Why is the exhaust temperature high on one main engine unit?

On a healthy multi-cylinder engine the exhaust temperatures sit in a fairly tight band, so a single unit reading well above its neighbours is one of the clearest diagnostic clues an engineer gets. Most plants treat a deviation of roughly 30 to 50 degrees C above the average of the other units as the point to investigate, and a steadily widening deviation matters more than the absolute figure. The fault is almost always local to that cylinder: its fuel injection equipment, its exhaust valve, or its share of the load.

Work through the likely causes in order of probability:

• A faulty injector: dribbling, carbon trumpets on the nozzle, low opening pressure or poor atomisation burn fuel late, and the afterburning shows up directly as high exhaust temperature.
• Incorrect injection timing or a worn fuel pump on that unit, which shifts combustion later in the stroke.
• A leaking or badly seated exhaust valve. The escaping hot gas raises the reading while peak pressure and compression fall, and you may hear a rhythmic hissing from the valve.
• An overloaded cylinder, taking more than its share because its fuel rack or pump index is set higher than the rest.

Confirm before you condemn anything. Check the thermocouple and its connection first, because a decaying probe can mimic a real fault. Then take indicator cards or PMI readings and compare peak pressure and compression pressure against the other units: high temperature with normal or high peak pressure points to overload, high temperature with low peak pressure and normal compression points to the injector or timing, and low compression points to the exhaust valve or rings. Note the fuel pump index for each unit at the same load. Fouled scavenge ports, a dirty air cooler or a fouled turbocharger also raise temperatures, but those tend to lift several units together rather than one.

The remedy follows the finding: renew or overhaul the injector, verify pump timing, or overhaul the exhaust valve, and rebalance the load afterwards so no unit runs hot. Resist the temptation to simply pull back the fuel rack to equalise temperatures without cards, because you can hide a developing defect that way. If the temperature keeps climbing or exceeds the maker's limit, cut the fuel to that unit, reduce power and inform the bridge, and keep an eye on the scavenge spaces, since prolonged afterburning and blow-by are a classic route to a scavenge fire. A single-unit deviation that survives an injector change deserves a full unit inspection at the next opportunity.