How To Do And Interpret An Engine Compression Test (Ford 3.0L, 3.8L)

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.

You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (Ford 3.0L, 3.8L) (at: autotecnico-online.com).

Symptoms Of Low Or No Engine Compression

In the majority of cases, engine compression problems will cause one of two problems:

• The engine does not start.
• The engine starts and runs, but runs with a misfire (rough idle).

On OBD II equipped vehicles, low compression problems will usually set a misfire trouble code:

• P0300: Random Cylinder Misfire.
• P0301: Cylinder #1 Misfire.
• P0302: Cylinder #2 Misfire.
• P0303: Cylinder #3 Misfire.
• P0304: Cylinder #4 Misfire.
• P0305: Cylinder #5 Misfire.
• P0306: Cylinder #6 Misfire.

You're also gonna' see one or more of the following:

• Bad gas mileage.
• Engine burns oil when accelerating the vehicle on the road.
• Rough idle (or misfire) that goes away once the engine is accelerated.

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 three recommendations to you:

TEST 1: Engine Compression Test

You'll be working around a cranking engine, so take all necessary safety precautions and use common sense.

The engine's temperature is a consideration to keep in mind when doing a compression test (if the engine starts and runs).

But keep in mind that the engine in your Ford (or Mercury) can not be hot or you run the risk of damaging the spark plug hole threads.

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 10 minutes.

If your vehicle doesn't start, then don't worry, don't worry about the engine being warm -you can still perform the engine compression test.

NOTE: If you don't have a compression tester and need to buy one, take a look at my recommendations here: Which Compression Tester Should I Buy?

OK, let's get testing:

1. 1

   Disable the fuel system. You can do this by removing the fuel pump relay.
   
   The reason you need to disable the fuel system is to keep the PCM from injecting fuel into the engine cylinders as you perform the compression test.

2. 2

   Disable the ignition system. This can be done by simply disconnecting the ignition coil or the ignition coil pack from its electrical connector.
   
   It's important to disable the ignition system so that you won't have spark firing off at the spark plug wires (while you do the compression test).

3. 3

   Remove all of the 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!

4. 4

   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.

5. 5

   Ask your helper to crank the engine once the compression gauge is set up and hand tight in the spark plug hole.

6. 6

   Once the needle stops climbing on your compression gauge, have him or her stop cranking the engine.

7. 7

   Record on paper the value at which the needle stopped and the number of the engine cylinder on a piece of paper.

8. 8

   Repeat steps 4 thru' 7 on the remaining five cylinders.

OK, let's make sense of all of the numbers you recorded for each cylinder:

CASE 1: You got 0 PSI on all six cylinders. This is not good. The engine won't start if all six cylinders have 0 PSI compression.

0 PSI compression across all six cylinders is usually the result of one the following conditions:

1. Broken timing chain.
2. Blown head gasket.
3. Blown engine.

CASE 2: One or more cylinders had low compression. Up to a certain point this is normal. But if the low compression value or values are lower than 15% of the highest, then those cylinders will misfire.

Thankfully, we can easily figure out if these cylinders (with the low compression values) are causing a problem by doing a little math. Go to: Interpreting The Results Of The Engine Compression Test.

Interpreting The Results Of The Engine Compression Test

The cylinders will eventually suffer wear and tear (as the engine accumulates miles) and produce lower compression values. This doesn't happen overnight, but it will happen.

Now, uneven compression across the cylinders will not cause engine performance problems if the variation (in the compression values) does not vary more than 15%.

If in TEST 1 some of the cylinders have a lower compression value than the rest, then this section will help you figure out if those compression values are lower than 15% of the highest value. Because if they are, they're causing a problem.

There are one of two ways to find out: You can calculate this 15% difference with pen and paper or you can use my low compression calculator. You can find the low compression calculator here: Online Low Engine Compression Calculator (at: easyautodiagnostics.com).

If you want to manually calculate the 15% difference, here's what you'll need to do:

• STEP 1: Multiply the highest compression value by 0.15 (this is the decimal value of 15%).
• STEP 2: Round the result to the nearest one (for example: 25.6 would become 26).
• STEP 3: Subtract the result (the number that was rounded) from the highest compression value.
• ANSWER: The result of this subtraction is the lowest possible compression value any cylinder can have.

Now, let me give you a more specific example: Let's say that my engine compression test produced the following compression readings:

Cylinder	Pressure
#1	165 PSI
#2	95 PSI
#3	155 PSI
#4	175 PSI
#5	165 PSI
#6	175 PSI

My next step is to do the following calculation:

• STEP 1:  175 x 0.15 = 26.25.
• STEP 2:  26.25 = 26 (rounded to nearest one).
• STEP 3:  175 - 26 = 149.
• ANSWER:  149 PSI. Any cylinder with this compression (or lower) value will misfire.

Since cylinder #2 is only producing 95 PSI, I can now conclude that it's 'dead' and causing a misfire.

To find out if the lowest compression value you got from your engine compression test is within a good range, you'll need to do the same calculation. Of course, you'll need to use the highest compression value you got and not the one in the example.

Once you've found the 'dead' cylinder, the next step is to find out what's causing the low compression value. For this step, go to: TEST 2: Wet Engine Compression Test.

TEST 2: 'Wet' Engine Compression Test

You might be asking yourself, “What's a wet compression test?”, it's the same compression test you did before with the difference being that you're gonna' add about two tablespoons of oil to the engine cylinder that showed a low compression reading in the 'Dry' compression test.

The engine oil that you're gonna' add to the cylinder will help determine if the low cylinder pressure or pressures you recorded in the 'Dry' compression test are caused by worn piston rings or worn cylinder head valves.

Depending on whether the compression pressure rises (on your compression tester) or not, you'll be able to say that the problem lies in the piston's rings or in the 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.
   
   The amount should be about 1 to 2 tablespoons of oil.

2. 2

   Install the compression tester onto the cylinder. 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:
   
   1.) The compression value will go up (from the one you recorded before).
   
   2.) The compression value will stay the same.

Let's take a look at your test results:

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 3.0L/3.8L Ford vehicle.

CASE 2: The compression value stayed the same. This test result tells you that the low compression in this specific cylinder is due to bad cylinder head valves.

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.

More 3.0L, 3.8L Diagnostic Tutorials

You can find a complete list of 3.0L, 3.8L Ford diagnostic tutorials in this index: Ford 3.0L, 3.8L Index Of Articles.

If this tutorial did not help you solve your misfire condition, misfire code, or rough idle condition, I recommend taking a look at the following specific Ford V6 article(s):

At easyautodiagnostics.com, you'll find the following articles:

• How To Test The Ford V6 Ignition Coil Pack.
• How To Test The Ignition Control Module (Older 3.0L And 3.8L With A Distributor).
• How To Test The MAF Sensor.
• How To Test The Throttle Position Sensor (TPS).