Chapter 3 part 3 Compression testing Smoke testing Engine noises Oil Pressure problems Cooling system problems
Compression testing • Can be used when there is • easy access to the spark plug • holes • But may require the help of • a second person • A screw in tester can be more • expensive but does not need a • second person. And is good • when access is difficult.
Test procedure • Preferably the engine should be warmed up • The battery must be fully charge • Remove spark plugs ( carefully) • Disable the ignition system and if possible fuel system • Install the compression gauge into the spark plug hole • Hold throttle wide open on carbureted engines • Crank the engine through at least 4 compression strokes • Record each cyl.’s reading
Understanding compressionreadings If all cyl.’s have good compression and there are no symptoms Then your done. Variations of no more than 20% between cyl.’s is considered Ok in most situations. If 2 cyl.’s next to each other have low compression, this is Usually an indication of a blown head gasket. 1 or more low cyl.’s with no pattern is usually burned exhaust Valves. And will have a rough idle .
Wet compression test If a cyl. shows low results your will need to Perform a wet compression test. Squirt about 1 tablespoon of oil into the cyl. That is low the do another compression test On it. If the compression now shows normal then Piston ring problem is indicated.
Running compression test Test the volumetric efficiency This test is used when a misfire or loss of power Can’t be identified with a cranking compression test A typical running test result will be about half the result of a Cranking compression reading. Can you tell me why this is?
How to perform a running test • Perform a cranking test and record results • Install all spark plug except the one you put the compression • tester in • 3) Start the engine and depress and hold the release on the gauge • 4) Allow the gauge to stabilize and note the reading • (should be about half of cranking results)
Snap compression test After the running compression test, perform the snap test Momentarily snap the throttle to w.o.t. Then allow to return to idle With the throttle wide open and The piston still at a relatively low speed This result should be higher Snap compression readings should Be about 80% of cranking results
Understanding snap compression test results Lower snap readings mean the intake system is restricted Possible causes if on 1 cyl. Worn cam lobes Broken valve springs Problems in all cyl.’s Can be traced to a restriction in the air intake System, or a plugged exhaust system
Smoke testing Can be used to find Vacuum leaks Oil leaks Cooling system leaks Exhaust leaks
Vacuum leaks • Engine off • Throttle closed • Disconnect pcv or brake booster hose • Connect smoke machine to this hose • Fill intake with smoke • Look for smoke escaping to locate the leak • Be sure to look under vehicle and inside vehicle
Finding exhaust leaks Use a cone adapter and Plug any other outlets It takes about 2 minutes To fill the exhaust system Smoke allows you to find Leaks before they are heard Manifold leaks can be found By using a mirror Cracks are easier found with the engine cold
Coolant and oil leaks Cooling system must be completely drained first Oil leaks Plug off pcv and breather hoses first Or smoke will come out intake through them Connect smoke machine to oil dip stick tube watch for smoke leakage
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