MerCruiser Overheating 350 (5.7L) 305 (5.0L) 1993-1997

MerCruiser Overheating 350 (5.7L) 305 (5.0L) 1993-1997

We often get asked the question ‘what is causing my MerCruiser 5.7L or 5.0L engine to become too hot?’. This article’s focus is to explain the many reasons why a MerCruiser overheats. Keep in mind that there may be more than one cause to an overheating issue.

When attempting to diagnose an overheating MerCruiser 5.7 or 5.0 liter V8 engine, it’s important to inspect both mechanical and cooling system components. Also, make sure the engine is actually overheating by ensuring the temperature sender and gauge are working properly.

Download Online Factory Service Repair manual PDF

Never perform repair work or maintenance to a MerCruiser or sterndrive without a manual present. A repair manual contains critical instructions that, if not followed correctly, can render the boat hazardous to its occupants, or result in costly repairs.

Engine Overheating Mechanical

  1. MerCruiser engine RPM below factory specifications at wide-open-throttle WOT (engine laboring). A few possible causes could be broken or incorrect size propeller. Ensure the false bottom is not full of sea water. Excessive growth on boat bottom can also cause overheating problems.
  2. Wrong ignition timing can cause a MerCruiser to overheat. Timing is either retarded or too far advanced.
  3. Check exhaust system for damage or restrictions.
  4. Inspect spark plugs. Ensure the correct heat range spark plugs were installed.
  5. Lean fuel mixture. Download a MerCruiser repair manual and refer to the “carburetor malfunctions” section of the book.
  6. Inspect spark plug wires and ensure they haven’t been crossed (wrong firing order).
  7. Sticking distributor advance weights.
  8. Not enough lubrication to moving mechanical parts. Test for defective oil pump, check for plugged oil passages, inspect for low oil level.
  9. A blown head gasket(s) can cause a MerCruiser to run hot. For reference, a compression test will usually not detect a blown head gasket, as most engines will operate at normal temperature below 3000 RPM, but will overheat when the engine speed is above 3000 RMP.
  10. Valve timing off. Make sure the timing chain hasn’t jumped and that it’s installed properly.

Engine Overheats Cooling System

  1. Seawater shutoff valve fully or partially closed (if equipped).
  2. Faulty temperature thermostat.
  3. Check exhaust elbow water outlet holes for blockage or plugging.
  4. Check for low coolant level.
  5. Seawater pump fails to force a sufficient amount of water due to worn pump impeller.
  6. Incorrect mixture of antifreeze. Correct mixture should be 50/50, maximum 60/40 (40 percent water, sixty percent antifreeze).
  7. Inspect heat exchanger cores for blockage.
  8. Be sure the water hose is not reversed at the water distribution block. Download a MerCruiser repair manual and refer to water flow diagram.
  9. Check for faulty seawater pump.
  10. Check for an obstruction in front of the seawater pickup on boat bottom. Obstructions can cause turbulence, thus causing air bubbles to be forced into cooling system.
  11. Check for a clogged seawater pickup.
  12. Engine circulating pump malfunction.
  13. Check for kinked, collapsed or plugged seawater inlet hose.
  14. Inspect the hose between seawater pump inlet and seawater pickup. If the hose is loose it will suck air into the cooling system, or cause the pump not to prime.
  15. Blockage to sea strainer or it’s installed incorrectly.
  16. Make sure the drive belt is not worn, loose or broken.

This MerCruiser overheating troubleshooting guide pertains to the 5.0L and 5.7L engines manufactured between 1993 thru 1997. It includes the models listed below, in the comment section:


    • The information listed applies to the following models:

      MCM Alpha

      1993-1995 5.0L 305
      1993-1995 5.0LX 305
      1993-1995 5.7L (4 Barrel)
      1996-1997 5.7L (2 Barrel)
      1996-1997 5.7LX
      1996-1997 5.7L EFI
      1996-1997 5.7LX EFI
      1996-1997 350 Magnum
      1996-1997 350 Magnum EFI Gen+

      MCM Bravo

      1993-1995 5.0 305
      1993-1995 5.7 (4 Barrel)
      1996-1997 5.7L (2 Barrel)
      1996-1997 5.7LX
      1996-1997 5.7L EFI
      1996-1997 5.7LX EFI
      1996-1997 350 Magnum
      1996-1997 350 Magnum EFI Gen+
      1994-1995 350 Magnum EFI/MP
      1996-1997 350 Magnum MPI Gen+

      MIE Ski

      1993-1995 5.7L Competition (4 Barrel)
      1996-1997 5.7L Competition (2 Barrel)
      1993-1997 350 Magnum Tournanment Ski
      1994-1996 350 Magnum EFI Tournament Ski
      1997 350 Magnum EFI Gen+ Tournament Ski
      1995 350 Magnum EFI/MP Tournament Ski
      1996 350 Magnum MPI Tournament Ski
      1996 350 Magnum MPI Gen+ Tournament Ski (Black Scorpion)

      MIE Inboard

      1993-1997 5.7L
      1997 5.7L EFI Gen+

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