How To Test The Engine Compression (3.8L Ford FWD)

In this tutorial, I'll explain how to test the compression of the Ford front wheel drive (FWD) vehicles equipped with a 3.8L V6 engine.

These vehicles include: 1991-1995 3.8L Ford Taurus, 1991-1995 3.8L Mercury Sable, and 1995-2003 3.8L Ford Windstar minivan.

Also I'm going to explain how to find out if the compression of a certain cylinder with low compression is causing a cylinder misfire or rough idle.

You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (1991-2003 3.8L Ford FWD) (at: autotecnico-online.com).

Symptoms Of Low Or No Compression

When an engine experiences compression issues, it can manifest in two main ways:

1. Low Compression in Some but Not All Cylinders: This means that the pressure within certain cylinders is lower than it should be, which can lead to various performance problems.
2. Zero PSI Compression in Some or All Cylinders: This is a severe issue where there is no compression at all in certain cylinders, indicating potential damage or failure.

Regardless of whether the compression is low or zero, you'll notice several symptoms that indicate there's an issue with the engine's compression:

• Hard Starting or No Start: Difficulty starting the engine, or it may not start at all.
• Poor Engine Performance: The engine may run rough, feel sluggish, or lack power.
• Misfiring: This is when one or more cylinders fail to ignite their air/fuel mixture properly, causing jerking or shaking.
• Reduced Power and Performance: The vehicle may struggle to accelerate or maintain speed.
• Excessive Oil Consumption: Compression issues can lead to oil leaking into the cylinders, causing increased oil consumption.
• Audible Engine Noise: You may hear unusual noises like knocking or tapping, indicating internal engine mechanical problems.

On vehicles with OBD II diagnostics (1996+), specific codes may be triggered related to cylinder misfires:

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

Important Tips And Suggestions

TIP 1: Don't remove the spark plugs if your 3.8L Ford vehicle's engine is hot.

You run the risk of stripping the threads, of the spark plug hole, if you remove the spark plugs with a hot engine. This is a nightmare I wouldn't wish on my worst enemy.

TIP 2: Be careful and take all necessary safety precautions as you perform the engine compression test (since the engine has to be cranked).

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 some recommendations to you:

TEST 1: Dry Compression Test

In this section, we're gonna' check the compression of all 6 cylinders. Checking them all will help us to find out which cylinder or cylinders are dead (due to low engine compression).

Let me tell you that depending on the age and mileage of your Ford's engine, the compression of each cylinder probably will not be similar.

Up to a certain point, this is normal. Now if the values vary too much between themselves, then this could lead to an engine cylinder misfire or rough idle condition.

Don't worry, I'll explain how to interpret your test results to find out if those dissimilar values are within a normal range or not.

Alright, these are the test steps:

1. 1

   Disable the ignition system by disconnecting the ignition coil from its electrical connector. This will prevent the ignition coil from sparking during the test.

2. 2

   Remove the spark plugs.
   
   When removing the spark plugs, be careful not to drop any of them on the floor, or you run the risk of having the spark plugs porcelain insulator crack and then you'll have a misfire on your hands.

3. 3

   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.

4. 4

   Have your helper crank the engine till the needle on the compression gauge stops climbing.

5. 5

   Record the value at which the needle stopped and the number of the engine cylinder on a piece of paper.
   
   Repeat this test step on the remaining steps 3-5 on the remaining cylinders.

Let's take a look at what your compression test results mean:

CASE 1: You got 0 PSI in the majority of the cylinders. This tells you that your Ford's engine has serious internal problems. You should check for a:

• Blown head gasket.
• Blown engine.

Any compression value below 100 PSI (even if it does not 0 PSI) means internal mechanical engine trouble.

CASE 2: All cylinders have compression but their values are not the same. It's normal for each cylinder's compression value to vary slightly from one another. But if they vary too much, you'll have a bonafide misfire or rough idle condition on your hands.

To find out, the next step is to go to: Interpreting The Compression Test Results.

Interpreting The Compression Test Results

On engines with a lot of miles, it's normal to see some differences in compression numbers from cylinder to cylinder. Small variations usually don't affect how the engine runs.

The trouble starts when one or more cylinders drop too far below the rest. That's when you'll run into a rough idle, a misfire, and a check engine light shining nice and bright on the instrument panel.

The good news is figuring out whether those low readings are a problem isn't complicated. In this part, I'll walk you through the process step by step.

The general guideline is that the weakest cylinder should not be more than 15% lower than the strongest one (the highest PSI you wrote down in TEST 1). If it is, that cylinder will misfire and your Fiesta's engine will idle unevenly.

You can run the numbers with my Online Low Engine Compression Calculator (at: easyautodiagnostics.com), or you can do the math by hand.

To show you how to work it out manually, let's look at these sample results:

• Cylinder #1: 175 PSI
• Cylinder #2: 165 PSI
• Cylinder #3: 160 PSI
• Cylinder #4: 120 PSI
• Cylinder #5: 165 PSI
• Cylinder #6: 160 PSI

Here's the math:

• Take 15% of the highest value: 175 × 0.15 = 26.25 (rounded to 26).
• Subtract that from the highest value: 175 − 26 = 149.
• That means the lowest acceptable compression is 149 PSI.

Looking at the results, cylinder #4 only has 120 PSI, which is well below 149. That makes it the problem cylinder and the cause of the misfire.

Once you've identified the weak cylinder, the next step is to track down why it's low. For that, go to: TEST 2: Wet Compression Test.

TEST 2: Wet Compression Test

In this test section, we're gonna' add a little bit of oil (two tablespoons) to the cylinder that gave you a low compression value test result in TEST 1. This will help us to find out what's causing the low compression value.

To be a bit more specific, what generally provokes a low engine compression (or a 0 PSI) value is worn piston rings or worn cylinder head valves.

Now, if the low compression is due to worn piston rings, then adding oil cause them to seal better and the compression value will shoot up.

If the low compression is due to severely worn or damaged cylinder head valves, then the oil will have no effect on the compression value. In other words, adding oil to the cylinder will not cause a change in the compression value you recorded in TEST 1.

Alright, let's get started:

1. 1

   Add a tablespoon (or two) of engine oil in the cylinder you need to retest. Use a funnel to make sure that the oil reaches the inside of the cylinder.

2. 2

   Once you've added the oil, install the compression gauge, and hand tighten it.

3. 3

   Have your helper crank the engine till the needle stops climbing on the compression gauge.

4. 4

   You'll see one of two results:
   
   1.) The needle will climb higher than the previous compression number you recorded for this specific cylinder, or...
   
   2.) The needle will not move at all or stay at the same number you recorded earlier.
   
   What ever value your compression tester reads, write it down again.

5. 5

   If you have another cylinder test, repeat steps 1 thru' 4 on it now.

Let's take a look at what test results mean:

CASE 1: The compression value went up after adding motor oil and retesting. This tells you that the low compression problem is due to worn piston compression rings.

Here's why: The motor oil you just added helped the piston rings to create a tighter seal. With the piston rings now sealing the compression inside the cylinder, the compression value on your compression tester went up. This type of test result only happens when the problem is due to worn piston rings.

CASE 2: The compression value DID NOT go up after adding oil and retesting (in other words, it stayed the same). This result tells you that the low compression value registered in this cylinder (in the dry test) is due to worn/damaged cylinder head valves.