Engine Noise


   No definite rule or test will positively determine source of engine noise; therefore, use the following information only as a general guide to engine noise diagnosis.

1. Use a timing light to determine if noise is timed with engine speed or one-half engine speed. Noises timed with engine speed are related to crankshaft, rods, pistons, piston pins, and flywheel. Noises timed to one-half engine speed are valve train related.

2. The use of a stethoscope can aid in locating a noise source; however, because noise will travel to other metal parts not involved in the problem,  caution must be exercised.

3. If you believe noise is confined to one particular cylinder, ground spark plug leads, one at a time. If noise lessens noticeably or disappears, it is isolated to that particular cylinder.

4. Try to isolate the noise to location in engine: front to back, top to bottom. This can help determine which components are at fault.

5. Sometimes noises can be caused by moving parts coming in contact with other components. Examples are: flywheel or coupler; exhaust flappers rattling against exhaust pipe; crankshaft striking (pan, pan baffle, or dipstick tube); rocker arm striking valve cover; and loose flywheel cover. In many cases if this is found to be the problem, a complete engine tear down is not necessary.

6. When noise is isolated to a certain area and component, removal and inspection will be required. Refer to proper sections of service manual for information required for service.

7. If noise cannot be distinguished between engine and drive unit, remove drive from boat. Run a water supply directly to engine. Run engine without  the sterndrive to determine if noise is still there.

Valve Cover Area
1. Valve cover area, timed to one-half engine speed, noise could be confined to one cylinder or may be found in any multitude of cylinders
        A. Rocker arm striking valve cover
        B. Rocker arm out of adjustment
        C. Worn rocker arm   
        D. Bent pushrod
        E. Collapsed lifter

Cylinder Area
   Cylinder area, may be confined to one cylinder or found in more than one cylinder, timed to engine speed
       A. Sticking valve
       B. Carbon build-up
       C. Connecting rod installed wrong
       D. Bent connecting rod
       E. Piston
       F. Piston rings
       G. Piston pin
       H. Cylinder worn

Camshaft Area
Camshaft area, front of engine, timed to one half engine speed
       A. Crankshaft timing sprocket
       B. Timing chain
       C. Fuel Pump
       D. Valve Lifter
       E. Cam Bearings
2. Camshaft area, center of engine, timed to one half engine speed
       A. Fuel Pump
       B. Valve Lifter
       C. Cam bearing
3. Camshaft area, rear of engine, timed to one half engine speed
       A. Distributor gear
       B. Valve lifter
       C. Cam bearings
4. Camshaft area, throughout engine, timed to one half engine speed
       A. Loss of oil pressure
       B. Valve lifters
       C. Cam bearings

Crankshaft Area
1. Crankshaft area, front of engine, timed to engine speed
       A. Crankshaft timing sprocket
       B. Timing chain
       C. Main bearing
       D. Rod bearing
2. Crankshaft area, center of engine, timed to  engine speed
       A. Crankshaft striking pan or pan baffle
       B. Main bearing
       C. Rod bearing
3. Crankshaft area, rear of engine, timed to engine speed
      A. Loose flywheel cover
      B. Loose coupler
      C. Loose flywheel
      D. Main bearing
      E. Rod bearing
4. Crankshaft area, throughout engine, timed to engine speed
      A. Loss of oil pressure
      B. Main bearings
      C. Rod bearings

Miscellaneous
1. Engine spark knock
      A. Advanced timing
      B. Low octane fuel
      C. Engine running hot
      D. Carbon deposits in engine
2. Popping through carburetor
      A. Wrong ignition timing 
      B. Carburetor set too lean
      C. Faulty accelerator pump
      D. Vacuum leak
      E. Valve adjustment
      F. Valve timing
      G. Burned or stuck valve
3. Hissing
      A. Vacuum leak
      B. Leaking exhaust (manifolds or pipes)
      C. Loose cylinder heads
      D. Blown head gasket
4. Whistle
      A. Vacuum leak
      B. Dry or tight bearing in an accessory
5. Sparks jumping
      A. Leaking high tension lead
      B. Cracked coil tower
      C. Cracked distributor cap
6. Squeaks or squeals
      A. Drive belt slipping
      B. Dry or tight bearing in an accessory
      C. Parts rubbing together
7. Rattling in exhaust pipe area
      A. Exhaust shutters

Radio Noise
1. A "popping" noise that will increase with engine RPM.
    Noise will stop as soon as engine is turned off.
    A.  Ignition System - wrong spark plugs; cracked distributor cap;
           cracked coil tower; leaking spark  plug wires;
    B.  Moisture on ignition components
2. "High pitched whine" in the radio
    A.  Alternator - poor brush contact on the slip rings
3.  "Hissing or crackling" noise when instruments
      are jarred with ignition on
    A.  Instrumentation - loose connections, or antennae wire
          are routed too close to instruments
4. Varying unexplained noises Accessories
        Bilge pump, bilge blower; fish finder, depth locator;
         cabin heater motor, etc.
  *  Disconnect one at a time until noise disappears.

We have a section for Radio Noise solutions.

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