How To: Compression Test
- Ratchet wrench.
- 5/8'' spark plug Socket.
- Extensions for the ratchet wrench.
- Motor oil (for the ‘Wet’ compression test part).
- Spark plug wire puller.
- Compression Tester
Disable the fuel system. You can easily do this by simply:
Removing the fuse labeled INJ 1, which is a 15 A fuse in the underhood fuse block.
Or disconnecting the fuel injectors from their electrical connectors.
Or, disconnecting the fuel pump relay.
This step is important, so don't skip it.
Disable the ignition system. You can easily do this by simply:
Removing the fuse labeled INJ 1, which is a 15 A fuse in the underhood fuse block (this fuse feeds the fuel injectors and the COP coils with power).
Or disconnect the COP coils from their electrical connectors.
This step is important since it will prevent damage to the ignition coils, so don't skip it.
Remove all 8 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 plug’s ceramic insulator to break, and this will cause a misfire!
Install the compression tester. Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder. Hand tighten the compression gauge only! Do not use any type of tool to get it tight.
When ready, crank the engine... as you observe the needle 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. Record this compression reading on a piece of paper. Include the number of the cylinder this reading belongs to. Now repeat steps 1 thru' 6 on the other cylinders.
Interpret the results. After testing all cylinders and having written down all of your compression test readings, now you need to interpret the results...
Interpreting The Results Of The Engine Compression Test
CASE 1: If you got a reading of 100 PSI or less (less being 0 PSI) on all of the cylinders you tested, you've got serious engine mechanical problems.
It's absolutely rare for the engine to get to the point of 0 PSI compression on all cylinders, but if it were to, this would usually means a broken timing chain.
What is common, if you have a very high mileage engine... is low compression across the board. Low usually means anything under 120 PSI (although the service manual says 100 PSI is the minimum). If your engine has reached this point, it's also smoking from the quart of oil it's burning every few days. The other symptoms you'll see is that the idle will be very rough.
What is NOT normal if the pressures vary by 15% or more. That's right, the individual cylinder compression readings of each engine cylinder can not vary more than 15% and this is how you can find out:
- Grab a calculator and multiply the highest compression reading that you recorded by .15.
Let's use the following compression readings to explain the point:
- Cylinder #1 175 PSI.
- Cylinder #2 160 PSI.
- Cylinder #3 165 PSI .
- Cylinder #4 95 PSI.
- Cylinder #5 160 PSI.
- Cylinder #6 165 PSI .
- Cylinder #7 170 PSI .
- Cylinder #8 165 PSI .
- The next step is to do the math: 175 x .15= 26, 175-26= 149.
- So then, 149 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.
- This means that cylinder #4 is the one causing the misfire
The next step is to do a ‘Wet’ compression test on the dead or low compression cylinder.
A ‘Wet’ compression test will help you to find out 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.
The ‘Wet’ compression test is done exactly the same as the ‘Wet’ compression test, the only major and significant change is that you'll add a small amount of engine oil (about 1-2 teaspoons) to the cylinder, that had low compression, to create temporarily seal.
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.
If you're wondering why the compression value shot up, it's because the engine oil that you added (to this low compression cylinder) helped the piston compression rings seal the compression within the cylinder. The end result being that your compression test gauge registered a higher compression value.
CASE 2: The compression value stayed the same.. This confirms that the problem is in the cylinder head valves.
No, if you're wondering why this means that the cylinder head valves are worn, here's the answer: It's due to the fact that engine oil can not help worn cylinder head valves seal the compression within the cylinder. So, if the compression value did not shoot up (from the result obtained in TEST 1), then you can conclude that the problem is in the cylinder head valves of that dead cylinder.