Q&A for How to Do a Compression Test

All cylinders are supposed to have the exact same compression, but that is rare in real-life scenarios. A max of 10% variability is usually considered okay, but more than that could indicate a problem.

Perform compression checks on all cylinders, writing down each number. When you're done, compare numbers. There will be differences. Some manuals indicate that numbers need to be within 10% of each other; other manuals may indicate 15% allowable difference. If any one or more numbers exceed the manual's specs, you likely have a problem.

It is more common to get one low cylinder to be honest but any major variance between the cylinders can indicate a problem.

The pressure (e.g. psi) developed on the compressing stroke of the cylinder's piston that occurs just before the fuel mixture is ignited in that cylinder. Good compression is an important part of the development of usable power from the burning fuel. Such a compression might be 150 to 170 psi.

That info can usually be found in your user's manual or online through the manufacturer's site.

The best way is to just go for a short drive. A car engine will normally reach full operating temperature after seven miles. Failing that, you could start the car and leave it idling for 10 minutes or so.

The wiring harness is what you're referring to. It plugs into the distributor.

If a cylinder had much lower compression than the others, that could indicate wear in that specific cylinder. That is to say, if all others were healthy, if compression is much lower than the needed amount across all cylinders, the engine is in need of a rebuild.

Removing the HT lead from the coil keeps the engine from starting while you're cranking the engine and checking compression. No spark will reach any of the cylinders with the HT removed.

If the coils are "live," and the plugs are still connected, they will get an electrical charge and could cause an electric shock. Otherwise, no damage will occur.

If the ignition system is functional except for the cylinder you are testing, the engine could start on the other cylinders. This could damage your equipment or yourself. Also, except for older vehicles, most everything now has electronic ignition. Activating these ignitions without spark plugs in place can damage the ignition system.

You must remove one because that is the same hole as the compression tester (gauge) uses to measure the compression. You should remove them all the get the best compression values as the remaining cylinders are not providing as much resistance by being compressed. If you try manually turning an engine with spark plugs in compared to all out, you'll find the latter is much easier.

Keeping the spark plugs in the non-test cylinders will cause more resistance as all the other cylinders will be being compressed. Removing all the spark plugs will minimise load on the battery and starter motor ensuring you get the best (highest) results possible.

Yes, if you plan to take a compression pressure reading, you must remove all of the spark plugs. Look for a series of black cables coming out of the top of the top end of the engine. Grasp each wire at the end, then twist it while simultaneously pulling it up to detach detach it from the engine block. The other end of the wires will still be plugged in, so you'll have to push them aside so they're off the engine. Label the wires so you know which spark plug each one connects to. The spark plug wires are generally well-routed to avoid confusion, but keep them separate anyway to reduce the possibility of switching them. Keep in mind that some vehicles have ignition coils instead of spark plugs, but they can be removed the same way.

You want to do a compression test while your engine is warmed up to its normal running temperature. That way, you'll get an accurate reading. If you haven’t driven the car lately, the engine will be cold. Start your vehicle as usual and keep the engine running for 5 to 10 minutes. Be careful not to overheat the engine by letting it run too long before the test. You will be able to feel the heat radiating from the engine when you go near it. Then, turn the engine off before you open up the hood. Make sure it is all the way off without any electricity or fuel flowing to the engine. Remove the key from the ignition to ensure the components can be safely removed from the engine bay.

It can vary based on the engine you're testing. Most engine cylinders fall in the middle of that range, usually around 125 PSI. But the exact rating could vary based on factors such as like the vehicle you have, the type of engine you’re testing, and its overall condition. For diesel engines, the optimal PSI is between 275 and 400. No matter what your engine's rating should be, the difference between the highest and lowest cylinder rating should be no more than 15 to 20 PSI. Big pressure differences are a sure sign of engine problems. If you see a result that looks out of the ordinary, figure out what engine cylinder it corresponds to.