Compression test ?
#1
07-20-2009, 07:29 PM
Compression test ?
Ok, I know, this may sound like a dumb/simple question for some of you but doing a compression test on a 3.8, do you do a dry test, wet test or both? How do you keep the engine from starting when you are testing? What test device is best to use? I am thinking I might have a head gasket leak into the cylinder. I keep losing water with no evidence of external leak and no water in oil. A slight evidence of white smoke from the exhaust with ambient temps around 70 degrees, but you have to be looking really hard to see it. Nothing like head gaskets that I have seen before where they leave a trail 20 feet behind of white smoke.
#2
07-20-2009, 08:30 PM
1) Important Remove all spark plugs, and ground ignition wires to prevent arching and overvoltage damage to coils. All plugs must be removed and left out for duration of test.
2) Remove FI fuses to avoid excessive fuel into cylinders
3) Attach pressure gage to #1 spark plug hole
Dry method (this will pick out a low cylinder, but won't tell you if it is due to rings, valves or head gaskets.
4) Crank engine with starter and read meter on 4th compression stroke. You will see a pressure increase as each time the piston goes into compression. Now read meter and record.
5) repeat for remainder of cylinders
Wet method This will indicate if valves or rings or head gaskets are bad.(do dry method first and then spray engine oil into the cylinder)
6) Repeat procedure for all cylinders and record readings.
Compare readings between both methods.
1) All cylinders must be within 10% of each other and the lowest cylinder must be equal or higher than specified reading for the particular engine.
2) Compare wet & dry readings for low cylinders. If wet reading is higher than dry then reading, the rings are bad. If both readings remain low, the valves or gaskets are bad.
Low readings between adjacent cylinders is an indication that the head gasket may have failed between the adjacent cylinders.
White smoke is usually an indicator of coolant into the combustion chamber. Among other things, this could be due to a bad head gasket or a bad intake manifold gasked. The compression test will not prove a bad intake gasket.
2) Remove FI fuses to avoid excessive fuel into cylinders
3) Attach pressure gage to #1 spark plug hole
Dry method (this will pick out a low cylinder, but won't tell you if it is due to rings, valves or head gaskets.
4) Crank engine with starter and read meter on 4th compression stroke. You will see a pressure increase as each time the piston goes into compression. Now read meter and record.
5) repeat for remainder of cylinders
Wet method This will indicate if valves or rings or head gaskets are bad.(do dry method first and then spray engine oil into the cylinder)
6) Repeat procedure for all cylinders and record readings.
Compare readings between both methods.
1) All cylinders must be within 10% of each other and the lowest cylinder must be equal or higher than specified reading for the particular engine.
2) Compare wet & dry readings for low cylinders. If wet reading is higher than dry then reading, the rings are bad. If both readings remain low, the valves or gaskets are bad.
Low readings between adjacent cylinders is an indication that the head gasket may have failed between the adjacent cylinders.
White smoke is usually an indicator of coolant into the combustion chamber. Among other things, this could be due to a bad head gasket or a bad intake manifold gasked. The compression test will not prove a bad intake gasket.
Last edited by z28pete; 07-21-2009 at 07:27 AM.
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