‘Wet’ Engine Compression Test
If you have found a cylinder with no or little compression, then the next step is to do a ‘Wet’ compression test. In a ‘Wet’ compression test you're gonna' add about two tablespoons of oil to the engine cylinder that showed a low compression reading.
The engine oil that you're gonna' add to the cylinder will help determine if the low cylinder pressure or pressures you recorded in the ‘Dry’ compression test are caused by worn piston rings or worn cylinder head valves.
Depending on whether the compression pressure rises (on your compression tester) or not, you'll be able to say that the problem lies in the piston's rings or in the cylinder head valves.
OK, this is what you need to do:
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.
Install the compression tester onto the cylinder.
- Do not use any type of tool to tightened the compression tester... hand tight is fine.
When all is set up, have your helper crank the engine.
- You'll get one of two results, either the compression value will go up (from the one you recorded before) or it will stay the same.
Let's take a look at what your test results mean:
CASE 1: The compression value shot up.. This tells you that the piston compression rings are worn out and thus the problem is in the bottom end (block) of the engine in your 2.3L/2.4L Quad 4 equipped GM vehicle.
To go into a little more detail: The reason the compression shot up is that the oil you added helped the piston rings (of the dead cylinder) to start sealing again. This effect doesn't last too long though, because as soon as you put everything back together and start the engine and the engine burns the oil... the compression will go back down again.
CASE 2: The compression value stayed the same.. This confirms that the problem is in the cylinder head valves.
To add a little more info, the reason why the oil made no effect on the compression value is that no amount of oil will help the valve seal better (agains its seat on the cylinder head). So, with this test result, you have indirectly confirmed that the low or no compression value is due to a bad or bent valve.
Why an Engine Compression Test?
This is one of the most overlooked tests to find the root cause of a misfire code, rough idle or an engine miss or a blown head gasket.
Over the years, I have solved many unsolvable misfire codes, rough idle, lack of power issues by doing a simple engine compression test and if you're faced with something similar... I highly recommend doing an engine compression test.
Which Compression Tester Should I Buy?
There are lot of engine compression testers to choose from and many places to buy them. I'm gonna' make two recommendations to you:
1) Which one to buy: The Engine Compression Tester that I have always used is the Actron CP7827 Compression Tester Kit My only complaint about this engine compression tester is that it does not come with a case to store it in.
Engine Compression Gauge Testers
2) Where to buy: You can buy an engine compression tester in any auto parts store in any neighborhood, in any city... but you'll be paying at least twice as much. Go to the above compression tester links, browse and compare, you'll see a big price difference!
Related Test Articles
You can find more 2.4L GM test articles by going here: GM 2.4L Index of Articles and at:
- How To Test For A Blown Head Gasket (GM 2.4L Quad 4).
- How To Test The Fuel Injectors (GM 2.4L Quad 4).
- How To Test The MAP Sensor (GM 2.4L Quad 4).
- GM 2.4L (Quad 4) Index of Articles (at: easyautodiagnostics.com).