Another fantastic and well articulated article from Jay Salser. Our timing lights are pointed in your direction. Thank you for all you do here at 1967beetle.com.
I commonly hear the words—“I had a vapor lock!”
We mostly think of vapor in conjunction with heat. So a BIG question arises when the “vapor lock” occurs under cool circumstances. This renders such a diagnosis suspect.
It is very easy to blame a poorly functioning air-cooled engine on a “vapor lock”. When someone calls to ask what can be done to cure a “vapor lock”, I ask lots of questions.
I want to hear how the car-engine acted. I want to hear about the circumstances that led to the problem. I want to hear about the ambient temperature.
My mind follows the Fuel System from the Tank to the Fuel Pump. And I literally ask the caller questions during my mental perusing of the Fuel System. It’s like a movie playing through my brain as I listen and ask questions.
Did the engine just quit?
Did the engine buck, then finally stall?
Or, maybe the caller has changed the Fuel Filter.
In any case usually no firm diagnosis is reached and the next time it happens, the same scenario plays out.
One person reported that his car stalled at the roadside. A passing motorist stopped to give aid. He produced a bottle of water and poured it over the Fuel Pump. Soon, the engine started and the driver resumed his journey homeward. The assumption—the Fuel Pump had suffered a “vapor lock”—even though the weather wasn’t even hot.
Recently, Frank Salvitti talked to me about the “vapor lock” which temporarily put his car out of commission. He had driven a few miles, parked his Beetle and gone into the store to make his purchases. When he came out—the car would not start. He said that he could not see any fuel in the Fuel Filter (mounted, still, in the engine compartment). Eventually, after the engine had cooled, he surmised, the car started and he drove home.
Here’s what I asked Frank to do. I asked him to get a Fuel Pressure Gauge to connect between the Fuel Pump and the Carburetor. In a few days, he reported Fuel Pressure in excess of 5 PSI. This is far too much pressure.
The Float Valve (commonly called the Needle Valve) in the top of the Carburetor cannot withstand such High Pressure. Gasoline forces its way into the Bowl and begins to overflow down the throat of the Carburetor. When this happens, not enough air can mix with the un-atomized gasoline and the engine is choking to death on raw fuel. It either stalls or won’t restart after having been turned off.
Until all of that raw gas has dispersed and evaporated.
Think of the Bowl of the Carburetor as a toilet tank. If we hold the float down, water continues to fill the tank until it finally overflows. We have generated “excessive pressure” on the tank float—overpowering the cut-off mechanism.
Sometimes the Pressure is so great that gasoline can be seen percolating in the filter (if it is connected between the Pump and the Carb). With the Air Breather removed, raw gas sometimes can be seen over-flowing down the throat of the Carburetor. This especially can be seen if the car has been parked nose-uphill.
First, let’s review how the Fuel Pump operates through the following photographs.