How Much Compression Do I Need?

Most Yanmars will leave the factory with around 500-600psi compression when they leave the factory. The engine will get hard to start when you get close to 400psi. Compare that to the average gasoline engine in your car or lawnmower which has only around 150psi. 

Why does a diesel Yanmar engine have so much compression? Diesel engines, in general, require much higher combustion pressures because they do not have spark plugs to ignite the fuel and air mixture. Diesel engines use extreme pressure to kick off the explosion. In fact if you google 'fire piston' there are some neat videos showing this process happening in a glass tube using just a little diesel fuel and a ball of cotton. 

So what causes low compression? There are really two common culprits. Catastrophic damage and wear. 

On the damage side- the most common causes are using starting fluid and overheating. Starting fluid is a great tool to help start stubborn diesel engines found in much larger machines but small diesel engines just can not handle the extreme forces created when starting fluid is ignited. You will typically see instant damage like a piston ring grove blown off the piston.... which causes low compression. This starts the vicious cycle of now needing more starting fluid to get the engine to start since it has lower compression and each 'hit' of the starting fluid causes more damage and further lowers the compression. 

The second biggest cause of damage is overheating. Overheating causes the piston to expand in the sleeve or block and creates accelerated wear on the piston rings. Excessive heat can also cause the piston rings to lose their spring tension and stop sealing correctly. 

Wear is a big cause of low compression and can happen slowly over decades of regular use or very quickly if an intake system leaks and sucks in a lot of dirt. As an air filter starts to get clogged the suction of the engine starts to try to pull in air from other sources like around the seals of the air filter or intake hoses. If there is a tiny leak in any of those places then unfiltered air is allowed to enter the engine. Unfiltered air often contains dirt which acts as sandpaper and quickly erodes the cylinders and piston rings. We have seen a good engine become ruined in under an hour when a customer was mowing with a hole in their dry-rotted intake hose. 

 How do you test engine compression? Take a look at this article