#29 Compression Test

Cylinder pressure is an excellent indicator of overall engine condition.  It can reveal worn piston rings or cylinder walls or uncover a head gasket leak between adjacent cylinders, among other things.

Compression is checked after warming the engine up by removing all spark plugs (5, in the case of this B5254S Volvo engine) to connect a pressure gauge in each hole one-by-one.

It’s also important to disable the spark generator by disconnecting the control wires from the ignition coil

and disabling the fuel pump by pulling its fuse.

Crank the engine so that max pressure is attained for each cylinder, then record the value.  I observed no pressure increase when the throttle was held open, which many insist be done.

Tests reveal rather low values (132,140,137,133,135 psi) but they are at least very consistent, which is the most important thing.  On a 15 year old car with 150K miles, we expect some wear in the cylinders so compression will be lower than factory specs with a new, tight engine.  Engine seems to have plenty of power and is not consuming or burning oil, so low compression is not apparently a big issue.

Top row numbers are under normal (dry) conditions.  Bottom row is pressure reading after adding a bit of oil to the tops of the pistons (wet).  Wet readings are good, close to new engine specs.  The big increase from adding oil to the cylinder indicates piston ring wear, which doesn’t really jive with the lack of smoke or oil consumption.  So we’ll have to see how this shakes out.  Will check compression again after performing internal engine cleaning (Seafoam) to see if things improve.

I did observe that the plugs were fairly new and in decent condition but set to a very wide gap (.048″).  They should be .028″, which I set them at.  Em reports a bit more power and presumably mileage too with proper gap.  Plugs will be replaced with new ones after engine cleaning.

Tags: compression, gap, plug, spark