Mazda Trucks 1987-1993 Repair Guide

Determining Engine Condition

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Anything that generates heat and/or friction will eventually burn or wear out (for example, a light bulb generates heat, therefore its life span is limited). With this in mind, a running engine generates tremendous amounts of both; friction is encountered by the moving and rotating parts inside the engine and heat is created by friction and combustion of the fuel. However, the engine has systems designed to help reduce the effects of heat and friction and provide added longevity. The oiling system reduces the amount of friction encountered by the moving parts inside the engine, while the cooling system reduces heat created by friction and combustion. If either system is not maintained, a break-down will be inevitable. Therefore, you can see how regular maintenance can affect the service life of your vehicle. If you do not drain, flush and refill your cooling system at the proper intervals, deposits will begin to accumulate in the radiator, thereby reducing the amount of heat it can extract from the coolant. The same applies to your oil and filter; if it is not changed often enough it becomes laden with contaminates and is unable to properly lubricate the engine. This increases friction and wear.

There are a number of methods for evaluating the condition of your engine. A compression test can reveal the condition of your pistons, piston rings, cylinder bores, head gasket(s), valves and valve seats. An oil pressure test can warn you of possible engine bearing, or oil pump failures. Excessive oil consumption, evidence of oil in the engine air intake area and/or bluish smoke from the tailpipe may indicate worn piston rings, worn valve guides and/or valve seals. As a general rule, an engine that uses no more than one quart of oil every 1000 miles is in good condition. Engines that use one quart of oil or more in less than 1000 miles should first be checked for oil leaks. If any oil leaks are present, have them fixed before determining how much oil is consumed by the engine, especially if blue smoke is not visible at the tailpipe.

COMPRESSION TEST



See Figure 1

A noticeable lack of engine power, excessive oil consumption and/or poor fuel mileage measured over an extended period are all indicators of internal engine wear. Worn piston rings, scored or worn cylinder bores, blown head gaskets, sticking or burnt valves, and worn valve seats are all possible culprits. A check of each cylinder's compression will help locate the problem.

A screw-in type compression gauge is more accurate than the type you simply hold against the spark plug hole.

Although it takes slightly longer to use, it's worth the effort to obtain a more accurate reading.

  1. Make sure that the proper amount and viscosity of engine oil is in the crankcase, then ensure the battery is fully charged.
  2.  
  3. Warm-up the engine to normal operating temperature, then shut the engine OFF .
  4.  
  5. Disable the ignition system.
  6.  
  7. Label and disconnect all of the spark plug wires from the plugs.
  8.  
  9. Thoroughly clean the cylinder head area around the spark plug ports, then remove the spark plugs.
  10.  
  11. Set the throttle plate to the fully open (wide-open throttle) position. You can block the accelerator linkage open for this, or you can have an assistant fully depress the accelerator pedal.
  12.  



Click image to see an enlarged view

Fig. Fig. 1: A screw-in type compression gauge is more accurate and easier to use without an assistant

  1. Install a screw-in type compression gauge into the No. 1 spark plug hole until the fitting is snug.
  2.  


WARNING
Be careful not to crossthread the spark plug hole.