Why does my 1997 MerCruiser 8.2 MPI misfire above 2,800 rpm and lose power?

Your symptoms point first to ignition breaking down under load or fuel pressure falling as demand rises, rather than a normal engine speed limit. Check the distributor cap, rotor, plug leads and plugs, then measure fuel pressure as the fault occurs and compare it with the healthy engine.

Identify engines and fault conditions

A petrol engine can idle smoothly yet misfire badly under propeller load. Higher cylinder pressure makes it harder for a weak ignition system to fire the plugs. A restricted fuel supply may cope at low speed but fall behind as you open the throttles.

Record both engine serial numbers before ordering parts or checking specifications. A 1997 installation described as an 8.2 MPI may differ in block generation, ignition equipment and fuel system. The boat's year alone cannot identify the equipment fitted, especially after nearly three decades of repairs.

• Confirm whether only one engine misfires and note the other engine's rpm at the same boat speed.
• Record whether the fault starts immediately at 2,800 rpm or after several seconds under load.
• Note temperature, oil pressure, warning buzzer activity and whether the tachometer needle jumps as the misfire starts.
• Establish whether the fault began suddenly, followed servicing or developed gradually over a season.

The 3,000 rpm it currently achieves is not necessarily its specified wide-open-throttle speed. Check the operating range for the engine serial number before making propeller decisions.

Inspect the ignition first

With the ignition isolated, remove the distributor cap. Look inside for moisture, green corrosion on the terminals, hairline cracks and thin black carbon tracks. Inspect the rotor for burning and damage, then check the centre contact and coil lead. Start here on an older petrol installation exposed to damp engine-room air.

Inspect every plug lead, especially where it passes close to exhaust manifolds or rubs against brackets. Look for hardened insulation, burnt sections and loose terminals. If the problem followed a tune-up, check lead routing and firing order against the correct engine manual.

Remove the spark plugs and label each by cylinder. An unusually wet or black plug can point to a cylinder needing investigation, as can one that is exceptionally clean. Check the specified plug type and gap for that engine serial number; do not assume a generic automotive big-block specification applies.

A marine technician can use an ignition scope or suitable spark-testing equipment to establish whether the spark fails under load. Resistance checks alone cannot prove that a lead or coil works under cylinder pressure.