| Compression Test
Description How to compression test your engine (3S-GE, 3S-GTE and 5S-FE).
Date Mon Mar 21, 2005 11:00 am
Type Text How-To
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How to compression test your engine (3S-GE, 3S-GTE and 5S-FE).
|Compression Testing Your Engine
by David Kaye
Tools required: Compression Tester, available from just about any auto parts store for around £20. Time required: About 3/4 hour total
Why would anyone want to do this?
Knowing the compression of your engine can give you insight into the shape of your piston, rings, cylinder walls, head gaskets and valve seats. Getting a good baseline now gives you something to compare to down the road, in the event your engine starts to act up. If you modify your car and/or drive it hard/race it, you should probably perform this procedure once in awhile. Your next plug change could be a convenient time to perform a compression test.
Before beginning this procedure, engine should be warmed up, and preferably not too hot. I usually do it after I've returned from a short trip (to the grocery store, for example) and the car has had a chance to cool down a bit (still pretty warm, but not too hot).
Remove all your spark plugs.
Prop open the throttle body intake butterflies to the wide open position. I usually disconnect the return spring, then prop it open with the handle of a pair of pliers or large screwdriver. I usually keep a clipboard handy, with a sheet of paper for recording the results. Down the left side, I list cylinders 1 through 4. Across the top I list test #1, #2 and #3. Don't forget to record the date. If the tool is not already assembled, install the additional fitting with "O" ring, onto the flexible shaft that matches the spark plug hole size. Make sure the fitting is on the flexible shaft is quite tight. You don't want the adapter coming loose once it's installed into the spark plug hole. Then install the flexible shaft into the first spark plug hole and screw it on tight. Now crank over the engine about 10 times. You'll hear it as it hits on the only cylinder with compression. Jump out and record the measurement on the gauge, then release the compression by pressing the compression release button. I usually test each cylinder two times, to be sure I got a good accurate reading. After testing all cylinders, sometimes I'll come back to the ones with the lowest pressure and test them again.
That's it! Put everything back together.
Evaluating the results:
Not all your cylinders will have exactly the same reading unless you are really lucky. In fact, you may see 5psi to 10psi differences from one cylinder to the next. All of them should be fairly close though. A rule of thumb is that there should be no more than 10% to 15% variance from the cylinder with the highest compression reading to the cylinder with the lowest reading.
Compression Pressure, (TB Open)
3S-GE 12.5kg/cm2 (178 psi or more)
3S-GTE 11.5kg/cm2 (164 psi or more)
5S-FE 12.5 kg/cm2 (178psi or more)
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. I got the following tip off the message boards and have not done it myself, but it sounds like an interesting way to examine a suspect cylinder further.
Take an old spark plug, remove the inside from it (the electrode and top metal part) so it is hollow. Attach a length of rubber tubing to it. Set the engine at the top of the compression stroke for cyl #1. You should be able to see piston at the top of cyl.(make sure it is not the exhaust stroke). Blow compressed air into the tubing then listen for where it comes out. If you hear it in the exhaust, you've got a bad exhaust valve. If you hear it in the intake i.e. thru the throttle body, you have a bad intake valve. If you hear it in the crankcase, you have bad rings or a cracked piston. If your coolant starts bubbling, you have a bad head gasket.
Wet testing should be done if you have some cylinders that have pressure readings less than 75% of the highest cylinder. After testing compression as noted above, get an oil can or similar and squirt about a teaspoon of 30-40 wt oil in each spark plug hole. After this is done, wait about 5 minutes (for the oil to flow down and into the ring lands) and then test again. If the numbers go up then your rings in that cylinder are worn.
Hope this helps
MK11 Turbo 1990
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