Low Compression and a Cool Cylinder on a Fuel-Injected Flat Six
A six-cylinder, horizontally opposed, fuel-injected engine in a Part 91 single has 1,180 hours since major overhaul. The owner reports a light roughness at cruise and about 2 gallons per hour more fuel burn than usual. At 65% power, cylinder #4 exhaust gas temperature sits roughly 90 °F below the other five at the same fuel flow, and its cylinder head temperature runs 30 °F cooler. A differential compression test gives #4 at 46/80 with air plainly audible at the exhaust stack, and #2 at 74/80 with air heard at the crankcase breather. Borescope on #4 shows a narrow bright crescent on the exhaust valve face with the remainder of the face dark, normal crosshatch on the barrel, and an undamaged piston dome. The operator wants to fly the airplane home, four flight hours away, and do the work there.
1. Interpret both compression readings. What does each leak path tell you, and how do you rank the urgency of #4 against #2?
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The differential test pressurizes the cylinder at top dead center on the compression stroke and measures how much of the reference pressure the cylinder holds. Where the air escapes identifies the leak path, not just its size.
- #2, 74/80, air at the breather: leakage past the rings into the crankcase. Some ring leakage is inherent in a running-in engine and 74/80 is comfortably above the usual 60/80 minimum. This is a normal finding, recorded and trended, not a defect.
- #4, 46/80, air at the exhaust stack: leakage past the exhaust valve and seat. This is below the manufacturer's minimum and, more importantly, the leak path is a hot-gas path.
Ranking: #4 is the urgent one. Ring blow-by leaks combustion gas into the case and is self-limiting. Exhaust valve leakage erodes the valve and seat progressively; a valve that stops rotating and tulips can liberate a head into the cylinder. The bright crescent on the valve face supports uneven seating or loss of valve rotation, consistent with the compression result and with the low EGT and CHT on that cylinder (reduced trapped charge, less heat released).
2. The operator argues that the low EGT and CHT prove the cylinder is 'just lazy' and safe to fly home. Evaluate that reasoning.
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The premise is right but the conclusion is backwards.
Low EGT and CHT on one cylinder at matched fuel flow are consistent with that cylinder trapping less charge and therefore releasing less heat. That is exactly what an exhaust valve leak produces: the compression stroke loses mixture out the exhaust, and the power stroke loses combustion pressure the same way. The temperature reading is a symptom of the leak, not a guarantee of safety.
A partially restricted injector nozzle can also drop EGT on one cylinder, so the temperature alone is ambiguous. The differential test resolves the ambiguity: the audible exhaust leak is direct physical evidence of a valve/seat problem, so the roughness is a compression problem, not a mixture distribution problem.
Also weigh the second-order effects. The remaining five cylinders are carrying the power, running hotter and at higher BMEP, and the failing valve is being eroded a little more each hour. Four flight hours is not a conservative margin for a valve that is already visibly marked.
3. What inspection and repair path do you authorize, and what are your return-to-service criteria?
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Before further flight: remove #4 for inspection. The borescope finding is enough to justify pulling the cylinder rather than re-reaming the guide in place and flying it.
On the bench:
- Inspect the valve face and seat for burning, tuliping, and contact pattern.
- Measure valve stem-to-guide clearance; a worn guide is a common reason a valve stops rotating.
- Check the valve stem for scoring, the rocker and pushrod for wear, and the cam lobe for distress.
- Refinish or replace the valve and seat, lap to a full contact pattern, and verify installed spring height.
Return to service: compression at or above the manufacturer's minimum with no exhaust leakage; correct torque on through-bolts and hold-down nuts; a run-in and full-power ground check with EGT and CHT spread within limits at matched fuel flow; and a documented trend of oil consumption. Because a valve problem can originate in cooling or mixture distribution, note in the log whether the other cylinders' guides were checked. One good ground run is not sufficient evidence - require a follow-up compression check and EGT/CHT trend after several hours.