Whether it’s a lawnmower, chainsaw, or car, good compression is vital to the performance of your engine. This article will provide a detailed discussion about engine compression and its importance. What causes poor engine compression? How do you prevent it? A step-by-step guide on how to test engine compression.

The internal combustion engine needs to be completely sealed in order to function optimally. If your engine is not properly sealed, you will experience poor performance, increased fuel consumption, and burning oil.

When any of the components that seal the engine fails, the compression will be affected. There are three main parts of the engine that need to be in good condition for good compression:

  • Valves: engine valves open to allow gas and air into the combustion chamber, then close, creating a seal within the combustion chamber. Once combustion is complete, the exhaust valves open to allow burnt gas to escape. If the valves are worn or don’t seat properly, they will not completely seal the engine, resulting in a loss of compression.
  • Engine rings: Two sets of rings fit into grooves on the piston. One ring provides a seal to prevent oil from entering the combustion chamber, these are called oil rings. The second ring (compression rings) ensures an airtight seal within the cylinder. These rings provide the compression needed for optimal engine performance.
  • Head gasket: The valves are seated inside the engine head and the pistons move inside the block. A gasket is placed between the head and block. When the head bolts are tightened, the gasket is compressed to seal the engine. This ensures good compression and prevents water from entering the engine. A damaged head gasket may affect engine compression and could cause water to enter the combustion chamber.

How do you know if your engine needs a compression test?

An engine compression test will help you determine if any of the above-mentioned components may be damaged.

If you see excessive smoke from the exhaust pipe, this is probably caused by damaged rings or valves. Damaged valve stem seals can cause smoke when accelerating or difficulty starting the engine when cold. Damage to the rings and valves will invariably lead to a loss in engine power.

Steam or water coming from the exhaust pipe is indicative of a damaged head gasket. You may also notice rust on sparkplugs and a loss of coolant. The engine can easily overheat if the head gasket is damaged. You will generally experience difficulties starting the engine when cold.

A damaged head gasket may not always cause coolant loss and overheating. Similarly, it may not cause a loss of compression. Depending on how the head gasket is damaged, it may cause one or both scenarios: coolant loss, combined with overheating and/or a loss of engine compression.

The most common cause for weak engine compression is damaged valves or compression rings.

How to Use a Compression Gauge

OTC5605 Deluxe Compression Tester Kit

Step 1

Disconnect the fuel pump and ignition. The easiest way to do this is to remove the fuses to the fuel injection and ignition systems.

Step 2

Remove the spark plugs. Removing all the sparkplugs before you begin may speed things up but can cause confusion later. You need to make sure that you reconnect all the sparkplugs in the correct order. If you’re not sure, disconnect one spark plug, test the compression, then reconnect it before moving onto the next cylinder. I prefer doing things this way, even if it takes a little longer.

Step 3

Turn the threaded end of the compression gauge into the sparkplug hole. Always start slowly and gently as you don’t want to damage the sparkplug thread. Tighten the gauge firmly by hand. Don’t use a wrench, the compression gauge uses an O-ring to seal it. The O-ring does not need much force to seal and can be damaged if overtightened.

Step 4

Turn the key and allow the engine to crank for about 5-seconds. Check the reading on the compression gauge. Engine compression specs vary. Generally, around 100 Psi is good. Make a note of the compression for the first cylinder and then move on to the next one.

Step 5

Once you’ve tested the compression for all the cylinders, compare the results. They should all be similar. If any cylinder has significantly lower compression than the others, you can be pretty sure that the rings or valves for that cylinder are damaged. In order to fully diagnose the situation, you’ll need to remove the cylinder head. This is a fairly complex procedure and does not fall within the scope of this guide.

Quick Tip

If you find a cylinder that is below 100 Psi, pour a little oil (approx. 1 teaspoon) into the sparkplug hole and redo the compression test on that cylinder. If the compression improves after adding some oil, the problem is most likely a damaged ring. If it remains the same, you can assume that the problem is probably a valve that is not sealing properly.

Prevention is Better Than Cure

Repairing an engine with bad compression is a complex and expensive undertaking, it is obviously preferable to avoid this. While it’s impossible to avoid engine wear entirely, you can prevent premature wear that could cause a loss of compression.

The best way to keep your engine in top condition, and reduce wear, is to change the engine oil and filter regularly. Use a good quality synthetic oil for the best protection. Carbon and micro-shavings of metal in the oil will damage the engine over time. By replacing the oil, you reduce this unwanted wear.

Use an engine oil that is best rated for your climate. The oil viscosity should match the driving conditions. It may be best to use a different oil in winter to the one used in summer.

Choose the best oil grade for either season. This is indicated by the numbers on either side of the letter ‘W” for multigrade oils EG: 5W30. The first number indicates cold start oil viscosity, for a cold climate a lower viscosity is better. The number after the “W” indicates the high-temperature viscosity. For hot climates or high-performance (high-temperature) engines, a higher number is preferable.

Excessive heat is one of the biggest causes of engine damage. If you notice that the temperature gauge is higher than it should be, stop, switch the engine off, and allow it to cool before continuing. When the temperature is consistently high, you need to diagnose the issue to prevent heat damage to the engine.

Periodically check the valve clearance, as valve “knocking” is a common cause of valve damage.