Testing the compression of your 2.6L Isuzu is not hard to do. In this 'How To' article, I'll show you how to do it and more importantly, how to interpret the results of the test.
Why do an engine compression test? Well, it will help you in two specific scenarios:
- A hard to diagnose engine miss (misfire).
- You might have already replaced a ton of stuff (spark plugs, wires, fuel injectors, etc) and the engine is still missing... and the problem may just be a cylinder with very low compression.
- A ‘cranks but does not start’ condition.
- An engine with no compression will not start.
OK, to make it as easy as possible to navigate this article, here are its contents at a quick glance:
- Important Tips And Suggestions.
- TEST 1: Dry Compression Test.
- Interpreting The Results Of The Engine Compression Test.
- TEST 2: Wet Compression Test.
- Why an Engine Compression Test?
- Which Compression Tester Should I Buy?
- Related Test Articles.
Important Tips And Suggestions
TIP 1: Engine temperature is an important factor in the getting the most accurate result from your compression test (if your vehicle starts and runs).
I suggest that you perform the compression test with a slightly warmed up engine. My recommendation is to let a completely cold engine run for about 15 minutes and no more.
TIP 2: If your vehicle DOES NOT start and run... the above suggestion does not apply to you.
TIP 3: Take all necessary safety precautions as you do the compression test, since the test has to be done with the engine cranking. Your safety is your responsibility... so use common sense and think safety all of the time.
TIP 4: All of the spark plugs need to be removed from the engine for the compression test and this must never be done with a hot engine.
Why? The spark plug threads in the cylinder head can easily get damaged (as in getting stripped) and this is a nightmare you do not want to experience.
TEST 1: Dry Compression Test
OK, we're ready to get this show on the road...
Before I forget, let me tell you that you should test each cylinder twice before moving on to test the next one.
Also, it's important that you write down what the compression value is for every cylinder. The illustration on the right will help you to identify the number of each cylinder.
OK, this is what you'll need to do:
Remove the spark plugs from a slightly warmed up engine (if it starts and runs). Remember, the engine can not be hot.
When removing the spark plugs, be careful not to drop any of them on the floor, or you run the risk of having the spark plugs porcelain insulator crack and then you'll have a misfire on your hands.
If the engine does not start... don't worry about it being warmed up.
Disable the fuel system by removing the fuel pump fuse. Disabling the fuel system will prevent fuel from being injected into each cylinder when the Test is performed.
Disable the ignition system by disconnecting the distributor's electrical connectors.
Disabling the ignition system is also very important, since it'll prevent damage to the ignition coil when the engine is being cranked.
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.
Have your helper crank the engine till the needle on the compression gauge stops climbing.
Now, record on paper the value at which the needle stopped and the number of the engine cylinder on a piece of paper. Release the pressure on the gauge and repeat this step one more time.
Repeat this test step on the remaining 3 cylinders.
Let's take a look at what your test results mean:
CASE 1: If Your Car Does Not Start and the results of the engine compression test are 0 PSI on all 4 cylinders then you have one of the following conditions:
- Broken timing belt.
- Blown head gasket.
- Blown engine.
Any compression value below 100 PSI (even if it does not 0 PSI) means internal mechanical engine trouble.
CASE 2: If Your Car Does Start, the very first thing you'll notice is that the compression values you wrote down for each cylinder are slightly different from one another. This is normal.
What is NOT normal is if the values vary too much. The cool thing is that we can find out if the variations in the values, you wrote down, indicate a problem (with that cylinder) or not.
The rule of thumb is that they can not vary more than 15% from each other and if they do... you're gonna' have a genuine misfire condition on your hands or possibly a No Start Condition (if more than one cylinder is affected).
How do you figure this out?... This is how you do it:
- Grab a calculator and multiply the highest compression reading that you recorded by .15. So, let's say that cylinder #4 gave you the highest reading of 170 PSI. Well 170 X .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.
To make better sense of the above calculation... let's say that my Mitsubishi Lancer produced the following compression test results:
- Cylinder #1 175 PSI.
- Cylinder #2 170 PSI.
- Cylinder #3 165 PSI .
- Cylinder #4 120 PSI .
The next step is to do the math: 175 x .15= 26, 175-26= 149. So, now I know that cylinder #4 is the one causing the misfire!!