Interpreting The Results Of The Engine Compression Test
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:
- Broken timing belt.
- Blown head gasket.
- Blown engine.
CASE 2: If Your Car Does Start, you'll notice that the compression values for each one is slightly different from one another (after testing the compression of all cylinders). This is normal.
What is not normal is if the values vary too much. 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.
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.
I'll give you one more specific example, just to make sure you understand how this 15% stuff works... let's say that my Ford Focus 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!!
‘Wet’ Engine Compression Test
Once you've found the cylinders with little or no compression, the next step is to add about two tablespoons of oil to that cylinder.
The engine oil that you're gonna' add to the cylinder will help 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 Ford 1.9L, 2.0L.
The reason why, in case you're wondering, is that the oil is helping the worn out compression rings (on the piston) to seal the compression pressure. This is the reason why you're seeing the pressure value jump up on your compression tester.
CASE 2: The compression value stayed the same.. This confirms that the problem is in the cylinder head valves.
The reason why is that no amount of oil can help the valve seal against its valve seat (on the cylinder head).