Well, I know a Porsche is not quite as good a Nighthawk, but it's
close.............. thought we could suffer through.
----- Original Message -----
From: James O'Gorman
To: [email protected]
Sent: Tuesday, April 19, 2011 6:37 PM
Subject: Re: [Nighthawk Lovers] Compression test vs. Cylinder leak down (more
than you wanted to know)
That was a good read - you just had to pull one for a Porsche though, didn't
you?
On Apr 19, 2011, at 8:57 AM, Gene Henry wrote:
The real poop: Cylinder leak down testing: from
http://www.carcraft.com/techarticles/116_0406_cylinder_leakdown_tester/index1.html
More poop: from
http://www.dietersmotorsports.com/Compression-leakdowntest.htm
Compression test vs. Cylinder leak down
How, why and what do they mean?
When the time comes to purchase a used vehicle the first question that
comes to most prospective buyers minds is, “How’s the compression?” This has
been the standard engine condition test since the internal combustion engine
was invented.
But what exactly is a compression test; and what do the readings tell us?
A compression test measures how much pressure the piston creates in the
cylinder when traveling from bottom dead center (BDC) to top dead center (TDC)
with the valves closed. The reading is taken at the spark plug fitting in the
cylinder head.
Because we are trying to recreate normal operating conditions there are a
few parameters that need to be met before performing the test.
• The engine must be at or near operating temperature.
• All the spark plugs must be removed. (It is advisable to loosen all the
spark plugs ½ turn, and then start the engine for 15 seconds. Then completely
remove the spark plugs from the cylinder head. This blows out any carbon that
might get broken loose and caught between the valve and valve seat. If this
were to happen you could get a false, low compression reading.)
• The throttle must be all the way open (WOT)
• The ignition and/or fuel system should be disabled
It is important to disable the ignition/fuel system for safety reasons and
engine health, as well as operator health.
Most Porsches produced after 1983 have a single DME relay, which can be
removed to disable fuel and spark. On earlier 911s the 14-pin engine plug in
the left rear corner can be removed and then bridge connector 1 & 14 to crank
the engine.
Insert the compression gauge into the proper spark plug hole and crank the
engine 5-6 times noting the first and final readings. Repeat on each cylinder.
More important than the exact number of times the engine is cranked, is the
consistency between cylinders. If you crank #1 cylinder 6 times, all cylinders
must be cranked 6 times. Another concern is the battery condition. The engine
must crank at the same speed for all cylinders.
The most important factor in a compression test is consistency between
cylinders. A good rule of thumb is to have less than 10% variance between
cylinders. So if an engine has an average compression reading of 150-psi there
should be less than 15psi between the highest and lowest cylinders.
Lets say for instance that we have the following compression readings:
1-150, 2-180, 3-140, 4-145
1,3 & 4 are all within the 10% specification but 2 is 20% higher than the
average of the other cylinders. Does this mean number three is high or the
others are low?
To get to the bottom of this question we need to start by examining the
spark plugs. Do they all look the same? Is #2 badly carbon fouled? If you have
one or more cylinders with high compression and oil burning, the high
compression can be caused by the build-up of carbon on top of the piston.
In our test case, the high compression of #2 cylinder can be caused by
excessive carbon build-up on the piston. Now to prove it!
Cylinder leakdown test-
While a compression test is a dynamic test (engine moving), a cylinder
leakdown test (C.L.T.) is a static test (engine at rest). The compression test
measures how much pressure the engine can produce while cranking; in contrast
to the C.L.T., which measures how much pressure is lost in the engine. In a
C.L.T. the engine is placed on TDC of the cylinder in question and using a
similar type of connector as the compression test, we fill the cylinder with
pressure. The tester then measures the volume of air needed to maintain a
predetermined pressure in the cylinder. This reading is expressed in a
percentage. Good cylinder leakdown readings should be below 5-8%.
The great thing about C.L.T. is that it deals with how well the cylinder is
sealing and nothing else. The readings are not affected by carbon deposits, cam
timing, or even engine cranking speed.
Another great feature of the C.L.T. is the fact that you can hear where the
air is leaking out of the cylinder. When a cylinder has high percentage of
leakage, first check the oil filler cap. Do you hear a hissing sound? If so,
you may have pressure leaking by the rings. Is there air escaping out the
exhaust? Is it escaping out the intake system? Then a burned valve may be the
problem. If two adjoining cylinders have similar low readings and you hear
leakage out the other cylinder, then a failed head gasket may be the problem.
Being able to pinpoint the exact source of the compression loss will tell
you where the problem is; and not just that you have one. This knowledge will
greatly assist you in the next step… the repair.
----- Original Message -----
From: Kurt Nolte
To: [email protected]
Sent: Monday, April 18, 2011 11:47 PM
Subject: Re: [Nighthawk Lovers] Need some help - I think the GS-1000 has
a major issue
How much pressure do you use for your leakdown tests? Just curious.
Kurt
On Apr 19, 2011 12:23 AM, "[email protected]"
<[email protected]> wrote:
and my buddy was just talking about his leak down testers today.... As
a matter of fact there sitting on my nav station right now.....
sorry, not constructive, just an observation
paul
-----Original Message-----
Date: Monday, April 18, 2011 10:06:47 pm
To: <nighthawk_lovers@googlegrou...
From: "Gene Henry" <[email protected]>
Subject: Re: [Nighthawk Lovers] Need some help - I think th...
Cylinder Leak-Down Tester
Item #94190 Only:$39.99 (less 25% Easter Sunday!)
Hope this will help....
Hello All,
Discovered today in the 140 odd miles since I did an oil
change the GS-1000...
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