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:
- Test 1: ‘Dry’ Engine Compression Test.
- Test 2: ‘Wet’ Engine Compression Test.
- Why an Engine Compression Test?
- Which Compression Tester Should I Buy?
- Related Test Articles.
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
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.
- Of course, this only applies if your Ford pickup (van, car, or SUV) cranks and starts.
- If your specific Ford 5.0L engine (4.9L engine or 5.8L engine) doesn't start, not only can you not warm up the engine... but it doesn't matter... you can continue with the other steps.
- If your Ford 5.0L engine (4.9L engine or 5.8L engine) does start... let the engine run for about 20 minutes and no more. If the engine has been running for an extended amount of time... let it cool down for at least 45 minutes.
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.
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.
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.
- Your job is to keep your eyes on the compression gauge and when the needle stops climbing... have your helper stop cranking the engine.
- Write down the number at which the needle stopped (and the number of the engine cylinder) and then repeat steps 1 thru' 6 on the other cylinders.
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:
- Grab a calculator and multiply the highest compression reading that you recorded by .15.
So, let's say that cylinder #4 gave you the highest reading of 170 PSI. Well 170 X .15 gives you 26 (25.5 rounded off).
- Now, the next step is to subtract 26 from 170... which gives us 144 PSI.
- So then, 144 PSI is the lowest possible compression reading that any one of the rest of the engine cylinders can have. Any compression reading below this.. and that engine cylinder will misfire.
- If you see 0 PSI in the majority of the cylinders or even values of 90 PSI or less in the majority of them, you now know that you have a serious engine mechanical condition that will give a No Start Condition.
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:
- Cylinder #1 175 PSI.
- Cylinder #2 170 PSI.
- Cylinder #3 165 PSI.
- Cylinder #4 40 PSI.
- Cylinder #5 160 PSI.
- Cylinder #6 165 PSI.
- Cylinder #7 155 PSI.
- Cylinder #8 160 PSI.
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!!
My Engine Doesn't Start
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.