Engine Diagnosis: Engine Compression
An engine runs right only when every cylinder squeezes air and fuel just right. If one cylinder pushes out less power, the engine can jerk or lose strength—telling you something’s off. …
An engine runs right only when every cylinder squeezes air and fuel just right. If one cylinder pushes out less power, the engine can jerk or lose strength—telling you something’s off. Most folks don’t get that keeping all cylinders balanced in their squeeze is key to smooth rides. I found this out when my car started coughing and stalling—just a quick compression check showed me exactly what was wrong. Fun fact: Testing compression often can catch trouble early, saving you big bucks on repairs later. Think of it as a checkup for your engine—keeping things tight helps it last longer and use less gas. Stick around, and you’ll see how simple it is to make sure your engine’s cylinders work together like a well-tuned band.
This depends on the mixture of air and fuel thus maximizing the energy produced, this occurs when the pistons in their ascending stroke compress the mixture of air and fuel in the combustion chamber, if there were a leak in part of that mixture of air / fuel would result in excessive fuel consumption and at the same time a loss of power.
Symptoms to identify compression problems
When an engine has problems with compression, some or several of these problems may occur:
- Excessive smoke of any color
- Due to the lack of power, it is necessary to accelerate more than normal.
- Increased fuel consumption
- When the car is stopped, the revolutions are very variable.
- Problems with starting the car
- Frequently shuts off
- Finally, you may consume more water or coolant than usual.
Causes or engine compression problems
The cause of a compression failure is that the combustion chamber is not well sealed and/or there are leaks, which can be caused by the following:
- Spark plugs: They can be misplaced, not adjusted or with damaged threads.
- Valves or springs: A damaged valve or spring does not allow good seating at the top of the engine, allowing leaks. Burned, worn or scratched valves and springs are often the cause.
- Rings: If the piston rings are loose in the piston or piston liner, leakage may occur.
- Gasket: It could be damaged, badly adjusted, badly fixed or badly selected which will allow leaks.
- Block: If it has cracks in any of the cylinders then there could be some leakage.
- Bad synchronization: if it is not the correct one, they may not seat the valves correctly or not do it on time, this is usually due to a worn band.
Procedure for measuring compression
The compression of the engine must be done with a manometer, this check gives good information on the state of the engine. In order to measure the compression the following steps must be followed:
- Reach the normal operating temperature of the engine.
- Remove the spark plug wires.
- Remove some of the spark plugs and place the pressure gauge taking care that it covers all the hole where the spark plug goes.
- Start the engine for a few seconds with full throttle, i.e. start the engine normally. Consider that the car will not start.
- Write down the pressure registered in the manometer.
- Replace all parts that have been removed.
The pressure given in each cylinder must be very similar between them and the variation must not be more than 10%. This measurement must coincide with the range indicated by the car manufacturer.
Conversions
In case you need to convert Bars to Psi it is necessary to multiply the bars by 14.5038. As an example, if we have 5 Bars, we multiply 5 x 14.5038 = 72.519 Psi.
Otherwise, to convert from Psi to Bars, multiply the Psi by 0.068947. As an example, if we have 100 psi, we multiply 100 x 0.068947 = 6.8947 Bars.
Results – Problems with compression
As mentioned above, the difference between the reading of one piston and another should not exceed 10%. If the difference is greater the diagnosis can be:
A low compression reading of all the cylinders can mean that the cylinders are “washed”, that is to say that the engine has been injected with a lot of fuel and this has eliminated the oil from the walls.
The oil is very necessary as it creates a sealing effect between the piston, the rings and the cylinder walls. Faced with a lack of oil, the compression of the engine escapes into the crankcase.
If the smoke coming out of the vehicle is bluish and there is normal operation but sometimes loses power, it means that the rings are worn. To check this, you can put a small amount of oil in each cylinder and repeat the test.
If the compression increases significantly, the problem is the rings. If the reading has no major variation, it is necessary to check the camshaft and crankshaft of the engine and it is recommended to check the chain or timing belt.
If the compression reading is very low or zero on a cylinder it is likely that there is damage such as:
- Having a hole in the piston due to a broken connecting rod
- Leaking or stuck valve
- Broken valve spring or bent rod
- Excessive wear of the camshaft which does not give the necessary opening to the valve
- Low or no compression on two adjacent cylinders may indicate that the gasket is not working or is damaged by improper adjustment This may also indicate that the camshaft is damaged in the area between the two adjacent cylinder valves.
The accumulation of carbon can cause the compression of one cylinder to come out much higher than another, this can be corrected by removing the head of the engine and performing a deep cleaning at the top of the piston and in the opening area of the valves.