How to Test the Engine Compression
(GM 4.3L, 5.0L, 5.7L)

‘Wet’ Engine Compression Test

So, you've found the ‘dead’ cylinder or cylinders... what next? The next step is to do a ‘Wet’ compression test and find out if the low compression value (or 0 compression value you got) is due to bad cylinder head valves or bad piston compression rings.

This involves adding a few drops (2 tablespoons) of engine oil to the cylinders with the low engine compression result and repeating the compression test.

The results you obtain from this second ‘Wet’ compression test will help you determine if the low compression you recorded in the ‘Dry’ compression test are caused by worn piston rings or worn cylinder head valves.

OK, this is what you need to do:

  1. 1

    Add a small amount of engine oil to the cylinder that reported low compression or no compression in the ‘Dry’ compression test

    1. The amount should be about 1 to 2 tablespoons of oil.
  2. 2

    Install the compression tester onto the cylinder.

    1. 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, either the compression value will go up (from the one you recorded before) or it will stay the same.

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 GM 4.3L, 5.0L, or 5.7L equipped vehicle.

Here's why: The engine oil helped the piston rings seal better, thus bringing up the compression value almost back to normal. If the problem were in the cylinder head valves... the engine oil you just added wouldn't make a difference at all (on the compression value).

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

Here's why: If the cylinder head valves and their seats are worn out (or maybe even bent from a broken timing belt), no amount of engine oil is gonna help seal the compression in, in the cylinder. So, if the compression value, for the specific cylinder you're testing did not go up (after you added oil to it)... then this is a dead giveaway that you've got cylinder head valve damage.

Why an Engine Compression Test?

This is one of the most overlooked tests to find the root cause of a misfire code, rough idle or an engine miss or a blown head gasket.

Over the years, I have solved many unsolvable misfire codes, rough idle, lack of power issues by doing a simple engine compression test and if you're faced with something similar... I highly recommend doing an engine compression test.

Which Compression Tester Should I Buy?

There are lot of engine compression testers to choose from and many places to buy them. I'm gonna' make two recommendations to you:

1) Which one to buy:  The engine compression tester that I have always used is the Actron CP7827 Compression Tester Kit My only complaint about this engine compression tester is that it does not come with a case to store it in.

Engine Compression Gauge Testers

     

2) Where to buy:  You can buy an engine compression tester in any auto parts store in any neighborhood, in any city... but you'll be paying at least twice as much. Go to the above compression tester links, browse and compare, you'll see a big price difference!

Related Test Articles

To see the 4.3L, 5.0L, and 5.7L specific articles here at troubleshootmyvehicle.com, go to: GM 4.3L, 5.0L, 5.7L Index of Articles.

Here's a sample of the tutorials you'll find in the index:

  1. How to Test the ‘Spider’ Fuel Injector Assembly (4.3L, 5.0L, 5.7L).
  2. Troubleshooting the Fuel Pump (GM 4.3L, 5.0L, 5.7L).
  3. Coolant Leaking From Intake Gaskets (GM 4.3L, 5.0L, 5.7L).
  4. How to Diagnose Misfire Codes (GM 4.3L, 5.0L, 5.7L).
  5. MAP Sensor Test (P0106, P0107, P0108) (GM 4.3L, 5.0L, 5.7L).

If you need and/or want to test the distributor ignition system to see if it's the source of the misfire condition (or misfire code: P0300, P0301, P0302, P03030, P0304, P0305, P0306, P0307, P0308), you can go to this link (found at easyautodiagnostics.com):

  1. How to Test a Misfire / No Spark-No Start Condition GM 4.3L, 5.0L, 5.7L Distributor Ignition System (1996-2004).

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