In this article, 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.
OK, to make it as easy as possible to navigate this article, here are its contents at a quick glance:
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).
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. Let's get started:
Warm Up the engine. It's standard operating procedure to perform the compression test with the engine slightly warmed up but not hot.
Disable the fuel system. You can do this by simply disconnecting the fuel pump relay.
Disable the ignition system. This can be done by disconnecting the ignition coil from its electrical connector.
Remove all six (or eight spark plugs) 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!
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.
Crank the engine once the compression gauge is threaded into the spark plug hole with the help of a helper.
Interpret the results after testing all cylinders and having written down all of your compression test readings, using the info in the next page...
Interpreting the results of your compression test is not hard and in this section I'll go into detail about it.
For starters, let me tell you that you're not gonna' see the exact same compression value on each of the 6 or 8 cylinders you tested on your Ford 4.9L engine, 5.0L engine or 5.8L engine and this is normal. What is not normal is to have one or several a compression values that are radically different than the average compression of the good cylinders.
Keeping the above in mind, this means that compression between cylinders should not vary more than 15%... If you're scratching your head and wondering how to do this, don't worry... this is how you can find out:
OK, before moving on... I'll give you one specific example on how to do this 15% calculation: For the sake of this conversation... let's assume that my 5.0L Ford Crown Victoria gave me following compression values when I did a compression test on all 8 cylinders:
The next step is to do the math: 175 x .15= 26, 175-26= 149. So, now I know that cylinder #4 is the one causing the misfire!!
If you're testing the engine compression to find out if the engine is shot: Then you'll know right away because most, if not all, of the compression readings will be at 0 PSI.
Also, if the majority of the readings are less than 90 PSI... you're engine is shot.
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