Engine compression tests and testings are a quick down and dirty way to figure out both quickly and cheaply whether to walk away from that used truck or SUV on the car lot.  Yet how is the procedure accomplished and what do the numbers and values mean? Alternatively when are the danger levels when its time to shove off laddie?

             Engine compression tests are simply a means of assessing how tight your engine components are fitting to get an idea of the overall wear on the motor itself. Your car or trucks engine is its whole reason of its existence of providing basic transportation.  Without a good working engine – your vehicle will not provide you with reliable transport. If the engine is not firing well – and is basically showing great signs of wear – fuel economy and economies will be on the low end of the scale.

Engine repairs are downright costly and expensive.  You could replace the power plant with another or a refurbished remanufactured one, or you put highly expensive, high labor cost repairs in the vehicle if it is worth it. However overall if you are car shopping its best to move on.  Who needs the headache and costs?

           The test is simply and quickly done at the spark plug holes. It’s essentially a relatively non-invasive procedure. Just the spark plugs themselves are removed out of their threaded holes – the plugs are simply and easily removed by unscrewing with a spark plug wrench 1, 2, and 3 and then after the test replaced.  Nothing to that.

With a hot engine and a full throttle pressed down.  A hose fitting is then attached and seals each cylinder one by one in sequence.  The hose fitting’s hose is simply attached to a simple compression meter gauge. It’s analogous to you blowing into a small hose with a measuring gauge at the end and seeing how hard you can blow to move the dial.  Each cylinder of the car’s motor is tested in sequence and the measurements marked down for later reference.

It may be that in a 6 cylinder motor all are fine, or 3 are fine and 3 not or any similar results.

          What does it all mean and how can you interpret the compression test measurements to correlate whether the car is a wise buy? 

            As a rough guide experienced auto wholesale buyers will state that an engine can be considered acceptable and within reasonable tolerances if the worst cylinder gives at least 620 kPa (equivalent to 9o psi pounds per square inch) and the remaining cylinders give readings of at least 75 percent of the highest readings.

              Abnormally high readings in one or more of the cylinders can mean that some exhaust valves are not opening fully or alternatively that there is excessive buildup of carbon inside the engine. From then on – a good mechanic should be able to diagnose and rule out carbon deposits as the cause to reassure you of the engine’s merit and overall state.

                     On the other side of the dial – low readings in one or more cylinders or readings from two adjacent cylinders that are least 20 psi (140 kPi) below the others can indicate other problems.  What the mechanic or auto technician will do to determine the causation is too simply and easily squirt engine oil into the spark plug holes and take yet another set of readings.  If it’s found that the new readings are higher than the first set it’s a dead give away that the piston rings show excessive wear and tear. Yet if the new readings stay about the same as the first set it’s a tip off that the valves are sticking and/ or burned. If two adjacent cylinders still read at least 20 psi below the others it can well mean and be a tip off of big time repairs upcoming – that the head gasket itself is damaged between those two cylinders.

                       What is the summary for you as an automobile buyer?  Auto engine compression tests area quick and inexpensive means of ruling out risky auto purchases.  Save yourself a lot of time, time, expensive repair bills and overall inconvenience. If you are seriously looking at a car to purchase and wish to rule out big time problems and issues in the near future and not too far down the road have your mechanic perform a simple compression test on the power plant.

Ted B. Cobblestone