Fuel pressure/flow issues with hot IO540

A pilot participating in a break-in test flight of a freshly overhauled Lycoming IO-540 normally aspirated engine encountered a classic vapor-related fuel delivery anomaly. During climb and cruise on a hot day, with the engine not yet fully broken in and running warmer than normal, fuel pressure and flow began a slow, progressive decline. The drop in fuel supply leaned the mixture further, which in turn drove cylinder head and oil temperatures even higher, creating a self-reinforcing thermal spiral. The crew's diagnostic instinct proved correct in the moment: activating the electric boost pump immediately restored fuel pressure and flow to normal values, and CHTs came back down. The pressure and flow trace shared in the post shows the gradual decay followed by an abrupt correction the instant the boost pump was switched on, a signature pattern consistent with vapor formation in the fuel line rather than a mechanical restriction or pump wear issue.

The underlying mechanism here is fuel vaporization in the lines between the tank and the engine-driven pump, most likely at the gascolator, fuel lines routed near the exhaust or engine case, or at the inlet of the mechanical pump itself. As fuel temperature rises under high underhood/cowl temperatures, its vapor pressure can approach or exceed the pressure in the line, causing bubbles to form. These vapor pockets reduce the effective liquid fuel volume the engine-driven pump can move, so both indicated pressure and flow fall even though the pump itself is functioning correctly. Because vapor formation is temperature-dependent and worsens as engine heat soaks into fuel system components, the problem compounds on hot days, at high power settings, and especially during a break-in period when higher-than-normal oil and CHT temperatures are expected due to increased friction and richer break-in mixtures generating more heat before rings seat. The boost pump resolves the symptom because it pressurizes the fuel ahead of the vapor-prone section, collapsing bubbles back into solution and pushing adequate liquid fuel volume through to the servo or carburetor regardless of local heating.

For working pilots and mechanics, this is a useful reminder that new or freshly overhauled engines can reveal fuel system marginalities that were previously masked, especially when overhaul work touches fuel lines, the gascolator, or the engine-driven pump and its mounting/cooling provisions. Blast tubes and heat shields on the gascolator and mechanical pump are known mitigations, and their absence or misrouting is a common squawk on both new-build and overhauled installations, particularly on airframes where the cowling was modified, baffling was disturbed, or heat shielding was omitted during maintenance. The practical takeaway for anyone conducting engine break-in flights or ferry flights in hot weather is to treat gradual fuel pressure/flow decay paired with rising CHT as an early warning sign, use the boost pump proactively rather than reactively, and treat persistent occurrences as a maintenance discrepancy requiring inspection of fuel line routing, insulation, and heat shielding rather than something to fly around indefinitely.

This scenario also reinforces broader trends across piston GA and light business aviation: engine and accessory overhauls increasingly involve parts sourced or reinstalled without full attention to OEM heat-shielding specifications, and warm-weather operations continue to expose fuel vapor lock issues especially in normally aspirated, gravity- or pump-fed systems without dedicated fuel coolers. Operators running IO-540s and similar high-output Lycoming/Continental engines in hot climates, at high density altitudes, or during initial break-in periods should treat boost pump use as a standard mitigation tool, brief it explicitly in checklists, and use fuel pressure/flow trend monitoring—as captured in the shared data trace—as an objective way to catch vapor-related degradation before it becomes an in-flight emergency rather than a bench-test curiosity.