5 Causes of Low Compression in a Car Engine (and How to Fix)

Car engine compression refers to when air and gas are mixed together in the cylinders of an engine. This process is required for the car to move and function. If there are any problems with the compression process, then you can expect to experience all kinds of car problems.

It will be easy to tell when you have a low compression problem because you may experience a misfire when you try to start the engine. Either that or the engine will offer poor performance as you’re driving the vehicle down the road. The worst-case scenario would be the car not starting if all the cylinders have no compression.

Generally speaking, if you have low compression in one cylinder, the engine will start but you’ll likely experience misfires and your vehicle will run rough. If you experience no compression in ALL cylinders, your engine simply won’t start.

Top 5 Causes of Low Compression in a Car Engine

There are many reasons why low compression might exist in a car engine. Sometimes there will be low compression in just one cylinder of the engine and other times, low compression may exist in ALL cylinders.

You just have to understand the main possible causes of low compression in a car engine and then fix or replace whatever is damaged. Below are the top 5 causes of low compression in car engines.

#1 – Holes in Piston

hole in piston

You probably know that the cylinders of an engine have pistons in them. These pistons are typically made from aluminum alloy and are supposed to be able to handle the power of combustion.

However, if there is overheating in the engine, then hot spots will get onto the piston. After a while, these spots will burn holes right through the piston. Once that happens, gases will leak through these holes and cause low compression to take place.

#2 – Leaky Valves

The top of each cylinder contains exhaust valves and intake valves. Air and fuel go into the intake valve for the combustion process. The gases which form from this go out of the exhaust valve.

If these valves become overheated, they could start to leak gas prematurely out of them. Once this happens, you have low compression.

More commonly, the valve seals can over time become worn which allows the gasses to escape, thus causing cylinder compression to go down.

#3 – Worn Timing Belt

bad timing belt

There is a timing belt or chain in every engine which connects the camshaft and crankshaft together. If the timing belt were to be broken or damaged, the camshaft would no longer be able to turn anymore.

This means it cannot open or close the exhaust valve or intake valve properly. As a result, the combustion in the cylinders will be ruined and no gases can get released. So, you have low compression because of it.

#4 – Head Gasket Failure

bad head gasket

There is a gasket between the area on top of the engine where the cylinder head is connected. If for some reason the head gasket becomes faulty and starts to break, then it will leave a tiny hole in between the cylinder and its head.

This is known as a blown head gasket and causes the gases in the cylinder to leak out of the hole in the gasket. Then you will have low compression and a poor performance. If the head gasket fails between two cylinders, it can cause a compression leak in both.

#5 – Bad Piston Rings

symptoms of bad piston rings and valve seals

Overheating can cause the rings of the piston to brake or become damaged. This will cause carbon gases to leak through the rings because they are no longer able to seal them inside of the cylinder. As you’re probably aware by now, when there’s this type of leak, low compression results.

How to Fix Low Compression

The first thing you need to do is use a compression gauge and test to see if there is actually low compression in your engine. This process will usually take 45 minutes so make sure you have some free time on your hands.

If you don’t have a compression gauge, then you can either purchase one or take the car to an auto repair shop and have them test the compression for you. If they find there is low compression, the next step is to inspect the cylinder, piston, valves, and gasket to see if any of them are damaged or broken.

From there, you can replace whatever is damaged. However, this will be a lengthy and costly job because it involves taking out the engine. Be prepared for that.

Here’s a good video explaining how to properly perform a compression test:

 

Comments

  1. Hi, I have a VW Polo 9n3 1.6 Petrol engine. There’s low compression in 3 of the 4 cylinders and a heavy misfire with the car cutting off under low revs. Would it be recommended to overhaul the engine or replace it with a another engine instead. What’s the best feasible option.

    • Honestly I would just rebuild with rebuild kit rather than overhaul cause it cost cheaper and its more guaranteed than a used engine.

      • I am dealing with a 2011 ford E150 we found multiple misfire cilynder or code P0300, P0302, P0303, P0304, P0304, P0307. We checked and found low compression. Also lately I have been loosing antifreeze coolant t to the point that the reservoir becomes completely empty, my question is from the information I’ve been reading, is it possible that the head gasket is causing the problem? I’ve noticed antifreeze in one spot near the head gasket.

  2. Hi! It appears my Dodge Ram 1500 4.7L has one low Compression cylinder #8.
    Can the Rockauto Engine Rebuilt Kit be used without pulling the engine out of the car?

  3. hi, my 2002 Mazda b3000 6cyl 3.0 liter has been running rough so changed plugs wires , coil.Still ran rough. took in and had compression check. found #6 had really low compression.They say I may need a valve job or worse.Body is in great cond. so it’s going to be a tough decision whether to do this.Is it worth the cost?

  4. I had head gasket fail due to spark plug electrode failing out and damaging 2 exhaust valves. I replaced valves that were damaged and reseated all valves replaced oil stem seals and put new gasket on. Now will not start. Piston 1 has 75psi p2 80psi p3 30psi 4 75psi. Timing is correct. Strong fuel smell in sump. When head went was doing 70mph. It blown the heater core out and shot water into the sump. Sorted these out but won’t start. Stuff to turn by hand with no plugs in. Is it worth fixing engine is 24 years old. Is it rings oil test or wet test is still same psi. Thanks for reading.

  5. Tom, I have the same issue with my Cadillac CTS 05. #1 has %95 leak. If I open oil cap I can feel lost compression escaping. to do farther diagnosis they have to tear down the engine. With the mileage I have in it 255k I am coming to the point to take tough decision. The car still drive strong but it wont pass inspection that’s due in couple weeks.

  6. I have a 2005 BMW torqua..stopped running cranks but no start. Shop said no compression to cylinders. Is the engine blown

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