Engine Overheats (Mechanical) - MerCruiser GM4 Service Manual

Engine Overheats (Mechanical)

1. Engine RPM below specifications at wide-
open-throttle (engine laboring)
2. Wrong ignition timing
3. Sticking distributor advance weights
4. Spark plug wires crossed (wrong firing order)
5. Lean fuel mixture
6. Wrong heat range spark plugs
7. Exhaust restriction
8. Valve timing off
9. Blown head gasket(s)
10. Insufficient lubrication to moving parts of engine
Index
1C- 32 - TROUBLESHOOTING
Cause Special Information
1. Damaged or wrong propeller; growth on boat bot-
tom;false bottom full of water
2. Timing too far advanced or retarded
5. Refer to "Carburetor Malfunctions" in this section
8. Jumped timing chain, or improperly installed
9. A blown head gasket(s) normally cannot be de-
tected by a compression check. Normally the en-
gine will run at normal temperature at low RPM,
but will overheat at speeds above 3000 RPM.
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 be
tween 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.
10. Defective oil pump, plugged oil passage, low oil
level
90-816462 2-695