10.1 Engine Fuel System Flow, Contamination, and Vapor

Fuel Flow Before Metering

Before fuel can be metered, it must be available as clean liquid fuel at the correct pressure. The engine fuel path normally begins at the tank outlet, passes through selection, strainers, pumps, filters, valves, and lines, then reaches the carburetor, fuel control, injector servo, flow divider, nozzle, or turbine fuel nozzle system. A restriction or air leak early in the path can look like a metering fault unless the mechanic follows the fuel from source to combustion.

Fuel pressure and flow are related but not identical. Pressure is available force in the line. Flow is the rate of fuel movement. A plugged filter can reduce both at high power. A partially blocked injector nozzle can leave upstream pressure normal or high while that cylinder runs lean. A leak downstream of a flow transmitter can show flow without producing power. The mechanic must ask where the instrument is located, what it measures, and whether the symptom is engine-wide or cylinder-specific.

Contamination affects cause and effect. Water can settle in low points, freeze at altitude, corrode components, or pass as a slug that interrupts combustion. Dirt can hold a valve open, clog a screen, wear a pump, or distort fuel metering. Microbial contamination is more associated with water presence and can create sludge or corrosion. Old hoses can shed particles internally. The corrective action is not just draining a sample; it may require finding how contamination entered and whether it reached precision components.

Vapor lock and vapor formation occur when fuel changes state or contains vapor in a section of the system. Heat, low pressure, high altitude, suction-side restrictions, and volatile fuel can contribute. Pumps move liquid effectively, but vapor reduces pump output and causes fluctuating pressure. A boost pump may suppress vapor by increasing inlet pressure, depending on system design. An exam answer that says add pressure to the suction side may be correct only when that matches the system instructions.

A practical fuel-supply troubleshooting sequence is:

1. Verify correct fuel grade, quantity, tank selection, and venting.
2. Drain sumps and inspect samples for water, debris, color, and odor using approved procedures.
3. Check filters, screens, boost-pump operation, engine-driven pump output, and line condition.
4. Compare fuel pressure, fuel flow, RPM, exhaust temperature, and power response.
5. Inspect downstream metering only after proving the supply path can deliver clean liquid fuel.

For knowledge-test questions, watch for high-demand conditions. A restriction may not appear at idle because fuel demand is low. During takeoff, climb, or acceleration, the same restriction can starve the engine. A blocked vent may run normally for several minutes before tank vacuum limits flow. Cause-effect timing is often the clue that separates a tank, vent, pump, filter, or metering problem.