IWOBBLED — Diesel Pre-Start Checks
The IWOBBLED mnemonic covers the essential pre-start checks for a marine diesel engine. Run through this checklist every time before you turn the key — it takes two minutes and could save your engine (or your boat).
Quick Reference
Isolate
Close the fuel isolation valve (seacock). This prevents fuel flowing while you inspect the engine. You'll re-open it before starting.
Water
Check the raw water seacock is open and the strainer is clear. Without cooling water the engine will overheat in minutes. Check the header tank level on indirect-cooled engines.
Oil
Check engine oil level on the dipstick — should be between min and max marks. Look for milky oil (water contamination) or very dark oil (overdue service). Check gearbox oil too.
Belts
Check alternator and water pump drive belts for tension, cracks, and glazing. You should be able to deflect the belt about 10mm with thumb pressure. A snapped belt means no charging and no cooling.
Bilge
Check the bilge under the engine. It should be clean and dry. Oil in the bilge means a leak. Water means a cooling system problem or stern gland drip (some is normal). Fuel smell means a fuel leak — do not start.
Levels
Check fuel level in the tank(s). Check coolant level in the header tank. Running out of fuel introduces air into the system and requires bleeding — a messy, time-consuming job at sea.
Electrics
Check battery voltage (should be 12.4V+ at rest). Inspect wiring for corrosion or loose connections. Ensure the battery isolator switch is on. Check the alternator belt (already done, but confirm).
Diesel
Open the fuel isolation valve. Check the primary fuel filter/water separator — drain any water from the bowl. Look for fuel leaks at injectors and connections. Sniff for diesel — any leak is a fire hazard.
Unisolate & Start
With all checks complete, ensure the fuel valve is open (you opened it in the "D" step), put the engine in neutral, and start. Immediately check for cooling water flow from the exhaust — you should see a steady stream within 10 seconds. If no water appears, shut down immediately and investigate.