Firstly, disable the ignition and injection (remove the purple main relay from the triangular relay box by the battery is probably the best bet). Make sure the battery is well charged.
Remove all the spark plugs and insert the compression tester in one plug hole. Try to arrange it so you can see it from the driver's seat or get someone to crank the engine for you.
Crank the engine with the throttle wide open and watch the tester. It should pulse up to a constant reading after 3 or 4 compression cycles of the cylinder and remain there. Note down the figure obtained.
Repeat for the remaining cylinders.
Typical values on a healthy engine are probably around 12-14 bar but this does depend on the engine type. More importantly, on a good engine the compression will be within 10% between cylinders and will be reached after 3 or 4 cycles.
Cylinders which make a low reading, or where the reading is slow to build up might be problematic.
If a cylinder is low, a teaspoon of oil into the plug hole followed by a retest might bring the reading up to normal. If this is the case, it points to piston ring seal as being the problem. If not, the head gasket or valves are suspect.
It can be a useful guide to an engine's condition of wear and is one step that can help diagnosis, but look at the big picture of symptoms as a compression test is rarely conclusive on its' own (unless there is damn all compression!).
Kevin
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Topic: Compression test (Read 624 times)