Low Compression in a Car Engine – Causes & Fixes

For the car engine to operate smoothly there has to be the right amount of air and fuel mixture.

This is compressed then ignited by the spark plugs. The resultant pressure moves the pistons and crankshaft.

However, there instances you will experience low compression in one of the cylinders. This causes misfires and problems accelerating. When you have low compression on all the cylinders, the car will not start. Low compression is a problem you need to resolve fast to prevent further engine trouble. But, what are the causes of low engine compression?

Reasons for low compression in your car engine

There are several different reasons that could cause low compression in your car. 

1. Worn pistons/rings

The most common cause of low compression is worn pistons or piston rings. Anytime the pistons or their corresponding rings have leaks air will leak from the crankcase into the compression cylinders. A sure way to determine that it is the pistons with issues is to pour some engine oil to the spark plug hole and then check for a rise in the compression.

Pistons are often made from aluminum alloy and are sturdy enough to withstand the high engine temperatures. However, with time they develop hot spots. These hot spots soon burn holes on the pistons causing gases to leak into the combustion chamber. The result is low compression.

2. Faulty Valves/Valve Seats

The valves are sealing the combustion pressure before its releasing it into the exhaust. If you have a leak on the valves or the valve seats, it the compression might leak out into your exhaust or the intake. You will need a leak-down-tester to identify any gas leakages from the valves in the air inlet manifold or exhaust pipe. You can then identify which of the two is leaking and replace the valves and fix the seats.

If the valves are okay you may need to check your decrepit timing belt. 

3. Wrong Camshaft timing

If you experience low compressions on all cylinders, it may be caused by a faulty fitted camshaft timing belt or chain. If you know that you got low compression after a timing belt replacement, check the timing. Anyways, if you experience low compression on all cylinders, check the timing.

These are found between the crankshaft and camshaft. Valves control the outflow of exhaust gases from the combustion chambers. When the belt or chain is faulty then the camshaft fails to perform its task and you have an accumulation of exhaust gases in the combustion chambers. This will eventually lead to low compression problems.

4. Worn out Head Gasket

Fuel and oil should never mix in the combustion chambers. However, the gasket can become cracked due to overuse. When this happens, pressure leaks from the chambers and you have low compression. You can use a pressure tester to read the cylinder pressure.

5. Cracked cylinder walls

The combustion cylinders need to be airtight to promote efficient air/fuel combustion. The high temperatures can cause some of the cylinder walls to crack. When this happens you will experience low compression. You can know that your cylinder walls are cracked by looking out for bubbles in the radiator. This is a sign that air is leaking from the cylinders to the coolant.

6. Cleaned cylinder walls

If your car was running without combustion on one cylinder for a while, the petrol might have washed away the oil from the cylinder walls and this might cause a lower compression on that specific cylinder. You can pour in a small amount of oil into the affected cylinder and then make a new measurement of the compression to check if it did improve or not.

How to Diagnose low compression

There are a few ways which you can use to find out where the problem is when it comes to low compression.

1. If you get low compression on all cylinders – check the timing belt and chain.
2. If you experience low compression on one cylinder – Try pouring in some oil in the affected cylinder to see if the results get better. If you get a higher pressure after pouring the oil, there are basically two reasons for this. The first one is that if your car running for a while with misfires, the fuel might wash the cylinder walls which cause the low compression. The second reason is that the piston rings are sealing poorly or were stuck. In some cases, this can fix the problem but in most cases, you have to replace the piston rings.
3. Remove the Oil Cap. Remove the oil cap while the engine is running, if you can feel a lot of overpressures there together with smoke, the compression is leaking through the pistons down into the crankcase and you might have to replace the piston or piston rings or repair a crack somewhere. There should be a slight underpressure in the crankcase at idle if everything is working properly.
4. Use a Leak-Down-Tester to check where the compression is leaking. Make sure that the camshaft is at the timing when both valves are closed, then put pressure into the cylinder and listen if you could hear the compression to leak out in the intake, exhaust or into the crankcase ventilation.

Conclusion

Having a low compression ratio is not good for your car. It means you will have more misfires from one cylinder. If there is no compression then your car will not start. The timing belt connects the crankshaft and camshaft. When it is loose or broken then the intake and exhaust valves will not open.

Most of the timing belts are made from rubber and you will need to replace one after every 110, 000 miles. The other case for low compression ratios is when you have worn out piston rings. Pistons have rings at the top and bottom. The rings also prevent oil from getting into the cylinders.

Pistons rings wear out with time and you will need to replace them whenever you start experiencing low compression ratios.

Magnus Sellén

Hello I'm Magnus, the owner and the writer of this website. I have been working with cars since I was 16 and I'm specialized with in-depth Automotive diagnostics. Also been driving drifting for the last 6 years. I'm here to give you answers to all your automotive questions and I hope that you enjoy our content.