An engine needs three factors to work: a spark, lubrication, and compression. Each is equally important in the function of an engine. The engine function is a controlled explosion and if there is no compression the explosion will blast everywhere and not pushing the piston, where was designed to go.
So the more compression the better, right? Or maybe you only want a little bit? Well, each car has its own compression setting for how it is designed to perform. Usually, a higher performance engine will need higher compression. But How do we find out? What is a compression test? What is good engine pressure?
Generally speaking, a compression test is a test performed on your engine to find out how much pressure is in your engine during operation. It is measured in pounds per square inch or PSI. If the test is passed each cylinder or piston will have a PSI within 10% of each other. Each car should have its own recommended compression setting.
When Should You Do A Compression Test?
- Smoky exhaust
- Sluggish acceleration
- Engine vibration
- Poor MPG
- Your engine uses more oil
- Your car is running hot
If there is variance in the compression readings it can tell you a lot of things, whether your car has worn piston rings, if there is a leak in your engine from blown head gaskets to blown manifold covers, to worn and leaking valves.
The test is performed with a pressure gauge stuck in the spark plug hole in your engine. It is a basic operation that anybody can do with minimal tools. There are some specifics that can be crucial and if you are unsure you should consult a mechanic or simply drop it off are your local trusted auto shop.
I’m not a mechanic and I have limited tools. But I don’t trust the local shop. I think they just try to take advantage of me. So, how do you check compression on an engine?
TIP: If a cylinder reads under 100 PSI, pour 1 teaspoon of oil into the spark plug hole and retest the cylinder
As a general rule, how much compression should a motor have?
100 PSI is a good starting point. Each cylinder should have a PSI reading within 10% of one another. So if one reads 106 PSI the others should all be within 10PSI of each other.
Higher performance engines will often have much higher compression settings to deliver more horsepower. For instance, a 1999 Porsche 3.4 liter engine (207 cubic inches) with 296 horsepower uses about 170 PSI. Check your owner’s manual or shop manual for your car’s specs.
Most modern cars (newer than 2010) will have compression specs of around 130-150 PSI.
So I did a compression test. But my compression test results are high. Is this an issue? Is my truck going to explode??
Any variation in compression may be an issue. High compression may be a cause of a valve sticking and not letting air escape or there is something in the cylinder chamber. Liquid such as oil may be trapped as a result of worn piston rings, or possibly exhaust is getting trapped.
These can cause carbon build up in the combustion chamber. This means the same amount of air is working in a smaller amount of space.
TIP: Most gasoline has additives added to prevent carbon build up. Higher quality gasoline and oil can minimuze carbon deposits in your engine.
- Engine shaking or vibrating a lot
- Car jerks or surges when stopped
- Check engine light is on
- Car misfires when cold
What about smaller engines? Say, 2.0 liters or smaller? The compression is different, right? How much compression should a small engine have?
100 PSI cold and 90 PSI warm.
But again this is very general. At a minimum, an engine won’t rotate if it can get at least 90 PSI. The air and fuel won’t be able to provide enough umpf to move the cylinder.
Think if you are pushing a door shut with two hands and one hand breaks through the door. Will you have the same amount of power? Or will your pushing ability weaken? It is the same general idea with engines.
If you do a compression test on a cold engine you will not get accurate results. The engine is made of mostly metal. This metal expands when hot and this will affect any compression test. This thermal expansion will create a lower compression reading than when a warm engine is at a normal operating temperature.
That is not to say you will not see a problem. A cold start testing will still show if you have a blown head gasket. Any major problems will not be hidden by engine temperature though minor problems may be misinterpreted when not tested at normal operating temperatures.
So now we know what is a compression test. We know what to look for and how to deal with a possible serious issue if it may appear. Compression issues can be dealt with if you stay on top of your engine. Of course, older engines may have more frequent issues but they may be easier to diagnose and fix. You may be able to do tests without a computer!
But I have a diesel truck. I know they are different than gasoline engines. So, how much compression should a diesel engine have?
Diesel engines are a bit different. With a gasoline engine, the spark plug is the igniter and it lights up the gasoline. But in a diesel engine, there is no spark plug. A diesel engine instead uses a lot of pressure to create the spark or ignition. So this requires A LOT of pressure.
What is good compression on a diesel engine: In the ballpark of 275 to 400 PSI.
Bad compression in diesel engines is caused by similar causes in a gasoline engine; leaking valves or engine gaskets, bad piston rings, camshaft wear…
Engines are designed to perform within certain ranges. They have compression settings, temperature settings, and much more. Today, cars are very complicated and a lot can go wrong. Some are so overengineered they cannot be worked on without a computer.
The good thing is that if you practice preventative maintenance and monitor your car’s performance any problems with your car can be eliminated before they lead to bigger problems in the future.
So no you know what is a compression test and hopefully, you will get out there and perform it on your own car. After all, we all have a little gearhead in each of us.
Thanks for reading and stay dirty.