How To Test Engine Compression (Toyota 1.8L)

Interpreting The Results Of The Engine Compression Test

It's not unusual to see the cylinder compression readings vary between each other, especially in high mileage engines. Up to a certain point, this is normal, since the cylinders can wear unevenly across the engine's life.

If the compression readings vary too much, typically 15% or more, then the engine will suffer engine performance problems.

You can find out if the compression values you got are causing a problem 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 the vehicle I tested 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

To find out if the problem is coming from the top end (intake or exhaust valves) or the bottom end (piston rings), we need to do a wet compression test.

All that we're gonna' do is add about two tablespoons of oil to the engine cylinder that showed a low compression reading in the 'dry' compression test.

The engine oil 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.

Here are the 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.
   
   Do not use any type of tool to tightened the compression tester. Hand tight is fine.

3. 3

   Crank the engine until the needle on the gauge stops climbing.

4. 4

   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 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 in your 1.8L Toyota.

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