Overheating? Is that what's troublin' ya' bunky?

Engine Overheats (Mechanical)

1. Engine RPM below specifications at wide open-throttle (engine laboring)    Damaged or wrong propeller; growth on boat bottom; false bottom full of water

2. Wrong ignition timing too far advanced or retarded

3. Sticking distributor advance weights

4. Spark plug wires crossed (wrong firing order)

5. Lean fuel mixture Refer to "Carburetor Malfunctions" in this section

6. Wrong heat range spark plugs

7. Exhaust restriction

8. Valve timing off Jumped timing chain, or improperly installed

9. Blown head gasket(s)
     A blown head gasket(s) normally cannot be detected by a
compression check. Normally the engine will run at normal
temperature at low RPM, but will overheat at speeds above 3000 RPM.


NOTE: Engines that are seawater cooled: Using a clear plastic hose, look for air bubbles between seawater pump and engine. If there are no bubbles present, install clear plastic hose between thermostat housing and manifold(s). If air bubbles are present at a higher RPM, it is a good indication there is a blown head gasket.

NOTE: See tip sheet for MC Alpha I Generation II sterndrive,

10. Insufficient lubrication to moving parts of engine.
      Defective oil pump, plugged oil passage, low oil level

Engine Overheats (Cooling System)

IMPORTANT: The first step is to verify if the engine is actually overheating or the temperature gauge or sender is faulty.
IMPORTANT: Best way to test gauge or sender is to replace them.

1. Loose or broken drive belt
2. Seawater shutoff valve partially or fully closed  (if equipped)
3. Clogged or improperly installed sea strainer
4. Loose hose connections between seawater pickup and seawater pump inlet (models with belt driven seawater pump only) Pump will suck air. Pump may fail to prime or will force air bubbles into cooling system.
5. Seawater inlet hose kinked or collapsed
6. Seawater pickup clogged
7. Obstruction on boat bottom causing water turbulence. Obstruction will be in front of seawater pickup, causing air bubbles to be forced into cooling system.
8. Defective thermostat
9. Exhaust elbow water outlet holes plugged, with rust from motor, salt, silt, sand, etc.
10. Insufficient seawater pump operation
11. Obstruction in cooling system such as casting flashes and rust, salt, etc.    Refer to water flow diagram for engine type being serviced.
12. Engine circulating pump defective
13. Also refer to "Engine Overheats (Mechanical)"
14. Wrong thermostat housing for model
15. Modified exhaust system, Incorrect hose routing for model (see 20.)

IMPORTANT: In addition to previous checks, make the following checks if engine is equipped with closed cooling.

16. Low coolant level
17. Clogged heat exchanger
18. Excessive air in system
19. Wrong antifreeze mix ratio
20. Check flow direction chart for correct hose routing

Sterndrive models
21. Water pump in sterndrive worn or burned
22. Water tube adapter warped or burned
23. Water hose between drive unit and transom cracked or pinched
24. Brass fittings for drive water tube, cracked or corroded.