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[Datsun 1200 encyclopedia]

Engine compression test

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Category: Maintenance And Daily Care

A compression test will check the condition of the engine cylinder sealing -- how well the compression rings are working. The exact compression reading you get will vary depending. Among many variables, the reading depends upon your camshaft, static compression ration, and (most importantly) the accuracy of the gauge. Some guages will read higher or lower than others. What is important is that the reading of each cylinder is close to that of the others.

Contents

Wet Compression Test

See step 2 in the instructions below.

Overview

New Compression Tester is only $20
th_sum-900009_compression_tester.jpg

To perform an Engine compression test:

  1. Perform Dry Test, following the details below. Ensure all four cylinders are within 15% of each other
  2. Perform Wet Test. Squirt or drip a bit motor oil into each spark plug hole. Spin engine a couple of revolutions to clear out any excess oil. Re-check compression again.
    • If the compression reading doesn't rise with the wet test, it's all good.
    • If it does rise, it indicates the ring sealing is not good.

Details:

  1. Put transmission in neutral and block the wheels
  2. Remove the spark plugs
  3. Disconnect distributor wire
  4. Block throtle open
  5. Connect compression tester (according to its directions) to spark plug hole for cylinder #1
  6. Crank engine 4-5 revolutions (or until the gauge reading doesn't rise any higher)

Some variation is OK, but all cylinders should be within 20% of each other. For example, if the peak cylinder reads 150 psi, all the other should read at least 135. OR if one reads 200 psi, all the others should be at least 170 psi. Two gauges will most likely give two readings, so don't worry about the exact numbers.

  • If one cylinder is 90 psi or below suspect bad rings or a burnt valve
  • If one cylinder is below 30 psi, a burnt/leakig valve is the most likely cause
  • If two adjacent cylinders are low, suspect a blown head gasket

Factory Service Manual

The 1973 B110 Factory Service Manual says:


Checking Engine Compression

Compression pressure test
NOTE: To test cylinder pressure, remove all spark plugs and hold tester fitting tightly in spark plug hole of a cylinder. The tester is used to determine whether cylinder can hold compression, or whether there is excessive leakage past rings, etc.

Test compression with engine warm, all spark plugs removed and throttle and choke valve opened. No cylinder compression should be less than 80% of highest cylinder's. Excessive variation between cylinders, accompanied by low speed missing of the cylinder, usually indicates a valve not properly seating or a broken piston ring. Low pressures, even though uniform, may indicate worn rings. This may be accompanied by excessive oil consumption.

Test conclusion
If one or more cylinders read low, inject about one tablespoon of engine oil on top of the piston in low readingg cylinders through spark plug hole. Repeat compression check on these cylinders.

  1. If compression improves considerably, piston rings are defective.
  2. If compression does not improve, valves are sticking or seating improperly
  3. If two adjacent cylinders indicate low compression and injecting oil on pistons does not increase compression, the cause may be a cylinder head gasket leak between the cylinders. Engine oil and coolant in cylinder could result from this problem.
Combustion pressure
12.5 to 14.5 kg/cm²
(178 to 206 psi) at 350 rpm

Reference: Section ET (Emission Control and Tune-up), 1973

Testing Out of Car

If the engine is separated from the transmission, bring a spare transmission, starting motor and battery jumper cables. Bolt it up and use you car's battery to turn the motor over.

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