A compression test is a basic part of a routine tune up for an air cooled Volkswagen engine. With these instructions you can check the engine compression yourself.
Compression is a measurement of the pressure that builds up within a cylinder as the piston rises and both valves are closed. For the fuel mixture to burn properly a certain range of pressure is required.
The two main factors that affect compression are the condition of the piston rings and the valves. Obviously, the valves need to be fully closed on the compression stroke to avoid loss of pressure.
Before you check the compression the engine needs to be at normal operating temperatures. Let your Volkswagen idle until the engine is completely warmed up.Turn off the engine and begin removing the spark plug connectors from each spark plug. Unless you are very familiar with the firing order mark each wire with something like masking tape so you can put them back on the correct plugs later.
Remove all the spark plugs and block the carburetor throttle so that it is fully opened.
Thread the compression gauge into each cylinder making sure that the pressure release button has been pushed and the reading is at 0. Crank the engine for about 2 seconds and write down the highest reading from the gauge. Here it is very helpful to have a friend who can sit in the driver’s seat and crank the engine when you yell. You may also use a remote starter to turn the engine.
Below is a chart that tells you what sort of compression pressures your air cooled VW engine should have. It is desirable to have high compression and relative uniformity among the cylinders. Variations from cylinder to cylinder should be no more than 14 to 20 pounds depending upon your engine’s compression ratio.
VW Beetle Compression Ratios
1954 to 1960 6.0:1
1961 to 1965 7.0:1
1967 to 1970 7.5:1
1971 to 1972 7.5:1 / 7.3:1
1975 to 1977 7.3:1
In most cases weak and/or uneven compression are caused by worn piston rings or bad valve seating. Of course improper valve adjustment could also be the culprit, but I am assuming the valves have already been checked and adjusted at this point (If not then see How To Adjust Air Cooled VW Valves). In worst cases there could be a damaged piston, cracked cylinder (jug), cracked head, leak between head and cylinder, or a burnt or broken valve.
Proceed to Step 2 of tune up procedure How To Adjust Air Cooled VW Valves
Written by David Slone, Copyright 2008 all rights reserved.