THE LAST RESORT FOR CBR600RR NAS21
LEAK DOWN TEST
An internal combustion engine makes power by first drawing air and fuel into the
combustion chamber. Next is the compression of the mixture and the addition of a
spark. The harnessing of the resulting contained explosion is ultimately what powers the
car. As an engine gets on in miles, the containment of this power can be lost due to
piston ring, valve or cylinder wall wear. Engine performance will suffer as a result.
Pressure reading
A common test of an engine’s ability to compress the air-fuel mixture is an engine
compression test. A pressure gauge is connected in place of the spark plug. The engine
is then cranked to create a pressure reading. A compression test is a good way to
check the engine’s ability to create pressure. The gauge reads the positive pressure
created by the cylinder.
Loss reading
An engine leakdown test is a compression test in reverse. Instead of measuring the
engine’s ability to create pressure, compressed air is introduced into the cylinder
through the spark plug hole. One gauge on the tester measures the pressure of the air
entering the cylinder and the other measures the percentage of the air escaping (or
leaking) from the cylinder. The loss percentage will indicate the condition of the cylinder
and overall condition of the engine.
Top dead center
Before sending air into the engine, the cylinder being tested must be placed at Top
Dead Center (TDC). The piston must be at the top of its travel. The intake and exhaust
valves must be closed. When the air is compressed into the cylinder, the leakdown
tester will measure any loss of air escaping past valves or piston rings. If the cylinder is
not at TDC, air escaping past an open valve will give a false reading.
Reading results
No engine will have perfect sealing with 0 percent loss. Five to 10 percent loss indicates
an engine in great to good running order. An engine between 10 and 20 percent can still
run OK, but it’ll be time to keep an eye (or ear) on things. Above 20 percent loss and it
may be time for a teardown and rebuild. Thirty percent? Major problems. The percent of
leakage should also be consistent across the cylinders. Any great differences indicate a
problem in that cylinder.
Hearing problems
Beyond getting an overall picture of engine condition, the engine leakdown test is an
excellent way to pinpoint where problems are before tearing down the engine. Listening
for where the air is escaping by ear can isolate the problem.
Intake valve : Air whistling out of the intake, carburetor or throttle body indicates
a leak at the intake valve.
Exhaust valve : Air heard hissing out of the tailpipe, turbocharger or exhaust
manifold means an exhaust valve leak.
Piston rings : Whistling or hissing out of the PCV valve, oil filler cap hole or
dipstick tube means the air is pushing past the rings. Suspect ring or cylinder wall
wear.
Head gasket : Air bubbles in engine coolant seen at the radiator filler cap could
mean air escaping into the coolant past the head gasket.
Cracked cylinder head : Bubbles in coolant or coolant being pushed up out of
the radiator neck can also indicate cracks in the cylinder head or cylinder walls.
Tools, supplies and instructions
You will need a compressed air source, a leakdown gauge kit, spark plug socket, basic
hand tools, vehicle service manual and a notepad to record results.
Step 1 : Remove the spark plugs and rotate the engine to put the cylinder to be tested
at TDC. Tip: Insert a long screwdriver or extension into the spark plug hole and turn the
engine by hand with a socket on the crankshaft. When the screwdriver stops rising or
falling, you’re at TDC. Don’t turn the engine backward if TDC is missed. Go around
again. In a four-cylinder engine, cylinders one and four and two and three are at TDC at
the same time.
Step 2 : Figure out which spark plug adapter works best and connect the gauge into the
hole. Put the vehicle in gear and set the parking brake to prevent the engine from
turning when air is compressed into the cylinder. Start with the regulator turned
counterclockwise to zero the incoming pressure. Connect compressed air. Turn the
regulator clockwise to pressurize air into the cylinder.
Step 3 : Record leakage percentage for the cylinder. Remove the oil dipstick, radiator
cap and oil filler cap. Open the throttle body or remove the air cleaner. Listen and
watch. Wherever air is escaping will indicate where the problem is. Disconnect the
gauge and move to the next cylinder.
Possible repair costs:
1. cracked cyllinder
