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Go Back   ClassicBroncos.com Forums > 66-77 Ford Bronco > Bronco FAQ > Engine FAQ's

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Compression Test Compression Test
Questions about Engine Compression Testing
What is a compression Test?
A compression test is a test performed to determine the internal health of the engine. You use a special tool that you either press into the spark plug hole or you screw into the spark plug hole. This tool measures the pressure the piston creates on the compression stroke.

What can a Compression test find?
With a compression test you can find out if one or many of the cylinders has low compression. This could be caused by any of the following:
Burned Valve
Burned Valve Seat
Physical Engine Damage (Burnt valve, Hole in piston, etc)
Blown Head Gasket
Worn Rings or Cylinder wall
Valve Train Troubles (Incorrectly adjusted Valves, broken valve spring, etc)
Jumped timing chain

What tools do I need to perform a Compression test?
Compression tester Something like This is what I use.
Small wrench or socket to unhook the coil
Spark Plug wrench
Paper and pen or pencil

How do I perform a compression test?
1. Warm the engine to normal operating temperature.
2. Disconnect the Coil so the engine will not run.
3. Remove a spark plug.
4. Insert the Compression test tool
5. Turn the engine over several times until the tester is not gaining any more pressure. (usually 2-6 revolutions)
6. Note the cylinder and the pressure on the paper.
7. Repeat steps 3-6 on all other cylinders noting the cylinder and pressure on each.
8. The recorded pressures should have no more than a 10%-15% variance.
9. Check your compression against the compression readings of a new engine. If your readings are less than 70% of a new engine with similar components you are most likely about to need a rebuild.
10. If you have one cylinder that has a low reading try adding 1 teaspoon of 30wt oil and run the test again. If you compression is higher on this run this means that you cylinder rings are wore on that cylinder.

If you have anything to add to this FAQ please respond below.  
zeeman1 on 05/30/08, 05:18 PM
I've got a few notes to add:

When your doing the test your going to want to remove all the spark plugs before you get started.

Also - If one cylinder is lower than the rest, you can try plugging the spark plug holes next to it with your finger. If the compression changes when you do this you've got a blown head gasket thats leaking between those two cylinders.
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