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Engine Compression Test w/- Gauge |
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OK, you followed TT #1 and suspect that you have compression variation. The next step is to actually record the compressions using a compression pressure gauge. A compression test will tell you specifically what condition the rings, valves and head gasket of the engine are in. Ensure the engine is at normal operating temperature and the battery is fully charged before you proceed. You can do the test on a cold engine, but the reading will be a little lower. The pressures will vary from engine to engine but the most important thing you're looking for is fairly equal compressions (variation less than 20psi) in a petrol engine and less than 60psi in a diesel. Petrol Engine. Begin by removing the plug leads. Note which leads go where because some leads may have one or two that are the same length and if you mix them up on re-installation the engine may not start or will misfire. If you are familiar with the firing order and HT lead layout then you only need to klnow which is the number 1 lead. You can then install the rest in firing order. Next clean around the spark plugs before you remove them. Compressed air is best if you have it, otherwise a bicyle pump will do a reasonable job. This is done so no dirt gets into the engine when you remove the spark plugs. When clean remove all the plugs and keep them in order for diagnostic purposes. For EFI engines: disable the fuel supply so that you don't have fuel spraying into the cylinders. (not in the scope of this article on how to do this) Now you must disable the spark. Not in the scope of this article to do this on an EFI engine. All other engines with a distributor and standard ignition coil it is a simple matter of removing the positive or negative terminal wire from the coil. You don't necessarily have to remove both. Install the compression gauge in the number one spark plug hole and zero the gauge. You must open the throttle fully as you perform each cylinder test. Crank the engine over at least 4-5compression strokes and watch the gauge. The compression should build up quickly in a good engine. Write down the highest reading obtained and repeat the procedure for the remaining cylinders and compare the results to your cars specifications if you know them. Otherwise just look for variation between them. As stated before there should be no more than 20PSI variation from the lowest reading to the highest. If you know what your compressions should be and all the readings are well below normal, add about 4-5 squirts of engine oil to each cylinder, through the spark plug hole, and repeat the test. If all the compressions read 30+ PSI higher than the previous reading then you need to replace the rings. If the compression doesn't increase significantly, then you have valve or head gasket troubles. Leakage past the valves may be caused by burned valve seats and/or valve faces. If two adjacent cylinders have equally low compression, there's a strong possibility the head gasket between them is blown. If the compression is unusually high, the combustion chambers are probably coated with carbon deposits. If that's the case, add some fuel additive to the fuel tank that will over a period of time remove this carbon build-up. Higher than normal compressions can also be caused by oil by-passing the rings. This is the same result as squirting oil into the bore. The spark plugs will show signs of oil burning. If one cylinder is lower than the others, add 4-5 squirts of oil into this cylinder and re-test the compression. Same as before, if the compression picks up, then the rings on this cylinder have failed. If no significant increase was noticed then the valves are the likely suspect. To confirm if the leakage is past the valves: Remove the air cleaner so you can listen to compression leakage coming back through the throttle butterly. Remember to hold the throttle open. Getting a friend to help is handy here because it is hard to hear any leakage from inside the car. Get your mate to crank the engine while you listen. If you hear a hiss on each compression stroke then your intake valve is leaking. Next have a listen at the tailpipe while the engine is being cranked. You won't hear the hiss anywhere near as loud as at the intake but you will still hear a slight hiss, if so then you have a burnt out exhaust valve or seat. After the test is complete, refit the plugs and leads, reactivate the fuel supply and ignition system as required. If you wired your throttle open, don't forget to remove it. Start the engine. The engine will blow a thick cloud of blue smoke for a while. After this settles take the car for a drive to recharge the energy in the battery that was used to crank it for the test. That's it. You should now, from the recorded reading be able to diagnose if you have mechanical engine problems.
Diesel Engine, Toyota Coaster 1HZ 6cyl. Begin by cleaning around the glow plugs before you remove them. Compressed air is best if you have it, otherwise a bicyle pump will do a reasonable job. This is done so no dirt gets into the engine when you remove the glow plugs. When clean remove all the glow plugs and keep them in order for diagnostic purposes. You will have to disconnect the fuel shut off solenoid at the injector pump before beginning the compression test. A high pressure compression tester with a range of glow plug and injector adaptors is needed for a diesel . The compressions are much higher in a diesel so high pressure fittings are needed to hold the compression.
Install the compression gauge in the number one hole and zero the gauge. Crank the engine over for 4-5 compression strokes and watch the gauge. The compression should build up quickly in a good engine. Write down the highest reading obtained and repeat the procedure for the remaining cylinders and compare the results to your engines specifications. Since compression is more important in a diesel, you should find out what the specification actually should be. A common allowable variation between cylinders is no more than 60PSI from the lowest reading to the highest. If your compressions are below normal, add about 4-5 squirts of engine oil to each cylinder, through the glow plug hole, and repeat the test. If all the compressions read 40+ PSI higher than the previous reading then you need to replace the rings. If two adjacent cylinders have equally low compressions, there's a strong possibility the head gasket between them is blown. If the compressions are unusually high, the cause could be oil by-passing the rings. This is the same result as squirting oil into the bore. In saying that though you would notice blue smoke from the exhaust. If one cylinder is lower than the others, add 4-5 squirts of oil into this cylinder and re-test the compression. Same as before, if the compression picks up, then the rings on this cylinder have failed. If no significant increase was noticed then the valves are the likely suspect. To confirm if the leakage is past the valves. Remove the air cleaner so you can listen to compression leakage coming back through the intake manifold. Gettting a friend to help is handy here because it is hard to hear any leakage from inside the car. Get your mate to crank the engine while you listen. If you hear a hiss on each compression stroke then your intake valve is leaking. Next have a listen at the tailpipe while the engine is being cranked. You won't hear the hiss anywhere near as loud as at the intake but you will still hear a slight hiss, if so then you have a burnt out exhaust valve or seat. After the test is complete, refit the glow plugs and start the engine. The engine will blow a thick cloud of blue smoke for a while. After this settles take the car for a drive to recharge the energy in the battery that was used to crank it for the test. That's it. You should now from the recorded readings be able to diagnose if you have mechanical engine problems. |
