There's no engine-driven fuel pump (like the injection system has) to pull on the fuel and reduce its pressure and making vapor lock more likely. It isn't likely due to the small head pressure all the way from the tanks, and the abundant airflow though the cowling keeps the FWF stuff cool enough. The old carbureted 172s used only gravity feed for fuel flow, so vapor lock wasn't an issue. Not an AD (at least not yet) so it's not mandatory. Some of them experienced vapor lock and Cessna issued an SB outlining the means of adding a return line and check valve. The earlier 172R and S models had no return systems.
#Engine vapor lock manual
Just hoping I don't ever have to run the wobble and do a manual gear extension. In my case- that archaic wobble is the solution.
#Engine vapor lock how to
How to prevent vapor lock? Got me- other than learning as much as you can about your bird. I know a guy that added an STC to the old E225 which required the removal of the fuel return line and the result was vapor lock at altitude when running car gas. It appears the extra (3 gph?) fuel transfers heat and cools the plumbing under the cowling.
The fuel return line is the biggie when in the air. The wobble with the large plunger and insides will push fuel any time. When I had a -35 with the electric fuel aux pump, it seemed like the fuel (near?) the pump would vaporize rendering it a noise maker. The wobble pump is the only way to start a hot engine on a hot ramp this time of year. (Ok, outside the low compression engine.) Two primary elements appear to be what keeps the system from vapor locking running car gas: the wobble pump and the fuel return line. The early bonanza was designed to run what was available- car gas. You'll hear the fuel boiling in those lines, and it's being forced out into the intakes so that it tends to flood the engine. Shut it down on a hot day in a quiet place and listen at the cowl opening in front. Injected engines have their injection lines above the engine. When they stuck electric pumps in the tank it went away. Lifting the fuel a few inches would do it. Old cars had this problem when they had only engine-driven pumps. Use Mogas and it gets worse, since Mogas has higher volatility, another way of saying that its boiling point is lower. Pull on the fuel on a hot day and you could get vapor lock as the fuel upstream of (below) the pump starts to boil and the pump gets nothing but vapor. A fuel pump sucking fuel uphill will lower the pressure on the fuel, as in a low-wing airplane with the boost pump off and the engine's pump pulling the fuel up. Hot days and heat under the cowl warms the fuel. The fuel in the airplane has to deal with heat and sometimes with pressure drops. If the water is warmer, it takes less vacuum. Remember the old water-in-a-bell-jar demonstration in school? Water at room temperature, with enough air sucked out of the bell jar, will boil. You can get a liquid to boil if you raise its temperature to its boiling point, or if you reduce the atmospheric pressure on it without raising its temperature. Also, cooling air blowing on fuel pumps or insulation on fuel lines can reduce the tendency to vaporize.
#Engine vapor lock free
There are precautions used to help combat vapor such as header tanks, a small tank near the engine (as close as practical) where the fuel flow is near free of turbulence and any vapor can be vented back to the main tank. While the atmosphere keeps the fuel in the tanks bubble-free, heating and the numerous bends in the lines and fittings create turbulence in the flow, the higher you climb the more likely the turbulence will create "vapor". Gasoline is sensitive to heat and pressure changes that along with the convoluted maze of tubing and fittings that the fuel has to travel through to get from the tanks to the engine is not ideal. The same thing happens when you hold your arm under water in the swimming pool and move it extremely fast, bubbles form in the area immediately behind your arm in the turbulent area. Decrease the pressure enough on a liquid and bubbles form. A hydraulic pumping engineer described it to me and I will paraphrase a bit.
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