How to do a compression test

[oledb4]

Does anyone know how to do a compression test? I called the Nautique of Orlando shop and they estimated it to be $100-$150 to have it done...It doesn't seem too hard, and I feel like I can save a little money here. Does anyone have a step by step guide or any advice??? Thx

[bhectus]

go to Autozone or Sears or where ever and get yourself a compression test kit. Should have a gauge with a hose attachment. You will do 1 cylinder at a time. Disconnect all the plug boots at the spark plugs. (may want to mark them so you don't have to figure out where to connect them again later.) Pull one spark plug at a time and hook up your gauge. Turn over motor with the key for a couple seconds for each one. Check reading. Repeat twice or 3 times to get a good reading and avg the 3 for each cylinder. That's how I do it.

[oledb4]

go to Autozone or Sears or where ever and get yourself a compression test kit. Should have a gauge with a hose attachment. You will do 1 cylinder at a time. Disconnect all the plug boots at the spark plugs. (may want to mark them so you don't have to figure out where to connect them again later.) Pull one spark plug at a time and hook up your gauge. Turn over motor with the key for a couple seconds for each one. Check reading. Repeat twice or 3 times to get a good reading and avg the 3 for each cylinder. That's how I do it.

It's that easy?? You don't have to unplug anything else?? So to make sure I understand, you unplug all the boots, but leave the actual spark plugs in. Then remove Spark plug from cylinder #1, hook up gauge, tune it over and get your reading. Do I then screw spark plug #1 back in when I check cylinder #2???

[oledb4]

go to Autozone or Sears or where ever and get yourself a compression test kit. Should have a gauge with a hose attachment. You will do 1 cylinder at a time. Disconnect all the plug boots at the spark plugs. (may want to mark them so you don't have to figure out where to connect them again later.) Pull one spark plug at a time and hook up your gauge. Turn over motor with the key for a couple seconds for each one. Check reading. Repeat twice or 3 times to get a good reading and avg the 3 for each cylinder. That's how I do it.

http://www.youtube.com/watch?v=2VyoyB4NgnM

This guy removes all spark plugs from the beginning...does it matter???

[bhectus]

ok, i gave you a little bit of bad info, for which i apologize. Let me correct my initial response with a more clear and concise and most importantly....correct answer.


I found this for you, which should help:

• 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.

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.

[oledb4]

ok, i gave you a little bit of bad info, for which i apologize. Let me correct my initial response with a more clear and concise and most importantly....correct answer.


I found this for you, which should help:

• 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.

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.

I appreciate you finding that for me. So my next question then is how do you disable the fuel/ignition system?? and then I must run the engine for a little bit before I run the compression test??

[Sell]

I would pull the fuel pump relay to disable the fuel system.

[Nautiquehunter]

I see you have a 86 Ski it should have a 351 ford engine? You want to disable the ignition to prevent random sparks around the engine.You can do this by removing the coil wire from the distributor and grounding it to the intake manifold. Also when you start cranking you will be able to hear each time the cylinder being tested is on compression stroke. You will notice the starter laboring to compress that cylinder you will also see the needle on your gauge move. Count each time the cylinder compresses, test each cylinder 4 compression strokes. Its important to test each cylinder the same number of strokes.
You are looking for the readings to be within 10% of each other . Why are you doing a compression teat? What is the problem with your engine?