Engine Roughness at Cruise
You are cruising at 8 500 ft in a normally aspirated, fuel-injected single. EGT was stable at 1 450 °F, CHT 380 °F, fuel flow 38 L/h, oil pressure mid-green, oil temperature 195 °F. Over two minutes the engine begins to run rough, EGT on number 3 cylinder drops about 150 °F while the others stay steady, fuel flow is unchanged, and the manifold pressure is steady. The roughness is mild but noticeable, and you are 40 min from the nearest suitable airport with terrain below. There is no fire warning, no smoke, and oil pressure and temperature remain normal.
1. What are the most likely causes of this symptom pattern, and which instruments point to which cause?
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A single-cylinder EGT drop with unchanged fuel flow and manifold pressure points to a problem local to that cylinder: a fouled, partially blocked, or failing injector nozzle; an ignition problem on that cylinder (plug or lead) - though ignition usually raises EGT on the affected cylinder as unburnt mixture passes, so a drop is less typical; a sticking or leaking valve; or a compression loss. A blocked injector reduces fuel to that cylinder, leaning it - but that would normally raise EGT, so a drop with unchanged total flow suggests fuel is going somewhere else or combustion is being lost. Unchanged total fuel flow with one cylinder down means the remaining cylinders are running richer, which lowers their EGT and can mask the problem. Steady oil pressure and temperature argue against a bearing or major mechanical failure. The absence of fire or smoke argues against a fuel or oil leak into a hot section.
2. What immediate actions would you take, and how would you decide between continuing and diverting?
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First, treat it as real: reduce power to a setting that smooths the engine if possible, enrich the mixture slightly to protect the affected cylinder from detonation, and check mags briefly on one magneto at a time to see if the roughness changes - a change points to ignition. Check fuel selector, boost pump, and tank contents. Monitor CHT closely: a rising CHT on the affected cylinder is a strong reason to land. If the roughness is mild, stable, and CHT and oil temperature remain normal, a controlled continue to a suitable airport 40 min away is defensible, with a diversion airfield identified en route. If roughness worsens, CHT climbs, or power output falls, divert immediately. The decision rule is: any trend toward worsening or any temperature excursion means land at the nearest suitable airport, not the original destination.
3. Why is it significant that total fuel flow is unchanged while one cylinder's EGT has dropped?
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If one cylinder is receiving less fuel, the fuel system is still delivering the same total, so the other cylinders are getting more than their share. That makes them richer, which lowers their EGT and can hide the fault, and it means the engine is not running at the mixture you think it is. Richer mixture also raises the risk of fouling and reduces power. More importantly, the unchanged total flow tells you the fault is not a simple loss of fuel supply to the engine as a whole - it is distribution or combustion within one cylinder. That narrows the diagnosis to injector, ignition, or valve/compression, and it means you cannot rely on the fuel-flow gauge to tell you the engine is healthy.