Before going to the trouble of trying to tune an engine, it is good idea to first check and see if it is possible to tune the engine satisfactorily. If compression is much below par in any or all of the cylinders it will be impossible to tune the engine without first having the cause of the low compression remedied. If any Volkswagen is found to have a compression lower than 65 pounds per square inch (psi) in one or more cylinders, it will be difficult to put the engine in proper running condition through tuning alone.
A noticeable lack of engine power, excessive oil consumption and/or poor fuel mileage measured over an extended period are all indicators of internal engine wear. Worn piston rings, scored or worn cylinder bores, blown head gaskets, sticking or burnt valves and worn valve seats are all possible culprits here. A check of each cylinder's compression will help you locate the problems.
As mentioned in the Tools and Equipment section of , a screw-in type compression gauge is more accurate then the type you simply hold against the spark plug hole, although it takes slightly longer to use. It's worth it to obtain a more accurate reading. Follow the procedures below.
- Warm up the engine to normal operating temperature.
- Remove all the spark plugs.
- Disconnect the high tension lead from the ignition coil.
- Fully open the throttle, either by operating the carburetor throttle linkage by hand or by having an assistant hold the accelerator pedal to the floor.
- Screw the compression gauge into the No. 1 spark plug hole until the fitting is snug.
- Ask an assistant to depress the accelerator pedal fully on both carbureted and fuel injected vehicles. Then, while you read the compression gauge, ask the assistant to crank the engine 4 to 6 revolutions using the ignition switch. Repeat the test two or three times until a consistent reading is obtained.
- Read the compression gauge at the end of each series of cranks, and record the highest of these readings. Repeat this procedure for each of the engine's cylinders.
The resulting compression pressure should be in the neighborhood of that specified for each of the Volkswagen engines. Compression differences between cylinders should be no greater than 15 psi, and any cylinder that is even 10 pounds less than the others should be given particularly close attention during subsequent compression checks.
When compression is low in one cylinder, it is due either to worn piston rings or to valve leakage in that cylinder. Squirting about a tablespoon of heavy lubricating oil into the spark plug hole will reveal the cause of the low compression. If, after squirting the oil into the cylinder, compression increases, the low compression is due to worn piston rings. If, on the other hand, squirting the oil does not result in a higher compression reading, the problem is most likely to be leaking valves. With regard to leaking valves, a valve may leak slightly in a compression test due to the presence of a particle of dirt on the seat of that valve. In this case, subsequent compression checks will probably reveal that the reading involving the dirt particle was unusually low.
It is an excellent idea to check and record compression readings in all cylinders at periodic intervals, especially if one is interested in keeping a close eye on engine wear and also wants to be forewarned of problems which might otherwise come as a surprise at an inconvenient time. Although compression readings on any particular engine will vary widely from one compression check to the next (due to oil temperature, viscosity, outside temperature, method of measurement, etc.) cylinders will generally show the same relative readings. For example cylinder number one may always be 3-4 lbs. higher than cylinder two, and cylinder three may usually be 5-6 lbs. higher than number four.