Doing a compression test on your Ford or Mercury Taurus, Sable, Windstar equipped 3.0L and 3.8L V6 is an easy test to do. This article will take you thru' the test and interpretation of the cylinder compression results in a step by step fashion.
By the way, this article is geared towards helping you solve a misfire condition or misfire codes (P0300, P0301, P0302, P303, P0304, P0305, P0306), since a low compression reading in only one cylinder, out of the six, will cause a miss, rough idle condition that no matter what gets replaced (like spark plugs, spark plug wires, fuel injector, etc.), the vehicle will continue to miss.
Here are the contents of this article at a quick glance:
- The Engine Compression Test.
- Interpreting the Results of the Engine Compression Test.
- Wet Engine Compression Test.
- Why an Engine Compression Test?
- Which Compression Tester Should I Buy?
- More 3.0L, 3.8L Diagnostic Tutorials.
You can find this tutorial in Spanish here: Cómo Probar la Compresión del Motor (Ford 3.0L, 3.8L) (at: autotecnico-online.com).
The Dry Engine Compression Test
You'll be working around a cranking engine, so take all necessary safety precautions and use common sense.
Also, this test article assumes that the engine starts and runs and that you're testing a misfire condition.
Now, if your vehicle doesn't start... the test steps still apply and will help you determine if the No Start is due to No engine compression.
OK, here's the test (but you can apply this info to an engine that does not start):
The engine's temperature is an important consideration to keep in mind! The engine in your Ford (or Mercury) can not be hot and it can not be completely cold.
So, if you have just turned the car or mini-van off from an extended run time, let it cool down for about an hour. If the vehicle is cold, start the car (or mini-van) up and let it warm up for no longer than 20 minutes.
If your vehicle doesn't start, then don't worry... this doesn't apply to you.
Disable the fuel system. It's important that no fuel be injected into the cylinders during the test.
You can easily accomplish this by removing the fuel pump fuse or the fuel pump relay. Once either of these components is removed, crank the engine to relieve the fuel pressure that could be stored in the fuel lines.
Disable the ignition system by disconnecting the ignition coil or ignition coil pack's electrical connector. It's important the the ignition system not create and deliver spark to the spark plugs.
Remove all of the spark plugs. Be careful and don't drop any of the spark plugs. Dropping them could cause their 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 (this is the spark plug hole closest to the drive belt). Hand tighten the compression gauge only! Do not use any type of tool to get it tight.
Now, have your helper crank the engine. Have him or her crank the engine till the needle on the compression tester's gauge stops climbing. It usually takes about 10 seconds or so of cranking for this to happen.
Record this compression reading on a piece of paper. Include the number of the cylinder this reading belongs to. Now repeat steps 1 thru' 6 on the other 6 cylinders.
After all is said and done, you should have six different compression readings written down. The next step is to interpret these results into useful information.
OK, let's make sense of all of the numbers you recorded for each cylinder. What we have to do is find out if the lowest compression reading you recorded is within a certain specification, because if it's too low, then that cylinder will not contribute to overall engine power... which will cause a miss, rough idle, or misfire condition. This is what you need to do:
- Grab a calculator and multiply the highest compression reading 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.
Let me give another example, so that you really see the logic behind the test... let's say that my Mercury Sable (or Ford Taurus or Windstar, etc.) gave me the following compression readings when I performed the engine compression test:
- Cylinder #1 175 PSI.
- Cylinder #2 160 PSI.
- Cylinder #3 155 PSI.
- Cylinder #4 165 PSI.
- Cylinder #5 30 PSI.
- Cylinder #6 170 PSI.
The next step is to do the math: 175 x .15= 26, 175-26= 149. So, now I know that cylinder #5 is the one causing the misfire!!