How To Test Engine Compression (2006-2012 2.4L Ecotec)

As your car's 2.4L Ecotec engine starts to age (and accumulate miles), the piston rings and the cylinder head valves start to wear. Eventually this wear and tear will cause the cylinders to produce low or no compression. The end result is a misfire.

Or the engine could suffer some sort of catastrophic failure and provoke all 4 cylinders to not produce compression. The end result is an engine that 'cranks but does not start'.

Thankfully, the compression test can reveal the cylinder or cylinders with low or no compression. And let me tell you, testing the compression on the 2.4L Ecotec engines is not hard, especially because the spark plugs (which need to be removed) are very accessible. In this tutorial, I'll explain how to do and interpret a compression test.

Symptoms Of Low Or No Engine Compression

As stated at the beginning of this tutorial, engine compression problems usually fall into one of two categories. Either all four cylinders have no compression and the engine doesn't start, or only one cylinder has low compression and the engine starts but with a misfire that's illuminating the check engine light (with a misfire trouble code).

Let's dissect each type of problem below:

Engine starts but runs with a misfire (rough idle):

1. Also known as an engine miss, rough idle condition.
   • Usually caused by very low compression in one cylinder or...
   • Uneven engine compression that varies more than 15% across all 4 cylinders.
2. Check engine light on with misfire codes:
   • P0300 Random Cylinder Misfire.
   • P0301 Cylinder #1 Misfire.
   • P0302 Cylinder #2 Misfire.
   • P0303 Cylinder #3 Misfire.
   • P0304 Cylinder #4 Misfire.
3. Bad gas mileage.
   • This is caused engine not producing its optimal power output since the air/fuel mixture isn't being combusted with adequate compression pressure.
4. Engine pollutes more.
   • If the air/fuel mixture isn't compressed within a certain range, it won't burn optimally and produce more unburned hydrocarbons escaping into the exhaust.

Your 2.4L Ecotec equipped car won't start:

This usually is caused by having 2 or all 4 cylinders with no compression. When this happens, you'll see:

1. The engine cranks very fast.
   • This fast cranking speed is very noticeable.
2. The Ignition System is sparking all 4 spark plugs.
   • This tells you that the no-start condition is not caused by a fault in the ignition system.
3. The fuel injectors spray fuel.
   • You can confirm this with a Noid Light test.
   • Also, you can confirm this, although indirectly, by removing the spark plugs and checking to see if they are fuel soaked (fuel fouled).
4. Fuel pump is working and providing pressure.
5. The most common causes of no compression on 2 or all 4 cylinders are:
   • Blown head gasket.
   • Broken timing chain.
   • Engine threw a rod.

Alright, let's get testing.

Where To Buy A Compression Tester And Save

There are lot of engine compression testers to choose from and many places to buy them. I'm gonna' make some recommendations to you:

TEST 1: Finding The Dead Cylinders

OK, to get started, we're going to test the compression of all four cylinders. In case you're only wanting to test just one cylinder, I still recommend that you test all four cylinders.

Why test them all? Because by testing all four cylinders we'll be able to find out which cylinder has the lowest compression value and which one has the highest. With this information we can then find out if the lowest compression reading you got is causing a problem.

If you don't own a compression tester and need help deciding which one to buy, take a look at my recommendations here: Which Compression Tester Should I Buy?

IMPORTANT: You'll be working around a cranking engine, so you have to be careful and stay alert at all times. Think safety all of the time!

This is what you'll need to do:

1. 1

   Disconnect all of the fuel injectors. This will prevent fuel from being injected into the cylinders as you crank the engine.

2. 2

   Remove all four spark plugs.
   
   NOTE: Don't remove the spark plugs from a hot engine! If the engine has been running for any length of time, let it cool down completely before removing them.
   
   As you're taking them out, be careful and don't drop any of them on the floor, or you could cause the spark plugs ceramic insulator to break, and this will cause a misfire!

3. 3

   Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder (this is the spark plug hole closest to the drive belt).
   
   Hand tighten the compression gauge only! Do not use any type of tool to get it tight.

4. 4

   When the tester is set up, ask your helper to crank the engine. Your job is to keep your eye on the compression tester's gauge.

5. 5

   Once the needle on the gauge stops climbing, have your helper stop cranking the engine.

6. 6

   Write down the compression value on a piece of paper. Include the number of the cylinder this reading belongs to.

7. 7

   Now repeat steps 3 thru' 6 on the other 3 cylinders.

Let's take a look at what your test results mean:

CASE 1: All 4 cylinders show a 0 PSI compression value on the compression tester. This test result tells you that you've got some serious internal engine problems.

The most common issues that cause a 'no compression' test result are:

• Blown head gasket.
  • To further test this, I recommend the following tutorial: How To Test For a Blown Head Gasket (GM 2.4L Quad 4) (the info in this tutorial applies to your 2.4L Ecotec equipped vehicle).
• Broken timing chain.
• Engine threw a rod.

CASE 2: All cylinders have compression but the readings are not similar. There's a chance that the cylinder or cylinders with the low compression reading are causing a misfire issue.

To find out if the low compression value is causing a problem go to: Interpreting Your Compression Test Results.

Interpreting Your Compression Test Results

If your engine compression test results show one or more cylinders with low compression values, the next step is to find out they're causing a problem.

To find out if the lower compression value is causing an engine performance problem, you need to find out if it's lower than 15% of the highest compression value you got.

You can do this (figuring out the 15%) in one of two ways: You can calculate this 15% difference with pen and paper or you can use my low compression calculator. You can find the low compression calculator here: Online Low Engine Compression Calculator (at: easyautodiagnostics.com).

If you want to manually calculate the 15% difference, here's what you'll need to do:

• STEP 1: Multiply the highest compression value by 0.15 (this is the decimal value of 15%).
• STEP 2: Round the result to the nearest one (for example: 25.6 would become 26).
• STEP 3: Subtract the result (the number that was rounded) from the highest compression value.
• ANSWER: The result of this subtraction is the lowest possible compression value any cylinder can have.

Now, let me give you a more specific example: Let's say that I got the following compression readings:

Cylinder	Pressure
#1	165 PSI
#2	95 PSI
#3	155 PSI
#4	175 PSI

My next step is to do the following calculation:

• STEP 1:  175 x 0.15 = 26.25.
• STEP 2:  26.25 = 26 (rounded to nearest one).
• STEP 3:  175 - 26 = 149.
• ANSWER:  149 PSI. Any cylinder with this compression (or lower) value will misfire.

Since cylinder #2 is only producing 95 PSI, I can now conclude that it's 'dead' and causing a misfire.

To find out if the lowest compression value you got from your engine compression test is within a good range, you'll need to do the same calculation. Of course, you'll need to use the highest compression value you got and not the one in the example.

Once you've found the 'dead' cylinder, the next step is to find out what's causing the low compression value. For this step, go to: TEST 2: 'Wet' Engine Compression Test.

TEST 2: 'Wet' Engine Compression Test

Once you've found the cylinder or cylinders with no or low engine compression, the next step is to do a wet compression test on them.

A wet compression test simply involves adding about 2 tablespoons of oil the cylinder and checking its compression again.

Let me explain, low or no engine compression in a cylinder is usually do to a problem with its piston rings or its cylinder head valves.

By adding two tablespoons of engine oil (to the low or no compression cylinder), we'll be able to find out if the problem is due to bad piston rings or bad cylinder head valve valves.

If the problem is due to bad piston rings, the engine oil will help to compress the air in the cylinder and the compression value on your test gauge will go up.

But if the problem is in the cylinder head valve, the oil will not be able to help them seal the air being compressed and the reading on your test gauge will remain the same as the reading from TEST 1.

OK, this is what you need to do:

1. 1

   Add a small amount of engine oil to the cylinder that reported low compression or no compression in the 'Dry' compression test.
   
   The amount should be about 1 to 2 tablespoons of oil.

2. 2

   Install the compression tester onto the cylinder.
   
   Do not use any type of tool to tightened the compression tester. Hand tight is fine.

3. 3

   When all is set up, have your helper crank the engine.

4. 4

   You'll get one of two results:
   
   1.) The compression value will go up (from the one you recorded before).
   
   2.) The compression value will stay the same.

CASE 1: The compression value, of the cylinder you added oil to, went up. This tells you that the problem causing the low or no compression value is bad piston rings on this specific cylinder.

CASE 2: The compression value, of the cylinder you added oil to, DID NOT go up. This test results tells you that the problem causing the low or no compression reading in TEST 1 is due to bad cylinder head valves.