In this section, you're gonna' do a ‘Wet’ compression test on the cylinder or cylinders that gave you a low compression reading.
The ‘Wet’ compression test is just a simple variation of the compression test you did in the previous page... the difference being that you're gonna' add about 2 tablespoons of engine oil to the cylinders with low compression.
The reason for adding engine oil, is that if the low compression is being caused by worn piston rings... the oil will seal the rings (in the affected cylinder) and cause the compression pressure to increase. This increase of pressure will be very obvious on your compression tester.
Now, if the problem is worn cylinder head valves... no amount of oil will seal them and thus the compression reading will stay the same as when you did the ‘Dry’ compression test.
OK, this is what you need to do:
CASE 1: The compression value shot up.. This tells you that the piston compression rings are worn out and thus the problem is in the bottom end.
CASE 2: The compression value stayed the same.. This confirms that the problem is in the cylinder head valves.
If this info really saved the day, buy me a beer!