In this tutorial, I'll show you how to test the engine compression and more importantly, how to interpret the results of the test on your 2.5L equipped Nissan Altima (or Sentra).
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, ignition coils, 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.
Contents of this tutorial at a quick glance:
- Important Tips and Suggestions.
- TEST 1: Dry Compression Test.
- TEST 2: Wet Compression Test.
- Which Compression Tester Should I Buy?
- Related Test Articles.
Important Tips and Suggestions
TIP 1: The engine in your 2.5L Nissan Altima (Sentra) should be warmed up to get the most accurate engine compression test result from your compression test. I want to stress the words ‘warm engine’ because the engine should not be hot. This applies only if your Nissan starts and runs.
TIP 2: You'll be working around a cranking engine as you perform the engine compression test so take all necessary safety precautions. Your safety is your responsibility... so use common sense and think safety all of the time.
TIP 3: 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
When doing an engine compression test, the first test is done ‘dry’. Which means you simply remove the spark plugs and check each cylinder's compression. The second test done is a ’wet’ compression test. In this second test, you add a few drops of engine motor oil to the cylinders that registered low or 0 compression.
OK, to get started 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.
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:
- Timing Chain Problem.
- 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 2.5L Altima 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!!