What is Vapor Lock?

[pwm74]

Driving my car after finishing some stuff on it and about 40min of driving it spuutered and died, let it sit for 5min and then it did it again. Someon told me it might be vapor lock, how true is this. i know good deal about mechanics but dont know what vapor lock is

[911pcars]

Vapor lock is when the fuel gets hot enough to boil (vaporize). The resultant air bubbles prevents the pump from effectively pushing the fuel where it needs to go (air is compressible; liquid is not). Historically, 911s don't have much of a vapor lock situation.

This cold be a fuel pump that's going dead or the ignition system crapping out for some reason. Check the tank and in-line filters and perform a pressure and volume test to see if this is a fuel system issue. Check for spark next time this happens to isolate the system.

Sherwood Lee
http://members.rennlist.org/911pcars

[Early_S_Man]

You don't mention what year or model your car is, but unless it is carbureted, it isn't going to be having vapor-lock problems!

Even though recent reformulations of gasoline have increased the partial-pressure, or vapor pressure of our gas, neither the CIS or DME-equipped cars have any potential for vapor lock, since the system pressures are over 30 psi!

Vapor lock as an automotive phenomenon ... originated in the days of carburetors and engine-mounted diaphragm fuel pumps, when fuel pressures were less than 6 psi, typically. And, earlier cars are apt to have the problem worse than ever on hot days with current fuel. But, electric fuel pumps can be relocated near the fuel tank, or a second pump can be added to minimize the pressure drop of long fuel lines, since fuel tanks and engines tend to be at opposite ends of the cars!.

[Pierre]

I've had similar problems. I don't believe the problem is fuel vapor lock. If your car is running down the road and sputters then quits, chances are that it is the CD unit (so I'm told). Check the neg. connection wire and all other connections in and around the unit. I'll bet that's your problem if your car is older like mine.('72 911e 2.4ltr). Hope that this helps!
-Pierre

[pwd72s]

"You don't mention what year or model your car is, but unless it is carbureted, it isn't going to be having vapor-lock problems! "

Warren, beg to differ on that comment. 1978 Porsche Parade, Snowmass, Colorado. Warm weather combined with CSI injection? 911's were sputtering to a halt all over the place. But for some reason the MFI cars were immune. I figure the 10,000 foot plus altitude was a factor, but really never knew why...

Paul

[Early_S_Man]

Paul,

I don't know what the problem with the CIS cars was in Colorado above 10,000 feet was ... but it WASN'T vapor-lock, since the CIS fuel pumps can put out more than 60 psi! And, the DME cars ... 40 psi!

[pwd72s]

Excuse me...CIS. But yeah, it was some kind of altitude thing. And Dr. Fuhrmann, who was there, referred to "vapor" lock, but it could have technically been something else? My friend Ben's '76 stalled on the entrance road to the Parade site. I told him I could "fix it", but had to go get some tools first. So, took him to our condo room, bought him a scotch drink or two. Took our sweet time. Back at his car some 45 minutes later, I told him to get in, crank it when I told him to. So, I paused 30 seconds or so, tapped the screwdriver a couple of times on the fan surround, asked him to give it a try. As I suspected it would, the car cranked right over. Later I told him the truth, that I'd done nothing but give the car time to cool down. Maybe at 10,000 feet, 60 psi wasn't enough to clear the vaporization? But the 200 PSI of the MFI was? Anybody else on the board go to the '78 Parade? If so, they'll remember the problem...mostly with the cars that entered the rally.
But I agree, pwm74's problem is most likely something other than vapor lock. You think maybe CD unit, or fuel related in another area? I'd suggest first checking the fuel filter, see if anything found there. At least it's cheaper than the CD unit....

[911pcars]

I wrote:
...."Historically, 911s don't have much of a vapor lock situation.

I mis-spoke. Carbureted 911s have had percolation problems with the fuel; hot fuel would expand in the float bowls and spill over into the engine. The excess fuel would prevent the engine from starting. The solution was to drill a hole to vent the float bowl and install insulator blocks under the carbs.

Vapor lock is usually a symptom of heat soaking which occurs when the engine sits for a time after running. Many manufacturers use temp. switches that leave the electric fan running after shutdown until underhood temps. drop.

If the engine cuts out while running, this is usually due to a lack of fuel or a symptom of an electrical or ignition malfunction.

Sherwood Lee

[pwm74]

Its a 3.6. I did not put the oxygen sensor back in yet,on custom exhaust. it also cant hold a idle. I think i am going to try some chevron fuel treament and change the filter, I just did all the other tune-up stuff. I also found out the other day I have about 1/4 throttle still to go, Hows that for free horsepower. Ithe front pedal is at the end of its adjustment. Does anyone know where to adjust the rest. its a 964 linkage. It must of stretch through the 3 years or so.

[81Turbo]

Having been in the refining business for over 20 years, I can assure you that it is possible (but far less likely) for fuel injected cars to experience vapour lock. This is not a phenomena of gasoline "boiling" but of the light hydrocarbons coming out of solution. Boiling involves a phase change which does not happen with gasoline (at least not in this case). All refiners try to maximize the amount of C4 (butane range material) in the gasoline for various reasons. The primary reason is that there is very little else they can do with it other than burning it as fuel gas. From a product standpoint though, the C4 (iso C4) has very good octane qualities and makes a great component to adjust Reid Vapour Pressure (RVP). This is the specification that changes in gasoline between summer & winter and basically just allows the gasoline to vapourize more or less easily. When the ambient temperature is lower more of the lower boiling range material will remain in solution. In older carbureted cars the problem is far more pronounced due to the lower operating pressure. Increase the pressure and the light ends will remain in the liquid, i.e the same effect as lowering the temperature but FAR more effective.

Warren & Sherwood hit the nail on the head and the only time you would normally see this type of problem on a fuel injected car is when the engine has been shutoff and radiant heat from the engine causes the fuel in the lines to heat up. No flow = higher temperature. Higher temperature = more light ends being liberated from the liquid. Less light ends in solution = poor firing caused by intermittent spray pattern through the injector.

I would expect that the problem experienced in Colorado at elevation was a result of 2 things, 1) air at higher altitudes is less dense than at sea level (lower mass / unit volume) and 2) higher elevations almost always have lower temperatures. This combination of changes is not a recipe for good combustion. This is one of the major reasons why cars have mass air flow sensors and fuel mixtures are adjusted "on the fly" to overcome the problem. This would be the reason the MFI cars had no problem at higher altitude burning the same gasoline as the CIS cars.

Years ago, gasoline was specified for a specific marketplace / region and the RVP of the gasoline was specified for that area. With all of the changes in the refining industry over the years many small regional refiners went bust. Product had to be produced in one area and marketed in another. The solution was to have bigger regional specifications, more trading between refiners and fuel systems that could cope!
One last thing, a pump can be a compressor but a compressor can never be a pump...

Todd Gillespie

[pwd72s]

Todd, Thanks for the great explanation! In the case of my friend's '76, his car wouldn't start after he'd gassed up the car. Most likely others wouldn't start after stopping at rally check points. "Sputtering to a halt" wasn't an accurate description, and for that I offer Warren my apologies. Can't recall whether the carburetor cars had trouble...

[J P Stein]

A fuel pump won't generate much pressure when it's trying to pump vapor. 914 D jet cars were/are notorious for this due to the FP location.
I know what happened in Colorado, IT WAS SATAN

[enk1]

PWM 74 -

On a 1971 911T I had many moons ago, a similar problem was caused by something as simple as a dirty fuel filter. You might want to check that out first.

[Wayne 962]

Warren, I also beg to differ with you on the CIS cars. Vapor lock can occur in many different systems, including CIS. My 914-6 has the fuel pump mounted near the engine, and it suffers from this greatly. I have to carry a can of compressed air with me to get the car started when it's warm (I'm working on this problem).

Anyways, your fuel accumulator is responsible for holding pressure in the system when the car is turned off. This unit is in the system specifically to help abate any warm-start vapor lock problems. If your accumulator's spring-loaded 'check-valve' is not working properly, then pressure loss in the system can help increase the chances of vapor lock. When you turn off the car, the accumulator helps to hold pressure in the system (never disconnect CIS fuel lines recently after the car has been running). Keeping pressure in the system helps to keep the fuel from evaporating (PV=nRT, physics 101).

-Wayne

[81Turbo]

Absolutely correct Wayne, the PVT relationship is the foundation of Fractionation (or distillation if you are more familiar with that term). To go back to the early comment about a pump not being able to generate pressure when pumping a vapour. This is also true however, this applies to what is on the SUCTION side of the pump. Heat is generated by pumping the same as in compression. What happens is the vapour in this case actually does boil (change phase) and bubbles are present. As the two phase material passes through the pump more & more material boils due to the heat until eventually cavitation occurs. This condition if not corrected will eventually lead to pump destruction. This is the reason why most cars have the fuel pump located as near as possible to the tank & as far away as possible from the engine.

There basically 2 types of pumps. Centrifugal pumps generate pressure by "throwing" liquid off the impeller at high velocity. The velocity is converted to pressure due to the decreasing area and shape of the volute casing. ALL centrifugal pumps work on a differential pressure across the pump, i.e. higher suction pressure=higher discharge pressure. In a car, suction pressure is a result of static head in the fuel tank. Rule of thumb is 2.2 ft. head of water (density 1 kg/L) = 1 PSIg. Gasoline has a density of ~.7 kg/L thus 2.2 ft. head of gasoline = ~ 0.7 PSIg. Note that this applies to any size tank. The same suction pressure would exist on a pump at the bottom of a 100,000 BBL storage tanks as exists in your fuel tank at equivalent static head height . Another aspect of centrifugal pumps is that they will only generate a specific discharge (shut in) pressure. From that point on no further increase in pressure will result.

Positive displacement pumps (normal application for car fuel pumps) work on a different principal. There a few different kinds of positive displacement pumps - reciprocating, rotary lobe, sliding vane, diaphragm, screw & gear. These pumps work only on the principal of forcing liquid through a smaller area to generate pressure. Positive displacement pump discharge pressures are not dependent on suction pressures. As long as they have liquid to pump they will continue pumping until either something lets go or the applied power is cut off. In industrial applications, all positive displacement pumps have a relief device on the discharge side to prevent damage. The same principal applies in automotive applications in the form of the return line to the tank.

Due to the properties of gasoline that I explained earlier, the possibility of vapour lock exists in all cars but is much less likely in fuel injected motors operating at higher pressures. Wayne indicated the root cause of the problem in his 914 as being the location of the fuel pump which I'm sure is correct. I would suggest though that the problem in this case has more to do with cavitation than vapour lock since the suction of the pump (low pressure side) is much more susceptible to vapourization.

By the way, Vapour Lock Protection is one of the many gasoline specifications. Typical specification on this is 57 - 60 Deg.C Summer, 53 - 57 Intermediate and 50 Winter. Years ago they actually carried out this test in Laboratories under ASTM (American Standard Test Methods). For the most part however the test has been done away with and a conversion table is used between the 10% distillation point of the gasoline & the Reid Vapour Pressure (RVP). Typical 10% distillation point of premium gasoline is ~65 Deg. C which does not change with season. RVP's range from 70 KPa (~10 PSI) in the Summer, 75 KPa Intermediate & 85 KPa Winter.

Note the P/T relationship here, the lower the ambient temperature the higher the vapour pressure of the gasoline.

Todd