A compression test can help you to nail down a hard to find misfire condition, since it's one of the most overlooked tests when diagnosing a rough idle or misfire condition.
The compression test will also let you know just how healthy (mechanically) the engine is on your Nissan 3.0L, 3.3L, 3.5L car, pick up, SUV or mini-van.
This article will walk you thru' the engine compression test and more importantly, I'll show you how to interpret the results of your compression test.
Contents of this tutorial:
The ‘Dry’ Engine Compression Test
If your Nissan vehicle starts and runs, it's important to keep in mind that the engine compression test should be done with the engine slightly warmed up.
If your vehicle doesn't run, then don't worry about doing this test with the engine cold.
One last thing, this test is done with the engine cranking, so be careful and take all necessary safety precautions. Your safety is your responsibility.
OK, let's get started:
It's important the engine be slightly warmed up for this test, yet it can not be hot (technically: normal operating temperature) So, if the engine is completely cold, start her up and let it idle for no more than 15 minutes. If the engine has been running for a long period of time, let her cool down for about 1 hour.
Disable the ignition system.
If your vehicle has a distributor, you can easily accomplish this by disconnecting all of the distributor's electrical connectors.
If your vehicle does not have a distributor (equipped with 6 Coil-On-Plug Ignition Coils), then you'll disable the ignition system when you remove then in the next step.
Remove all of the six spark plugs. Be careful and don't drop any of the spark plugs. Dropping them could cause their ceramic insulator to break and this will cause a misfire.
IMPORTANT: If necessary (on distributor equipped vehicles), tag each spark plug wire before you disconnect them from the spark plugs so that you'll know where they go when you re-install them.
Hand-thread the engine compression gauge into the spark plug hole that you've chosen to test first. Do not use any type of tool to get it tight, hand tight is enough!
Once everything is set, have your helper crank the car or mini-van till the needle on the compression tester gauge stops climbing. It normally takes about 10 seconds or engine cranking to get to this point. When the needle stops moving, you have reached the maximum compression pressure of that cylinder.
Record the reading on a piece of paper along with the cylinder the reading belongs to. Now, repeat this exact same test on the remaining engine cylinders. The illustration in the image above will help you to identify what cylinders you're testing.
Now, look at all of the four compression readings that you just wrote down. They should be within 15% of each other. Now, if you don't understand this 15% part, don't worry, the next part of this article will show you how to interpret what you have just done.
Interpreting The Results Of The Engine Compression Test
Now, I'm gonna' show you how to interpret all those compression pressure values you wrote down for each specific engine cylinder. Here's what you need to do with them:
Grab a calculator and multiply the highest compression reading recorded by 0.15 (.15 is the decimal equivalent of 15%). So let's say, for the sake of this example, that the highest reading was 165 PSI (Pounds per Square Inch). Multiplying 165 by 0.15 gives us 25 (24.75 rounded off).
Subtract 25 from the highest reading. In my example, the highest compression reading is 165. So subtracting 25 from this reading, I get: 140.
So then, 140 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.
Let me give you another, but more specific, example: Let's say that my 2002 Nissan 3.3L Frontier (or Nissan Quest, Pathfinder or whatever), produced the following compression readings:
- Cylinder #1 = 165 PSI
- Cylinder #2 = 85 PSI
- Cylinder #3 = 145 PSI
- Cylinder #4 = 160 PSI
- Cylinder #5 = 170 PSI
- Cylinder #6 = 165 PSI
The next step would be to apply the formula above and I get 145 PSI as the lowest possible reading (170 x 0.15= 25, 170-25= 145). So, now I know that cylinder #2 is the one causing the misfire!!