How to Do a Compression Check on Your Car Engine (It’s Easier Than You Think)

I’ve been turning a wrench since I was twelve years old. My father, an old-time body and fender man, almost always did his own auto repair work, and although I have never been a professional mechanic, with Dad’s help (in the beginning) I have rebuilt and/or replaced at least forty engines, transmissions, and various ancillary parts over the last fifty-five years.

While doing auto repairs on your own can be a scary proposition, leaving the work to an auto mechanic can be an expensive (and just as scary) alternative. There are ways, however, to make sure your repair shop doesn’t drain your entire bank account, or your peace of mind, when you find the family car in need of a little help.

One way is to present yourself to the mechanic of your choosing, in a way that tells him that you know, at least a little, about what is going on under the hood of your vehicle.

If your trusted car/SUV/pickup starts to show signs of being on its last legs: burning oil; overheating; tapping noises (coming from under the hood), the problem can often be coming from: burnt or broken piston rings; scored cylinder walls; burnt valves; or fouled or burnt valve seats. All of these can lower your engine’s compression and effect you car’s performance.

This usually means that the heads will have to be removed and rebuilt, and, in the case of a minor scoring of the cylinder: a hone will have to be run against the cylinder wall.

Doing a compression check before visiting your mechanic will accomplish two things. Most importantly: once you present your findings to your mechanic, it will instantly let him know that you understand considerately more about what is going on under the hood of your car than his average customer. (A side note here: if you are shopping for a mechanic, take your compression check readings with you, and, if any repair shop balks, or acts perturbed by the fact that you know what you are talking about, then it’s time to walk out of that shop and choose another place to have your work done. Many repair shops like their customers dumb, and ready to write big checks). The second thing that doing your own preliminary repair work will do is: continue to build your confidence for future home automotive repair work.

GETTING STARTED

What you will need: An old bath towel or an old unused (small) blanket; a cardboard box; a pocket knife or screwdriver; a sparkplug socket and driver (with a couple of assorted socket extensions); masking tape and magic-marker, and a compression gauge. Many of the large auto parts stores have tool lending programs. A few quick phone calls will find a gauge-for-loan near you.

Step one: (after opening the hood) Disconnect the negative battery cable (safety first when working near the fan or fan belts.

Step two: Lay your towel or blanket over the appropriate fender (it will protect your paint from that big belt buckle … you would be surprised how many professionals don’t take the time to do this). Pull the distributor wire from the center of the distributor cap (this is a redundant safety step). Finally pull all the spark plug wires from their respective plugs, labeling each with numbers that correspond to the position of each plug. It makes no difference what numbering system you use here because we are not concerned with engine timing (a later lesson). Stand in front of your engine and call the first plug on the right: number one, then continue to the rear of the engine to numbers two, three and four. Move across the engine (passenger side) to number five at the rear, then forward to number eight in the front. Make the obvious changes for a six cylinder (one to three, and four to six). Your masking tape and magic-marker works fine here.

Step three: Remove all of the spark plugs. You cannot run a proper compression check with any plugs still installed. Grab your cardboard box (a flat from a soft-drink or canned fruit or veggie product works great). With your pocket knife or bladed screwdriver, make eight X marks through the bottom of the box and shove the plugs into the holes in the same pattern as you removed them from the engine (numbering each one to match your numbering system).

Step four: Replace the negative battery cable.

Step five: (repeat eight times in your numbering order) Screw the compression gauge into number one spark plug hole and position it in the engine bay, away from any moving parts. Now, move to the driver’s seat and turn the ignition to START (holding it there until the engine has turned at least four to six revolutions). Return to the gauge and note the reading. Write this reading on the cardboard next to the corresponding plug. Push the pressure release button on the gauge and remove the gauge. After you have done this eight times, you have completed your first compression check. Easy, huh? Remove the negative battery cable again (I know, it seems like overkill on the safety issue, but, I had a boyhood friend lose a finger to a radiator fan).

Step six: this is a good time to install new sparkplugs if you wish, however, if you do end up taking your car in for repairs, DO NOT let your mechanic charge you for new ones after he completes your repairs. Always check your ITEMIZED bill carefully BEFORE writing your check.

FINISHING UP: Replace all of the spark plugs and wires. Replace the distributor wire to the center of the distributor cap. Fold up your blanky; clean and put away your tools; and once again, replace the negative battery cable.

Now, go back inside, out of the sun, and have a cold lemonade (or brewskie). You’re done.

An afterword on your gauge readings.

I have purposefully NOT given numbers for the readings from the compression gauge. There are too many makes and models and too many variables to list them all. What you are looking for in your numbers is: consistency throughout your readings. You may notice that the cylinders at the rear of the engine read a little lower than the rest. Speaking GENERALLY, those cylinders tend to run hotter then the rest (but not universally) and tend to show more heat related wear. If you have found one that has a really anemic reading, this may be the reason your faithful car is lagging in performance.

Your mechanic should be appreciative of the info on your cardboard box, and, like I said before, if it bothers him that you did a little preliminary work, then go elsewhere.

If you are interested in exactly what your compression readings should be, pick up one of the handy manuals, for your make and model, at the parts store that loaned you the compression gauge. They will appreciate it, and it will offer you a new look into the inner workings of your trusty ride.