Interpreting The Results Of The Engine Compression Test
If your Nissan vehicle does not start and your test results showed that the highest reading, of all four, is 90 PSI, you've got mechanical issues with the engine. This also indicates that the reason your Nissan Altima (or Xterra or Frontier) does not start... is low engine compression.
This low compression (across all four engine cylinders) can be due to several reasons. The most common are: broken timing chain, blown head gasket, or worn piston rings.
If your Nissan starts and you're trying to solve a misfire condition, then the following will help you to interpret your compression test results:
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 Accord (or Odyssey or Prelude) gave me the following compression readings:
- Cylinder #1 175 PSI.
- Cylinder #2 160 PSI.
- Cylinder #3 165 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, if you're curious or need to know if the problem of the low compression is due to worn valves or worn piston rings, the next step is to do a ‘Wet’ compression test and you'll find this test in the next page.
‘Wet’ Engine Compression Test
After interpreting the results of your ‘Dry’ compression test, and you have one or several cylinders with low compression, the next step is to do what's called a ‘Wet’ compression test.
This involves adding a few drops (2 tablespoons) of engine oil to the cylinders with the low engine compression result.
Now, you may be asking yourself... Why a ‘Wet’ compression test? Well, because it'll help you to 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 your test results:
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 2.4L Nissan Altima (Frontier or Xterra).
CASE 2: The compression value stayed the same. This confirms that the problem is in the cylinder head valves.
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 just about anywhere, but you'll end up paying more for it (especially at your local auto parts store). The above links will help you comparison shop. I think you'll agree it's the better way to save money on the compression tester!
Related Test Articles
You can find all of the Nissan 2.4L specific test articles here: 2.4L Nissan Index Of Articles
Here's a sample of the tutorials you'll find in the index:
- How To Test The Fuel Injectors (Nissan 2.4L).
- How To Test The Starter Motor (Nissan 2.4L -Frontier, Xterra).
- How To Troubleshoot A Blown Head Gasket (Nissan 2.4L, 2.5L).
If this info saved the day, buy me a beer!