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How to perform a
CYLINDER LEAKAGE TEST (CLT)
Most
of us are familiar with compression testing which helps, but does not
give you all of the information you need. In this process, the test gauge
is connected to the spark plug hole and the engine is turned over with
the starter. The gauge reads in pounds per square inch (PSI) and the reading
is compared with the specifications for that engine, previous test results,
and the readings from the other cylinders in order to determine the health
of the particular cylinder or engine. The gauge measures the engine's
ability to pump air at the starter RPM and changes in RPM can mean changes
in recorded pressure. It is not uncommon to have lower pressures for each
successive cylinder just because the battery is running down. If there
is a low reading, it is difficult to tell whether the problem is in the
rings, valves, head gasket, etc. (although the trick of squirting oil
into the cylinder to see if the problem is in the rings sometimes works).
Some performance modifications result in lower compression readings, at
least partly because the modifications are designed to improve the engine's
performance at high RPM, not at cranking speed. A worn cam may give a
lower reading, but so will changing to a high performance camshaft with
longer duration and significant overlap. Thus a racing engine in good
condition may give a lower compression test reading than a stock engine
in average condition.
Doing a leak
down test, particularly in conjunction with a compression test, will give
you a great deal of additional information and allows problems to be pinpointed.
The condition of the engine is determined by measuring the degree to which
a cylinder with valves closed leaks air. In simplified terms, if air is
pumped into the cylinder at 100 PSI and the gauge reads 97 PSI, then the
leak down percentage is 3%. Doing a leak down test is a fairly simple
matter. The tester is connected to an air source and is attached to the
engine through the spark plug hole. There are two types of instruments,
those with single and double gauges. The cost of the tester is usually
between $50 and $250 depending on whether it is the single or double gauge
and where it is purchased. The double gauge constantly measures the input
pressure and the cylinder pressure. The single gauge instrument relies
on checking the input pressure and then switching to measure the cylinder
pressure. Each requires some adjustment to be accurate. The engine should
be rotated so that the valves in the cylinder to be tested are closed
and the piston is at top dead center. Tests should be conducted when the
engine is warm.
Problems
are pinpointed simply by determining where the air is leaking out of the
cylinder. Air leaking out of the exhaust system (you can hear it in the
exhaust pipe) indicates a problem with the exhaust valve. Air coming out
of the carb or throttle body indicates a bad intake valve or seat. Air
going into the crankcase is leaking past the rings and does not indicate
a problem if the percentage is low enough. A leak where the air is going
into an adjacent cylinder or into the coolant indicates a blown head gasket
or cracked head.
A brand new
street engine might measure from 5% to 8% depending on the engine, manufacturer,
and degree of break in. A street engine that measures 10% to 20% per cylinder,
although indicating some wear, if there is consistency between cylinders
and if all of the air is leaking past the rings into the crankcase indicates
a reasonable street engine for daily driving that does not need any immediate
work. Any readings 30% or higher indicates a severe engine problem.
Using the
leak down test, especially in conjunction with a compression test, should
allow you to quickly determine the basic condition of any engine. If you
are not sure what the levels should be, differences in readings between
cylinders is a key indication of a problem. These are tests that can and
should be done by any competent garage. After testing, you will know whether
the top or bottom end really needs a rebuild.
A cylinder
that has poor compression, but minimal leakage, usually has a valvetrain
problem such as a worn cam lobe, broken valve spring, collapsed lifter,
bent push rod, etc.
If all the
cylinders have low compression, but show minimal leakage, the most likely
cause is incorrect valve timing. The timing belt or chain may be off a
notch or two. If the vehicle runs, do a vacuum test, very low steady vacuum
would verify the incorrect valve timing.
If compression
is good and leakage is minimal, but a cylinder is misfiring or shows up
weak in a power balance test, it indicates a fuel delivery (bad injector)
or ignition problem (fouled spark plug or bad plug wire).
USING THE TESTER
REGULATOR ADJUSTMENT
The tester requires an outside source of air pressure (70 to 200 psi) to be connected to the
"air inlet" connection. Before connecting this line to the air inlet connection, turn the CLT
REGULATOR KNOB to the left a couple of turns. Then connect the airline and turn the CLT REGULATOR
KNOB to the right in small steps until the gauge reads "0". Momentarily depress the valve in the
connection end of the "outlet to engine" hose. The gauge should rise, then return to "0". If it
does not return to zero, turn the CLT REGULATOR KNOB to the left again and repeat. When the Cylinder
Leakage Tester reads on "0" it is ready to use.
With the engine at normal operating temperature, this air pressure will be applied to a cylinder
when the piston is at TDC on the compression stroke. At this time both intake and exhaust valves
are closed and except for a small amount of air leakage past the piston ring gaps no air should
escape from a good cylinder. The amount that does leak out is measured on the % CYLINDER LEAKAGE gauge.
The Cylinder Leakage Tester uses the % Cylinder Leakage gauge, the CLT Regulator, a tester hose
and one of several adapter hoses for various types of engines.
PERFORMING THE CYLINDER LEAKAGE TEST
Make this test to determine the location of compression losses.
- Connect the remote starter switch to the starter solenoid. Turn ignition key ON.
- Ground the coil’s high-tension lead.
- Remove radiator and crankcase filler caps.
- Screw the proper adapter hose, finger tight, into the cylinder to be tested and put the whistle
on the end of the hose.
- Crank engine over slowly until whistle just stops sounding. Remove whistle.
- Connect the tester hose to the adapter hose.
Note: Watch the engine fan; if it turns, stop. Remove the tester hose, reposition whistle, and
repeat step 5. Do not attempt to crank engine with the tester hose connected.
- Listen for air leakage at exhaust pipe, radiator and carburetor and from the adjacent
cylinders. If the engine has a PCV valve, be sure to pinch off PCV hose when listening at carburetor.
- Look at gauge, record reading.
GOOD No "Air Leaking" heard in exhaust pipe, radiator, carburetor, adjacent cylinders and leakage
less than 20%.
BAD "Air Leaking" heard at exhaust pipe, radiator, carburetor, adjacent cylinders or more
than 20% leakage on a properly broken in engine, indicates a need for service.
- Air escaping at carburetor indicates a leak at the intake valve.
- Air escaping at exhaust pipe indicates a leak at the exhaust valve.
- Air escaping at radiator filler cap indicates a blown head gasket or cracked block.
- Air escaping from adjacent cylinders indicates a blown head gasket or cracked block.
- No "air leaking" heard in exhaust pipe, radiator, or carburetor, but gauge reads over 20%
indicates worn or broken rings or scored cylinder walls.
If additional cylinders are to be tested:
- Repeat steps 4 to 6 to bring each cylinder in firing order to TDC test position.
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