How To Test Engine Compression (3.2L Isuzu Amigo, Rodeo, Trooper)

Interpreting Your Compression Test Results

It is not uncommon to see different compression values across the engine's six cylinders, especially on high mileage engines.

Up to a point, a difference in compression readings will not cause engine performance problems.

However, if the deviation between the highest and lowest values is more than 15%, your Isuzu 3.2L V6 engine will suffer from a rough idle condition or a misfire problem.

You can calculate this 15% difference in one of two ways:

• Do the math with pen and paper.
• Use my low compression calculator.

If you want to use the low compression calculator, go here: Online Low Engine Compression Calculator.

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 I got the following compression readings:

Cylinder	Pressure
#1	165 PSI
#2	95 PSI
#3	155 PSI
#4	175 PSI
#5	160 PSI
#6	155 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.

Wet Engine Compression Test

Low (or no) engine compression in one or more (but not all) six cylinders of your Isuzu's 3.2L V6 engine is usually caused by worn cylinder head valves or worn piston rings.

The cool thing is that we can figure out the cause of the low compression reading without disassembling the motor. How? By doing a 'wet' compression test.

All the wet compression test involves is adding about two tablespoons of oil into the engine cylinder that showed a low compression reading in the 'dry' compression test and retesting its compression.

If the piston rings are worn, the engine oil that has just been added will cause the compression value to increase to the average compression value of the remaining cylinders.

If the cylinder head valves are worn (for that particular cylinder), the compression value will remain the same.

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.
   
   Remember, do not use any tool to tighten the compression tester. Hand tight is fine.

3. 3

   Have a helper crank your Isuzu's engine after you've set up the test.

4. 4

   The compression test will give you 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 interpret your test result:

CASE 1: The compression value shot up. This tells you that the piston compression rings are worn and thus the problem is in the bottom end (block) of the engine in your 3.2L Isuzu Amigo (Rodeo, Trooper or 3.2L Honda Passport).

CASE 2: The compression value stayed the same. This confirms that the problem causing the low (or no) compression reading you recorded earlier is in the cylinder head valves.

More specifically, you have burned out or broken cylinder head valves causing the low (or no) engine compression condition in that particular engine cylinder.

More Isuzu 3.2L Tutorials

You can find a complete list of tutorials in this index: Isuzu 3.2L Index Of Articles.

• How To Test The Isuzu 3.2L COP Ignition Coils.
• How To Test The Isuzu 3.2L Throttle Position Sensor.
• How To Test The Isuzu Manifold Absolute Pressure (MAP) Sensor.
• Ignition Control Module Test (1992-1995 3.2L Isuzu Rodeo and Trooper).
• How To Test The GM MAF Sensor (3.1L, 3.4L, 4.3L, 5.0L and 5.7L).
  • For GM MAF sensor equipped 3.2L engines.