On Mon, 17 Dec 2001, Peter Macholdt wrote:
> Bob,
Hello.
> I understand the principle of the leak down test. Is there a good how to
> source for the backyard mechanic?
The process itself is fairly straight forward... read on.
> How about any special equipment?
You need a "leakdown tester". These are available from various sources,
figure the cost to be between $60 and up. You get what you pay for.
Additionally, you need a compressor to provide air pressure to pressurize
the system.
In essence, the leakdown test looks like a gas pressure regulator valve
with two guages, line pressure and cylinder pressure.
The process of testing involved rotating the motor to TDC for the cylinder
that you are testing, removing the sparking plug and inserting the
leak-down tester hose in place of the spark plug. You then open the valve
and note the line pressure and the cylinder pressure. If there's a large
difference (typical street motors will get more than 5% leakdown), you
divide the two observed values and come up with a fraction. That's the
amount of leakdown. e.g. line pressure 100 PSI, cylinder pressure 98 PSI.
98/100=.98 ergo - 2 percent leakdown.
Now here's the nifty part - you listen to the tailpipe, carb throats and
crankcase and observe the coolant in the radiator and you can tell where
the presure is leaking to. If you hear it coming from the oil filler cap,
for example, then the leak is past the rings - similarly, noise from the
exhaust pipe would indicate a leaky exhaust valve seat or "hung valve".
Bubbles in the coolant = bad head gasket.
You then follow the leads of your discovery (or more likely discoveries!)
The leakdown test is very valuable for evaluating the condition of an
engine - I would never go by just the compression. A compression test is
quick and dirty for evaluating the motor. A leakdown test will likely be
more revealing.
If you don't want to part with $70 and you don't have a compressor, a
competent repair shoppe will have the necessary tools to do the test.
Figure an hour of billable time, maybe a bit more.
Note: when you pressurize the cylinder, you may need an assistant to hold
the brake pedal down FIRMLY (with the car in gear) or the motor will
rotate to the bottom of the stroke on the cylinder being tested. READ -
THE CAR CAN ROLL WITH THIS FORCE. Please exercise caution when performing
a leakdown test.
> Thanks,
> Peter
> '68 TR250
regards,
rml
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