Vapor Lock

[Guest imported_MrEarl]

I just got a 54 Special in from MN and it keeps cutting off on me after the engine gets hot.(Didn't even get it off the trailer before it exhibited this habit and let me know who's boss.) Prior to cutting off it will act as if it is starving for gas when I give it gas. Put it in gear and it dies. I cleaned the little after market fuel cleaner someone riged up behind the carb.No improvement.

What exactly is vapor lock. Where does it occur ie in the fuel pump, the carburator, fuel lines or where.I recall a friends 58 Chevy with a 348 back in high school. We had to pour cold water on the fuel pump to get it to go.

It has a Stromberg 2 barrel. I have a totally rebuilt 2 barrel Carter carb that I can replace it with if necessry.

I'm considering replacing the carb and the fuel pump later but would like to just get it running now. Any words of wisdom would be appreciated.

[Guest brh]

A couple of years and lots of dollars later I learned my vapor lock problem was a heat issue with the engine. It would get hot, start starving then eventually die. Found a blocked radiator, and I did not catch it because the engine has to get pretty hot before the idiot light goes on. Had the radiator done and problem went away. I would suggest you make sure the gage is correct if it has one, if it does not check this first (engine temp).

[TxBuicks]

Vapor lock can occur anywhere in the fuel system. When a fuel line, pump, or carb gets too hot, the gas will start to vaporize, causing a 'bubble' in the system. Check anywhere the fuel line runs close to the exhaust manifold or exhaust pipe. Many carburators have a metal spacer between it and the intake manifold to try to disperse heat.

If it happens quickly after start up, I strongly suspect it is not vapor lock because the environment has not had time to get hot enough. A clogged filter or weak fuel pump is more likely your problem.

[Guest Tom]

Lamar, since the car came from Minnesota, it most likely has gas in it with 10% ethanol unless the previous owner made an effort to fill it up at one of a few gas stations that sells non-oxygenated fuel. I have heard that gas with ethanol in it will cause problems with vapor lock. May be something to look in to.

[NTX5467]

Almost every gasoline blend has had varying amounts of ethanol in it for many years . . . just not the stated amount of 10% (with, as I understand it, 15% being the absolute max that can be tolerated without carb calibration changes) as we have now. Even if it was "winter gas" used in the summer, I somewhat doubt it would cause a problem as Lamar is experiencing--just my gut suspicion.

I concur with TxBuicks, the short run time Lamar mentions before the problem happens is not enough time for a vapor lock situation to happen, even in the summer time. What about a non-vented gas cap when the one it might need should be vented? Or an external vent being clogged with a non-vented gas cap?

As I recall, vapor lock usually happened out on the road, in 100+ degree (Fahrenheit) temperatures, bright sunshiney days, with the roadway having absorbed the sun's radiant energy and reflecting it back to the underbody of the car (usually black) . . . at higher-than-100 degree temperatures. In some cases, the fuel lines were hidden away from any significant air flow, so the radiated heat from the roadway was absorbed and not dissipated with air flow generated by vehicle movement. Water on the fuel pump, the classic ice cream cones on the fuel line, and clothespins as "heat radiation devices" for the exposed fuel lines all helped to get things going again.

In more modern times, when the weather suddenly turns warmer in the winter, driveability issues can happen from the more volatile winter gas vaporizing too quickly for the higher ambient temperatures which suddenly happen and the refineries didn't have enough response time build something more appropriate.

Thoughts and observations,

NTX5467

[Pete O]

I had a vapor lock situation in my '51 when I first got it because of a poorly placed aftermarket electric fuel pump. It was placed high up on the firewall, where it had to suck gas up to the carb. Sucking cause a lower pressure in the line, allowing the gas to vaporize at a lower temperature. Fuel pumps need to be down low, where they can push the gas along.

[Rooster]

And mount it as close to the tank as possible. These pumps like to push , not suck.

Lamar, it would appear to me it's definately a fuel problem.

Blockage somewhere, or maybe the accelerator pump in the carb is not working.

[Guest imported_MrEarl]

Rained most of the day so piddled in the barn. Hope to get to it tomorrow. I noticed that the hose between the fuel pump and gas line on the frame is rigged up with a rubber hose and a hose clamp. I'm wondering it this fitting may be sucking in air, contributing to the problem?

[Guest simplyconnected]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: MrEarl</div><div class="ubbcode-body">What exactly is vapor lock. Where does it occur...</div></div> Vapor lock happens when gas boils. Your carb is vented and has a float. You'll never notice a little pocket of 'air' that disburses in the bowl. But when you get a BIG fuel void, it's called vapor lock, because fuel isn't being delivered fast enough to run your engine. More air is pumped than fuel.<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TxBuicks</div><div class="ubbcode-body">Vapor lock can occur anywhere in the fuel system. When a fuel line, pump, or carb gets too hot, the gas will start to vaporize...</div></div> <div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Pete O</div><div class="ubbcode-body">Sucking cause a lower pressure in the line, allowing the gas to vaporize at a lower temperature...</div></div> Together, both of these conditions cause vapor lock.

Distilled water boils at 212F at sea level. In the Rockies, water boils at a much lower temp, (eggs won't ever 'hard boil' and every cake mix comes with different instructions for high altitude). In a pressure cooker, depending on the weight on top, water won't boil until it gets over 350F. Every pound of radiator cap pressure raises the coolant boiling point by three degrees.

Temperature with pressure are the key. (Ask any refrigeration guy.) In a classic car, the fuel pump sucks gas from the tank for nearly fifteen feet under negative pressure, then pumps it for the final one foot under positive pressure to the carb.

Lower pressure (sucking) drops the boiling point and a hot day added to the mix is a certain recipe for vapor lock, especially at 12,000 feet.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: MrEarl</div><div class="ubbcode-body">I'm wondering if this fitting may be sucking in air, contributing to the problem? </div></div> Fuel is 800 times more dense than air. Your fuel pump will suck air through a tiny leak much easier than sucking fuel. The result is the same as vapor lock, but the cause is much different.

Vapor lock is rare with today's gas. In the old days, if you had vapor lock, so did other cars around you. Modern cars are immune to vapor lock because they pump gas from inside the fuel tank at high positive pressure, and the lines recirculate back to the tank.

My '55 had rust so bad in the tank it would fill the screen at the pick-up, and gas would stop flowing. Soon as the engine stopped, the line would 'back wash' the rust off the screen, and it would run fine until more rust clogged the screen, again. When I finally pulled the tank, I cut a hole in it and pulled out seven handfulls of rust.

[NTX5467]

As the fuel tank is generally mounted higher in the vehicle than the fuel pump, it can have positive gravity feed of sorts so little negative pressure might be in the fuel lines as the engine is running. Unless the fuel system has a return line from a "pre-carb" filter/separator or a return line from the mechanical fuel pump itself, there would not be very many times when a significant vacuum might be in the fuel lines from the tank, I suspect. Clogged vents or a non-vented gas cap when it should be vented . . . plus the rust issues mentioned . . . would be contributing conditions to a negative pressure being in the fuel lines between the fuel tank and the fuel pump.

By observation, some vehicles' fuel system designs seemed to be more contributing to vapor lock issues, along with fuel volatility issues for particular blends and brands and their interaction with ambient temperatures. This would be BEFORE vehicle age issues would also enter into the situation.

I significantly doubt that Lamar's problems are from "vapor lock", but from more mechanical issues related to the fuel pump, fuel pickup, fuel tank issues (vents and/or gas cap) . . . even "dead or dying" fuel in the tank. Might even be an ignition issue!

We shall await "The Doctor's" further revelations and informed diagnosis "From The Barn" . . .

NTX5467

[Guest imported_MrEarl]

I replaced the fuel pump and fuel line btwn pump and chassis line. No better. I let it run 'til engine was hot/running temp (it was only in the upper 80's yesterday)) and it started doin the same thing and that is that while at idle it will start sputtering and when i push the throttle it sounds like it is starving for gas. It will eventually just cut off. I removed the gas tank cap and no improvement. Next I will attempt to run it from a bottle just off the fuel pump line to eliminate at least the tank and fuel line area. Or I may change out the coil first, that would be safer. Thanks everybody. <span style="font-weight: bold">WE'LL</span> get this figured out.

[Guest imported_MrEarl]

THIS JUST IN FROM THE BARN. By jove I do beileve we got it. but I may not know for sure 'til next summer as the temp today was only in the low 80's.

I believe Willis aka NTX5467 was spot on with " Might even be an ignition issue!"

I replaced the coil, coil wire and condensor this evening after work. Afterwards, I ran it at idle for over an hour trying to get it hot so I could test it. Either it was too cool or I fixed it. When Willis suggsted it may be an ignition issue I recalled going through much the same with a 54 Century a few years ago. I went about changing the coil first and it turned out to be a dirty gas tank then.

Thanks everybody for your help and suggestions

[Guest simplyconnected]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: MrEarl</div><div class="ubbcode-body">I went about changing the coil first and it turned out to be a dirty gas tank then.</div></div> When you have enough rust to stop flow, there is more rust than you think (and there is nowhere for it to go, so it continues to increase). Your coil is just fine. When they die, they don't heal themselves and you can smell raw gas for lack of spark. Glad you got it going again. - Dave

[old-tank]

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body"> " Might even be an ignition issue!"</div></div>

There have been times in the past when I drove these car for regular transportation and had problems with a coil or condensor that worked fine when cold and when warmed up started to give problems. The coil usually idled ok, but when accelerated to the point of bringing on the vacuum advance started cutting out. The condensor just died when hot.

Willie

[W_Higgins]

I'm just now finding this, but yes, a bad coil can be very deceptive (and very common). When they get hot they don't generate enough electrical pressure to jump the gap when under compression. Where they fool most people is when the cars dies they'll pull the coil wire, crank it, and see a spark and rule out the coil. The problem is that it takes more voltage to jump the same gap under compression than it does in the open atmosphere. When it cools off, resistance will drop, and it will start working again. Sometimes they fail gradually, sometimes quickly, but it sounds like you found your problem.