So after you've replaced the points, adjusted the valves and checked the timing on your high mileage engine, you've found that it still isn't putting the performance you'd like. It may be time to have the cylinder compression checked. If the rings are worn or a cylinder is low due to valve problems, this must be determined, otherwise all the tuning and adjusting in the world isn't going to make a difference.
It's time to get your compression gauge in hand. The following are the steps to take to get correct readings.
1. Make sure the battery is fully charged. A battery with a low charge will give low compression readings because of reduced cranking speeds.
2. Warm the engine to operating temperature to produce metal expansion and to reduce oil drag. A cold engine will give low readings.
3. After the engine is warm shut it off and loosen the spark plugs one turn. Restart the engine at a fast idle for about 5 seconds. This removes carbon debris from around the spark plug. This will keep small particles of carbon from being blown into the compression gauge.
4. Shut the engine off and remove the spark plugs noting which came from which cylinder. A Z-car engine will run with a plug or two firing at the wrong time. It'll even accelerate and go around the block a time or two. Don't ask how I know.
5. Block the throttle open.
6. In turn, carefully insert the compression gauge in each cylinder and with the starter, turn the engine over through five compression strokes.
7. Make a note of the readings from each cylinder. What was the pattern? Does the gauge pump up fast? Is the max reading obtained after five strokes? (Very important)
8. On all low readings, retake and if it is still low, inject 30 weight oil into the combustion chamber and retest.
9. Check the spark plugs from the cylinders with abnormally low or high readings. look for carbon buildup or oil deposits.
10. Replace the spark plugs and torque them to the recommended specs.
Once you've got the readings, it's time to evaluate them to determine just what the future of your engine is going to be.
If the gauge pumps up slowly, such as 30, 50, 70, 90, etc. to an almost normal reading, you probably have ring problems. A general rule is that a maximum reading should occur after two strokes. If oil is added to a low reading and the pressure improves, this will confirm worn rings. A persistent low reading generally indicates valve problems.
Oil lost through worn valve guides will not be determined through compression testing. Low readings can also be caused by a warped head and occasionally, two cylinders will have very low compression which can be the result of a blown head gasket. It is important to remember that the addition of oil to the combustion chamber will not improve the compression readings if your engine has burned, warped or stuck valves. High compression readings can be the result of carbon buildup or head surfacing.
Generally, if the variation between the cylinders is 10 to 15 lbs., this is acceptable, but always check the specs in the manual. It would seem that equal pressure is necessary to have smooth running engine, but it's been found that with only 70% of normal pressure is enough for running smoothly. Also, manufacturers specs usually list a bottom line acceptable pressure and a good engine will usually read 20 to 30 psi over the acceptable readings.
If you find that your engine has good compression, it may be time to go back over your tune-up steps. On the other hand, if you've found low compression readings, it's probably time to have that 200,000 mile engine rebuilt. ---------
[Compression Tester Data]
This is a quick two-person job. First, buy or borrow a compression tester. They are about $30 or less and screw into the sparkplug hole. Next, disconnect the coil and remove all the spark plugs (don't mix up those sparkplug wires!). The test is better if the engine is warm, but not required. Connect the tester to a sparkplug hole. Have a helper crank the engine about 6-8 times with the accelerator depressed to the floor. The gauge will increase and max out. Write down the reading for each cylinder.
What do these numbers mean? Well the compression should be something over 100psi, others would suggest up to 135psi. But the best information to gather is if all the cylinders are within 17psi from each other. A large difference between the cylinders readings indicates internal problems like broken rings, leaky or sticky valves or a combination of these. The downfall of the compression test is that is does not tell you exactly where the problem may be, but it will make you aware there is a problem. A leak-down test can be performed, which puts pressure into the engine to determine the source of the pressure loss. -----------
[www.svttexas.com/Tech/PDF/CompressionTest.pdf ]
A cylinder that has a low reading should be inspected further, as this could be an indication that something
is amiss (sticky or burnt valve, broken/cracked piston lands, broken or weak piston rings, leaking head gasket, etc.). If you come across a low reading cylinder, you should of course re-check your readings and make sure that the compression tester has been properly installed. Remove it and re-attach, then test again. Also, verify that you propped the TB butterflies open. If your TB is closed, your readings will be a bit lower. -----
[ www.ytmag.com/articles/artint35.htm]
...Using a good compression tester one can isolate this location. The tester is, basically, a pressure gauge with a hose on it that screws into the spark plug hole. They are available at many, if not all, automotive parts stores. After warming up the engine, remove the plugs and screw the tester into the first spark plug hole. Disconnect the coil secondary wire at the distributor and ground it on the engine somewhere. Then open the choke and the throttle all the way to provide unrestricted air passage into the intake manifold. Crank the motor over five or six revolutions or until the gauge needle stops rising. If the needle action came up only a small amount on the first stroke and little more on succeeding strokes, ending up with a very low reading, burned, warped or sticky valves are indicated. It is a shame to invest in assorted tune up parts only to have the motor still miss and run lousy because a burned valve or broken ring was not diagnosed. ----------