How To Test Engine Compression (4.9L, 5.0L, 5.8L)

In this tutorial, I'm gonna' show you the ‘ins and outs’ of doing a compression test on your Ford 4.9L engine (inline 6 cylinder), 5.0L engine, or 5.8L engine. More importantly, I'll show you how to interpret your compression test results.

Why do an engine compression test? Well, it may help you to find out if the engine is shot and/or if you have one or several ‘dead’ cylinders.

If you have no or very low engine compression across all cylinders, then this will cause a 'cranks but does not start' condition. If you have low engine compression one or several cylinders, this will cause an engine miss when the engine is idling or accelerating your Ford down the road.

I'm also gonna' include the test info you need to do a ‘wet’ engine compression test on your 5.0L engine (4.9L engine or 5.8L engine) and find out if the low compression test result you got is due to worn out cylinder head valves or worn engine piston rings.

You can find this tutorial in Spanish here: Cómo Probar la Compresión del Motor (Ford 4.9L, 5.0L, 5.8L) (at: autotecnico-online.com).

TEST 1: ‘Dry’ Engine Compression Test

How To Test Engine Compression (4.9L, 5.0L, 5.8L)

One of the things, that's gonna' help you to successfully interpret the results of your compression test, is knowing which cylinder is which.

And so, in the image viewer on the left, you'll see two images that will help you identify the number of the engine whether it's a V8 (5.0L, 5.8L) or an L6 (4.9L).

If you don't own a compression test gauge, you can borrow one from your local parts store (for a small cash deposit which they'll return when you return the it), or you can buy your own. For suggestions, take a look at: Which Compression Tester Should I Buy?

OK, to get this show on the road, I'll first explain the test steps. At the end of the test steps, you'll find two possible test results that will help you to interpret your specific test results.

NOTE: If the engine has been running for an extended amount of time, then let it cool down for at least 45 minutes before removing the spark plugs.

Let's get started:

  1. 1

    Warm Up the engine. It's standard operating procedure to perform the compression test with the engine slightly warmed up but not hot.

    If your specific Ford 5.0L engine (4.9L engine or 5.8L engine) doesn't start, don't worry. You can continue with the other steps.

  2. 2

    Disable the fuel system. You can do this by simply disconnecting the fuel pump relay.

    NOTE: It's important to disable the fuel system so that fuel is not injected into the engine cylinders while you're compression testing the engine.

  3. 3

    Disable the ignition system. This can be done by disconnecting the ignition coil from its electrical connector.

    NOTE: It's important to disable the ignition system since you can not have it firing off spark while you're compression testing the engine.

  4. 4

    Remove all six (or eight) spark plugs.

    As you're taking them out, be careful and don't drop any of them on the floor, or you could cause the spark plug's ceramic insulator to break, and this will cause a misfire!

  5. 5

    Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder. Hand tighten the compression gauge only! Do not use any type of tool to get it tight.

  6. 6

    Crank the engine once the compression gauge is threaded into the spark plug hole with the help of a helper.

  7. 7

    Once the needle stops climbing, have your helper stop cranking the engine.

  8. 8

    Write down the number at which the needle stopped (and the number of the engine cylinder) and then repeat steps 1 thru' 7 on the other cylinders.

Let's examine your test results:

CASE 1: All cylinders had 0 PSI. This test result tells you that the engine has an internal mechanical problem.

The most common cause of this condition is a broken timing chain.

Your next step should be to check the condition of the timing chain. I would also recommend that you check for a blown head gasket.

CASE 2: One or more cylinders had a low compression value compared to the others. This could be normal or it could be causing a problem.

To find out if the compression values are normal or not, go to: Interpreting The Compression Test Results.

CASE 3: All compression values were similar and above 120 PSI. This lets you know that a compression problem is not behind the no start or misfire problem you're trying to troubleshoot.