pump (which I assume is where your filter is), the vapor should blow out through the float valves and not back into the pump (so the pump wouldn't be vapor locked). But a leaky check valve (or valves) in the pump could be letting liquid go backwards through the pump (as well as being much more likely to not be able to clear any vapor in the pump).

Sudden stops from 75 mph were very common when I was driving to work every day. (In fact, that's how my previous TR3A got totaled. I stopped but the guy behind me didn't.) I have an override on the dash, but I've never needed it since I installed a thermostat for the fan. Since I have no mechanical fan, the electric one will come on within a minute of stopping anyway.

In my case, it appears the main problem is fuel boiling in the carb jets after shutting the engine off. Adding one of Joe's heat shields helped a lot, but did not eliminate the problem. My plan is to build a smarter fan controller that will let the fan run for a few minutes after shutdown.

I am using a mechanical pump. You are probably correct. Lately I've noticed that after sitting a week or so, the filter is empty as if the fuel were draining back to the level of the tank. Also, my pump doesn't have the priming lever, so I think it's time for a new one.

Thanks so much.

Vapor lock is always a combination of all the above. Anywhere you add heat to the fuel has a cumulative affect. Once the vapor lock "lights off" by the fuel reaching the boiling point, the resulting bubble forces it's way in all directions. Even if the pump is working with liquid fuel, since the hose/line is no longer being cooled by fresh liquid fuel flow, the fuel from the pump cannot keep up with the speed the fuel is boiling off. This is especially true when the pump is only functioning at cranking engine speeds. It very well may be venting the fuel "steam" out of the float needles, but there is not enough liquid being input to cool the lines and help the situation...until the entire line cools below the boiling point of fuel.

I have decided the brand and grade of fuel has the biggest influence on how low a temp the vapor lock will light off. As you can tell, an electric pump helps in 2 ways. First, it will pump at full rate even when the engine is only cranking. More flow means the lines will cool faster. Then, if you locate the pump properly, i.e. Near the fuel tank, it will not add to the heat being absorbed by the fuel on the way to the carbs, as a mechanical pump bolted to the engine bock does. Higher line pressure helps, in the same way water boils with less added heat at 10,000 feet vs. at sea level. On all my performance applications I run 14psi at the pump from the tank. I then use a regulator at the carb to knock the pressure down to what the carb can handle.

The final step to prevent vapor lock is to insulate the fuel lines from any source of heat. This would be wrapping the lines under the bonnet, adding heat shields below the carbs, and the like.

OK, this may sound a little crazy. I know at least one person on the BCF who constantly recommends removing the fuel filter. I tested my fuel pressure yesterday and noticed something I'd never seen before. The bubble of air in my in-line filter went away. The bubble was always about 1/2 of the filter. It seems I installed the filter perfectly level, and near the carbs such that some of it is higher than the carbs, and the bubble always filled the top half of the filter. I looked back through pictures until I found a picture where the fuel level can be seen, just after I shut off the engine, shortly after I installed the alternator.

So, I've been thinking about how air expands when it is heated. I looked up the Rayleigh-Plesset equation for bubble dynamics and found it much too deep for my understanding. But I can't help wonder if my bubble had an effect on my situation. In any case, I altered the arrangement so my filter is tilted, and the bubble is minimized.

My fuel pump provides over 2.5psi. The pressure is held to 2 psi for over an hour after the engine is shut off. I have often found the most complex problems require a very simple solution. And often the simplest solution is just a prayer. I wish I knew a way to simulate my car dying when I least expect it.

Rayleigh-Plesset is mostly about rapidly changing fluid dynamics (like in cavitation where the bubbles are being formed by a sudden change in pressure). In this case, I think the old ideal gas law is pretty close, which states that (absolute) pressure times volume is proportional to (absolute) temperature. If pressure is held constant, then volume is proportional to absolute temperature.

So if we look at say, 100F to 150F, that's 311K to 339K or only about a 9% change.

I could be wrong, tho.

I realize that you are probably not using webers, but this is what I used on my TR3 for fuel lines along with an electric fuel pump a fuel regulator and an electric fan. I never had an issue with vapor lock, over heating or fuel delivery problems after installing this setup no matter how high the outside temp was or how fast I was going.

TR3driver said:

Rayleigh-Plesset is mostly about rapidly changing fluid dynamics (like in cavitation where the bubbles are being formed by a sudden change in pressure). In this case, I think the old ideal gas law is pretty close, which states that (absolute) pressure times volume is proportional to (absolute) temperature. If pressure is held constant, then volume is proportional to absolute temperature.

So if we look at say, 100F to 150F, that's 311K to 339K or only about a 9% change.

I could be wrong, tho.

Click to expand...

That sounds like the equation for gas to gas. Don't we need liquid to gas...like the old steam equations maybe? I don't know how much gas vaporizes volume wise.

The plot thickens. Or is it just the fuel returning to a liquid state?

Today I drove the TR3 less than 2 miles to the cleaners, picked up, and returned to the garage -a little longer than required for the temperature gauge to be at the center point. The car started, restarted and ran perfectly but I opened the bonnet in the garage just for fun. I was stunned to see the fuel filter completely empty. I thought perhaps I stumbled upon the condition to create vapor lock. So I turned on the key, and hit the starter and the engine sprang to life as if it were warmed up. I let it sit and idle for at least 10 minutes. The filter remained empty, with occasional squirts of liquid fuel entering the bottom of the filter. It never stumbled or hesitated when I accelerated. It shut off without run-on. Just for fun, I opened the top of the front carb bowl, and could hear the pressure releasing as the float dropped. But the filter still appeared empty. I (re)secured the bowl cover, and restarted without hesitation, and verified the empty filter. After the electric fan cycled several times, I shut it off and took some pictures of the "empty" filter for the next couple hours.

At first (3:23pm) it appeared completely empty. An hour later (4:16pm) I could see liquid in the bottom. Later (5:01pm) a little more could be seen. Later (5:30pm) a little more. After 3 hours (7:00pm) the engine was warm to the touch and the fuel filter was nearly full again. I don't think the ambient temperature got over 93F today.

I don't understand why the filter appears to be empty, but the engine runs normal and the bowls are full. I don't understand why the filter refills itself, unless the fuel has changed to gas when heated (transparent), and returns to liquid (translucent) when cooled. I have another fuel pump that I can put on, but I am unsure that is going to keep me off the side of the road.

CJD said:

That sounds like the equation for gas to gas. Don't we need liquid to gas...like the old steam equations maybe? I don't know how much gas vaporizes volume wise.

Click to expand...

Good point, I was assuming that the "air" is mostly air and stays that way with little change in fuel vapor content. But it will actually be saturated with fuel vapor at whatever temperature, and we're talking about temps at least close to the boiling point(s) of the fuel. As Emily Litella would say, Never mind.

TexasKnucklehead said:

I let it sit and idle for at least 10 minutes. The filter remained empty, with occasional squirts of liquid fuel entering the bottom of the filter.

Click to expand...

That's wild. I have no explanation for that one; never heard of an engine that would run without a fuel supply. It might well start on what was in the bowls, but something had to be refilling them to keep it running. Hard to believe the fuel was boiling (turning to vapor), going through the filter, and then re-condensing before it got to the float valves!

This is just ordinary, local pump gas, right? Not Av gas or bought in Canada, anything like that?

On a totally unrelated side note: are the end pieces of that filter plastic? It looks very much like one that a friend had on his TR6. Driving down the road, the end piece on the inlet side cracked and started spraying fuel all over the place, even though the engine stayed running.

I would replace the pump, it sounds like it's pumping fuel most of the time but pulling some air in every once in awhile.

I can't say I am 100% convinced this is even a fuel problem.

Intermittent heat-related failures can be very misleading. The engine quits... you check things... you do 'something'... the engine starts again. It is natural to assume that whatever you futzed with while sitting at the side of the road was the cause of the failure - but in some cases just sitting there with the bonnet up for a few minutes is what made the difference.

I'm not saying it cannot be vapor lock - but were it I, I would try to recreate the problem (in a more friendly environment than a freeway median) and start checking everything (e.g. content of bowls & presence of spark for starters).

Might also want to test the fuel pump.

I have never experienced vapor lock in either of my TRs and I often drive in 100+ weather stop-n-go. I also never use an inline filter. I realize every engine is different and so my experience is merely anecdotal but you might want to know more before your start replacing parts.

re "you might want to know more before your start replacing parts" -I totally agree.

re "I would try to recreate the problem" -that's where I'm at too. But not knowing how I 'fixed' it or how it 'broke', I'm left guessing.

re "Hard to believe the fuel was boiling (turning to vapor), going through the filter, and then re-condensing before it got to the float valves!" It is hard to believe, but all kinds of people believe things that seem more outrageous. How could it be anything else when letting it cool allows the non-running engine to fill the filter? The fuel in the line would be at a higher pressure than in the bowls. There may even be a pressure differential on either side of the internal filter element. It is regular pump gas. The clear part of the filter seems to be glass, the threaded cap ends may be plastic or pot metal -something chrome and non-magnetic. I uploaded a video that I'd like you to see the squirts of liquid while the engine is running fine in my garage. https://youtu.be/yXUZ9R4Io5Q I'm having a difficult time believing the pump is somehow pushing air into the filter -since cooling results in a full filter.

I too could be wrong and this might have nothing to do with why I sat on the side of the road for what seemed an eternity. I'm pretty sure something happened that caused the car to not function as expected, and then something else happened that allowed me to continue my 5 hour drive home as hoped. Suggestions are welcome.

I was once driving to New York City on the parkway in my 1962 Corvair on a hot summer evening when the engine quit. I pulled off the road, raised the hood and did a quick looksee in the engine compartment. Nothing obviously amiss. Fuel gauge more than 1/2 full. Waited a few minutes, started it up and it ran fine.Never had the problem again in the several years I owned that car. vapor lock? Who knows.

That video sure looks to me like the pump is pushing mostly air (or perhaps fuel vapor, no way to tell) into the filter.

Also looks like fuel level inside the screen is staying much higher than outside, which I can't explain, but does explain how liquid fuel is getting through the filter.

Like Geo, I've never had anything I identified as "vapor lock" even in 115F heat (the driver overheated before the car did, we finally stopped and got an air conditioned hotel room to wait for the sun to go down). But I've certainly had things that left me on the side of the road and mysteriously cured themselves. Ran without a filter for decades, but finally added one after finding a bunch of very fine silt (probably ground up paint from the body shop not covering the fuel tank when they sanded the body down) in the carb jets.

Just last month I had a condenser fail, only part of the time. The engine would spit and cough and backfire; then clean up and run fine. Happened 3 or 4 times over a week or two before I hit on the condenser as being the problem. Even now, that bad condenser tests good with everything I know to try; but replacing it sure fixed the problem. At some point, I plan to try hooking it up to points & a coil on the bench and seeing what it looks like on the scope; but as usual I got distracted with other things.

Just to show how weird some problems can be; one of the Stags decided it didn't like to run uphill. Ran fine on the level, ran like crap going uphill. Finally traced that one to a broken wire at the starter solenoid! Seems the hot wire to the coil was swinging loose, and would sometimes touch the starter terminal (shorting out the coil) only if the car was tilted uphill.

TR3driver said:

That video sure looks to me like the pump is pushing mostly air (or perhaps fuel vapor, no way to tell) into the filter...

Click to expand...

Video? If you mean the 4 sequential photos above, I took it from the description that the chronology is read right to left (like Urdu).

George, post #16 includes a link to a video.

Funny, the right to left was entered from top to bottom and I assumed they would appear left to right.