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.
Contents of this tutorial:
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):
- 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.
- 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.
- 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.
- 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:
- The engine cranks very fast.
- This fast cranking speed is very noticeable.
- 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.
- 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).
- Fuel pump is working and providing pressure.
- 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.
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:
If the engine starts, let it warm up for about 10 to 15 minutes, before you start this test (but don't let it get hot!).
NOTE: If your 2.4L Ecotec equipped car doesn't start, then don't worry about the engine being warm.
Disconnect all of the fuel injectors. This will prevent fuel from being injected into the cylinders as you crank the engine.
Remove all four spark plugs.
As your 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!
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.
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. Once the needle on the gauge stops climbing, have your helper stop cranking the engine.
Write down the compression value on a piece of paper. Include the number of the cylinder this reading belongs to. Now repeat steps 1 thru' 6 on the other 4 cylinders.
After checking all 4 cylinders' compression, the next step is to interpret the results
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.
Our next step is to find out if the low reading is varying by more than 15% than the other cylinders (because if it is varying by more than 15%, then this cylinder is considered ‘dead’), this is how you can find out:
- Grab a calculator and multiply the highest compression reading that you recorded by 0.15.
- So, let's say that cylinder #4 gave you the highest reading of 170 PSI. Well 170 X 0.15 gives you 26 (25.5 rounded off).
- Now, the next step is to subtract 26 from 170, which gives us 144 PSI.
- So then, 144 PSI is the lowest possible compression reading that any one of the rest of the engine cylinders can have. Any compression reading below this and that engine cylinder will misfire.
Now, so that this calculation can make more sense to you, let's say that my 2.4L Ecotec equipped car gave me the following compression readings:
- Cylinder #1 165 PSI.
- Cylinder #2 170 PSI.
- Cylinder #3 175 PSI.
- Cylinder #4 30 PSI.
The next step is to do the math: 175 x 0.15= 26, 175-26= 149. So, now I know that cylinder #4 is the one causing the misfire!!
Now that you've found the cylinder with the low compression, the next step is to see if this low compression is caused by worn cylinder head valves or worn piston rings. For this test, go to: ‘Wet’ Engine Compression Test.