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26 June 2024 How to check the compression in a diesel engine
Is your engine acting up and losing power? It could be due to a loss of compression.
What is an engine compression test?
A compression test is intended to determine how much compression there is in an engine cylinder using a pressure gauge. Why should you check this? Compression plays an important role in the ignition process of internal combustion engines as it generates the high temperatures needed to ignite the fuel. High pressure is created by the downward and upward strokes of the car pistons inside the cylinders.
If there’s a loss of pressure in one or more cylinders of your diesel engine, the ignition system will therefore be seriously impacted, leading to misfires, power losses, poor acceleration, and more. Normal readings for a diesel engine typically range from 300 to 500 PSI depending on the motor. Other signs you need to perform an engine compression test may include excessively high crankcase pressure, starting problems that aren’t related to outdoor temperatures, and increased fuel or oil consumption.
What’s the compression ratio of a diesel engine?
Diesel engines typically have a high compression ratio of between 15:1 and 20:1, whereas petrol engines have a ratio ranging between 8:1 and 10:1. This is because diesel motors rely primarily on compression for ignition as they don’t use spark plugs.
If you aren’t sure what the compression ratio is, it’s basically the ratio of the volume of the cylinder and its head space when the piston is at the bottom of its stroke (largest volume) to the volume when the piston is at the top of its stroke (smallest volume).
How to do a compression test
Remember to follow safety precautions and specific instructions outlined in the vehicle's repair manual. Below is an outline of the steps for testing the engine pressure:
Tools needed:
- Compression tester kit (incl. Special adapter for diesel engines)
- Protective gloves
- Wrench or socket set - spark plug wrench or tool for glow plugs
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- Preparation
Park the vehicle on a level surface and switch the engine off.
Remove any components necessary to access the spark plugs or injector nozzles.
- Disable the ignition and fuel system
This usually involves disconnecting the ignition coil or fuel pump fuse.
- Remove the spark plugs (petrol only)
Unscrew the car spark plugs using the spark plug wrench and socket. Remove them from the port.
For diesel engines, you may need to remove the glow plugs or fuel injectors.
- Connect the compression gauge and adapter
Follow the product instructions carefully.
- Start testing
With the throttle fully open (for petrol engines) or the fuel system disabled (for diesel engines), crank the engine for several seconds. Note down the highest reading for each cylinder in the engine. Compare the PSI readings with those specified for the specific engine model. If it’s excessively high or too low, there is a fault that needs to be fixed.
- Reassemble the parts
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What causes low compression?
- Blown cylinder head gasket –
The head gasket connects the cylinders to the engine. If it gets damaged due to thermal or chemical damage or wear, there may be a gap or hole where air can leak out.
- Timing belt problems –
The belt is responsible for turning the camshaft and crankshaft at precise times. However, a worn or damaged timing belt may not be able to drive the camshaft properly. This may mean that the valves take too long to open or close, leaving room for the gases to escape and affecting ignition.
- Damaged pistons –
While pistons are designed to withstand high temperatures, they can become weaker over time. If the engine overheats, it may burn holes in the pistons, causing a decrease in pressure in the combustion chambers.
- Leaky valves –
Each engine cylinder has intake and exhaust valves to let fuel and air in and release the exhaust gases produced during combustion. Sometimes, these valves can be worn out or contaminated causing them to stick open and allow the pressure to escape. You may need to install a new valve retainer spring or clean the car inlet valves.
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