car compression test

Low Compression in a Car Engine – Causes & Fixes

In Engine by Magnus Sellén7 Comments

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 are instances when you will experience low compression in one of the cylinders. This causes misfires and problems accelerating. When you have low compression in 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

low compression car engine

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 into 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 seal the combustion pressure before its released into the exhaust. If you have a leak on the valves or the valve seats, 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.

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

3. Wrong camshaft timing

If you experience low compression 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 get 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 find out if 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 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 improved or not.

How to diagnose low compression

compression tester

There are a few methods 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 in the oil, there are basically two reasons for this. The first one is that if your car is 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 for the compression leaking out in the intake, exhaust or into the crankcase ventilation.
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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.

7 thoughts on “ Low Compression in a Car Engine – Causes & Fixes ”

  1. So I have this 09 toyota corrolla.
    Since I bought it almost 2 years ago Ive always had an issue with the check engine light.

    When the light comes on it’s usually around the time the car begins to misfire. And sure enough its a code for a cylinder.

    Not I’ve neglected the car by only switching out the coil the code called for.

    And that has been my method everytime the light comes on. To be cheap and just down right bad thinking. So this has happened 3 or 4 times

    Well fast forward to today. The light is on again. This time the one coil method didnt work. So instead I changed all 4 spark plugs…

    And the light wont go off. And the car has been driving horrible.

    Well I manged to then switch two coils.

    And the car doesnt jump or run rough like it did before. . before it was also hesitating.

    Well I had two street mechanics look at it. And well… I’m not sure about their credibility but they both determined that my engine is blown.

    But I’m convienced that it probably needs a tuneup. Ive driven it across the country 3x and used it for deliveries.

    Perhaps I’m in denial but I dont believe my engine is blown. Given how it improved a bit from the two coils i replaced and new spark plugs….

    But if it is I’m trying to figure out what tue cause was and what is my best route to take. Replace the engine? Repair the engine? Or just sell it as is…

    I could use your expertise. Thank you!

  2. Toyota corolla…the two outside holes have 170 lbs comp. The two inside holes have 90 lbs what could be the problem

  3. My car is 03 Toyota Camry V6. The engine of the car goes low after 15minutes drive. Also a little vibration when is not moving. The car stalls, movement not sharp and a burning smell after. Please what could be the cause?

  4. I ones took my Opel Zafira 1.8,2008 model for service and was told the compression is low on cylinder no3.The car was losing power and sometimes indicates that oil needs to be filled.

    My question is,can this be the cause of a cylinder head,because the guy said the head is over skeemed,but there were no signs of overheating.

  5. I keep calling mechanics asking if timing belt cause low compression and want it checked BUT they all say no timing belt not affect compression. How can I get a mechanic to check the timing belt before my engine quits working?

  6. I have a 1980 Ford camper van E 250 400 6.6 engine. Starts good runs good until it comes to a grade , really works to get up hills. It’s compression is 85 psi to 95 psi. No smoking, do you think it’s rings, valves, or timing.

  7. Hi guys l have a old 90s ford bantum 1.3 b3 engine is losing power when l engage number 2 gear and it’s making a knocking sound in the engine what could be the problem it doesn’t smoke

  8. Due to very high demand and a high amount of comments, you might have trouble getting your comment answered by me. If you want to get fast answers from a certified automotive technician you should ask your questions here: Ask A Mechanic

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