How To Interpret The Engine Compression Test Results
More than likely, your compression tests results vary a little (or a lot). If they vary more than 15%, then you're gonna' have a misfire or a rough idle problem on your hands.
Finding out isn't hard. You can use my online low compression calculator to find out what the lowest compression reading can be before the engine starts to misfire. The calculator is here: Online Low Engine Compression Calculator.
Or you can manually figure it out this way:
- Grab a calculator and multiply the highest compression reading that you recorded by 0.15.
Let's use the following compression readings to explain the point:
- Cylinder #1 175 PSI.
- Cylinder #2 160 PSI.
- Cylinder #3 165 PSI.
- Cylinder #4 95 PSI.
- The next step is to do the math: 175 x 0.15= 26, 175-26= 149.
- So then, 149 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.
- This means that cylinder #4 is the one causing the misfire.
The next step is to do a ‘wet’ compression test. For this test go to: TEST 2: Wet Compression Test.
TEST 2: Wet Compression Test
In this section, you're gonna' do a ‘wet’ compression test on the cylinder or cylinders that gave you a low compression reading.
The ‘wet’ compression test is just a simple variation of the compression test you did in the previous page, the difference being that you're gonna' add about 2 tablespoons of engine oil to the cylinders with low compression.
The reason for adding engine oil, is that if the low compression is being caused by worn piston rings, the oil will seal the rings (in the affected cylinder) and cause the compression pressure to increase. This increase of pressure will be very obvious on your compression tester.
Now, if the problem is worn cylinder head valves, then no amount of oil will seal them and thus the compression reading will stay the same as when you did the ‘dry’ compression test.
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 tighten 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: The compression value will go up (from the one you recorded before) or it will stay the same.
Let's interpret 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.
CASE 2: The compression value stayed the same. This confirms that the problem is in the cylinder head valves.
More 2.3L Ford Diagnostic Tutorials
You can find a complete list of 2.3L Ford tutorials in the following 2 indexes:
Here's a small sample of the tutorials you'll find in the indexes:
- How To Test The Ford Mass Air Flow (MAF) Sensor.
- How To Test The Ignition Module And Crank Sensor (2.3L Ranger, Mustang, B2300).
- How To Test The Engine Compression (2.3L Ranger, Mustang, B2300).
If this info really saved the day, buy me a beer!