One of the most over-looked tests, when troubleshooting a hard to solve misfire, is the engine compression test. Over the years, I've seen lots of folks throw their hands up in the air in frustration, because not matter what got replaced (on the car or pick up)... nothing solved the misfire condition or misfire code (P0300, P0301, P0302, P303, P0304). In this article, we'll explore the compression test as one of the key tests to troubleshoot a misfire code or codes.
This article supplements the information in the article: How to Diagnose Misfire Codes (GM 2.2L). Also, the info presented here applies to the GM 2.2L 4 cylinder equipped Chevrolet Cavalier, or Pontiac Sunfire, or Chevrolet Sonoma, or GMC Sonoma.
Here are the contents of this article at a quick glance:
- The Engine Compression Test.
- Interpreting the Results of the Engine Compression Test.
- ‘Wet’ Engine Compression Test
- Why an Engine Compression Test?
- Which Compression Tester Should I Buy?
- Related Test Articles.
You can find this tutorial in Spanish here: Cómo Probar la Compresión del Motor (GM 2.0L, 2.2L, 2.5L) (at: autotecnico-online.com).
Tools You'll Need:
- Compression Gauge Tester
- A Helper
- Pen and Paper
The Engine Compression Test
Each cylinder, besides needing Air, Fuel, and Spark, to contribute to engine power... also needs to be in good mechanical condition. If either the cylinder head valves or the engine piston rings are worn and letting the compression created on the power stroke escape... then that cylinder will misfire.
This test article assumes that the engine starts and runs and that you're testing a misfire condition. OK, here's the test:
The engine should NOT be at normal operating temperature and yet, it should be not be completely cold. So, if you have just turned off the engine, after an extended run time, let it cool down about 30 minutes. If the engine is cold, start’er up and let’er run about 15 minutes.
Disconnect all 4 fuel injectors from their electrical connectors. This will prevent fuel injectors from injecting fuel into the engine cylinders.
Disable the ignition system by disconnecting all of connectors that plug to the ignition control module/ignition coil assembly. This important, since the ignition system must NOT SPARK. If the ignition system sparks during the compression test, the ignition coils or ignition control module can be damaged.
Remove all of the four Spark Plugs, from your 2.2L Cavalier (or Sunfire, or S10 Pick Up, Sonoma, or 2.0L, 2.5L equipped vehicle). 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.
Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder (this is the spark plug hole closest to the drive belt). Hand tighten the compression gauge only! Do not use any type of tool to get it tight.
When everything is ready, have your helper crank the engine. It usually takes about 10 seconds of engine cranking to get the maximum compression reading. So, once the compression gauge's needle stops climbing, have your helper stop cranking the engine. 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.
Now, look at all of the four compression readings that you just wrote down. They should be within 10% of each other. Now, if you don't understand this 10% part, don't worry, the next part of this article will show you how to intepret what you have just done.
Interpreting the Results of the Engine Compression Test
OK, so you tested all four cylinders and you recorded the readings... what now? Well, those numbers will tell us if the misfire condition is due to a low compression reading in one of those four cylinders. Here's what you need to do:
Grab a calculator and multiply the highest compression reading recorded by .10. So let's say, for the sake of this example, that the highest reading was 160 PSI (Pounds per Square Inch). Multiplying 160 by .10 gives us 16
Subtract 16 from the highest reading. In my example, the highest compression reading is 160... so subtracting 16 from this reading, I get: 144.
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, to go into a lot more detail: Let's say that my GMC Sonoma (or Chevy S10 Pick Up, or Chevy Cavalier or Pontiac Sunfire) produced the following compression readings: Cylinder #1 = 165, Cylinder #2 = 95, Cylinder #3 = 155, Cylinder #4 = 170. The next step would be to apply the formula above and I get 153 PSI as the lowest possible reading (170 x .10= 17, 170-17= 153). So, now I know that cylinder #2 is the one causing the misfire!!