The Most Important Used Car Test You Should Do Before Deciding to Buy a Used Car

The Foundation of an Internal Combustion Engine

Before getting wrapped up into the vehicle inspecting of a used car when trying to decide if this is the right one for you, you need to start with a very basic test that lets you know right away if a used car has potential as a good buy; or, has potential as a money pit requiring major repairs down the road.

The foundation of any internal combustion engine’s ability to run is built on the stones of fuel, compression, spark, and timing. In fact, that is what makes those old V8 engines such a joy to own and work on---they are very forgiving and will chug along like a patient with a respiratory problem in an exam room until the doctor/mechanic figures out what’s going on when the integrity of any of those four stones are somewhat compromised.

However, when buying a used vehicle, we want more than just that “it runs” as seen in some advertisements. We want to know if the vehicle runs at least if not well, but will also continue to do so for long enough to get our money’s worth. And, a compression test can answer this question for you.

Compression Testing

A compression test basically is a procedure where the engine is made to spin through its cycles momentarily while a compression gauge is fitted into a single spark plug hole. As the engine spins, the gauge will measure what the maximum pressure is within the cylinder tested.

The significance of this measure is that the cylinder’s compression value is dependent upon the engine's valves, its valve seats, piston rings; and, whether these parts are wearing evenly. If any of these components are loose, leaky, open or burned, compression is lost and performance suffers significantly.

Roughly, a healthy cylinder reading should show compression over 100 psi per cylinder, with no more than 10 percent variation between the highest and lowest readings amongst the remaining cylinders tested. If the readings are relatively low in reference to the specs on the engine of your particular car; or worse yet, show too much variation between cylinders, then you can expect to need an engine rebuild in the near future.

In other words, should a potential used car purchase fail just this test alone, you should move on and find a better used car candidate for your home regardless of what any other tests reveal during an inspection.

How to Compression Test Older and Newer Model Used Cars

If you can service your own spark plugs, then you are capable enough to test the compression of an older model used car using a few simple tools every home garage should have. For newer models, it’s not quite as easy, but still doable with a few easy tricks as a DIY procedure for the home mechanic.

That said, here’s a useful YouTube video from the Scotty Kilmer channel that shows how to test the compression in older vehicles using a compression gauge; as well as, a clean and easy technique to determine the relative compression in newer model cars without getting your hands dirty. Based on these tests alone, you should be able to decide if that used car is a good buy or…a “good bye.”

Here's How a Real Mechanic Checks a Used Car Before Buying

And finally…

For more used car articles, be sure to check out the following links about “How To Spot A Used Car Being Flipped After a Hurricane” and “When Smelling Gasoline Around Your Car is an Immediate Problem.”

COMING UP NEXT: Labor Day Weekend New Car Deals!

Timothy Boyer is Torque News automotive reporter based in Cincinnati. Experienced with early car restorations, he regularly restores older vehicles with engine modifications for improved performance. Follow Tim on Twitter at @TimBoyerWrites for daily automotive-related news.