How To Test The Engine Compression (1995-2000 2.0L Dodge Stratus and Plymouth Breeze)

Interpreting The Results Of The Engine Compression Test

Slight variations in engine compression readings are normal. However, if a cylinder's compression is more than 15% lower than the highest reading, it could mean that cylinder is failing and might be the cause of issues like rough idling or misfires.

Calculating the 15% difference between the compression levels of cylinders isn't difficult at all. In this section, I'll explain how to do it so you'll easily find out if any cylinder is underperforming.

You can do this one of two ways: You can calculate this 15% difference with pen and paper or you can use my low compression calculator. You can find the low compression calculator here: Online Low Engine Compression Calculator (at: easyautodiagnostics.com).

If you want to manually calculate the 15% difference, here's what you'll need to do:

• STEP 1: Multiply the highest compression value by 0.15 (this is the decimal value of 15%).
• STEP 2: Round the result to the nearest one (for example: 25.6 would become 26).
• STEP 3: Subtract the result (the number that was rounded) from the highest compression value.
• ANSWER: The result of this subtraction is the lowest possible compression value any cylinder can have.

Now, let me give you a more specific example: Let's say that my engine compression test produced the following compression readings:

Cylinder	Pressure
#1	165 PSI
#2	95 PSI
#3	155 PSI
#4	175 PSI

My next step is to do the following calculation:

• STEP 1:  175 x 0.15 = 26.25.
• STEP 2:  26.25 = 26 (rounded to nearest one).
• STEP 3:  175 - 26 = 149.
• ANSWER:  149 PSI. Any cylinder with this compression (or lower) value will misfire.

Since cylinder #2 is only producing 95 PSI, I can now conclude that it's 'dead' and causing a misfire.

To find out if the lowest compression value you got from your engine compression test is within a good range, you'll need to do the same calculation. Of course, you'll need to use the highest compression value you got and not the one in the example.

Once you've found the 'dead' cylinder, the next step is to find out what's causing the low compression value. For this step, go to: TEST 2: Wet Engine Compression Test.

TEST 2: Wet Engine Compression Test

Now that you've found one or more cylinders with low compression, the next step is a "wet" compression test.

The "wet" compression test simply involves adding a bit of engine oil —around two tablespoons— into the low-performing cylinder and retesting its compression.

Once you get the compression value of this "wet" test, we'll compare it to the initial "dry" value you got in TEST 1. This will help you pinpoint whether the issue lies with the piston rings or the cylinder head valves.

OK, these are the test steps:

1. 1

   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.

2. 2

   Install the compression tester onto the cylinder.
   
   NOTE: Do not use any type of tool to tightened the compression tester. Hand tight is fine.

3. 3

   When all is set up, have your helper crank the engine.

4. 4

   You'll get one of two results:
   
   1.) The compression value will go up (from the one you recorded before).
   
   2.) The compression value will stay the same.

Let's take a look at what your test results mean:

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.

Here's why: When you add oil to the cylinder, it acts as a temporary sealant. It fills in the gaps where the piston rings are failing to seal properly against the cylinder wall. The added oil improves the seal and thereby increases the compression levels when you crank the engine for the test. This almost restores the compression value to what it should be if the rings were in good condition.

CASE 2: The compression value stayed the same. This confirms that the problem is in the cylinder head valves.

Here's why: When the cylinder head valves or their seats are damaged—perhaps even bent due to a snapped timing belt—adding engine oil won't improve the cylinder's compression. So, if you don't see an increase in compression for the cylinder you're testing after adding oil, it's a clear sign that the issue lies with the cylinder head valves.