How To Test Engine Compression (2000-2010 2.0L Ford Focus)

Engine compression problems usually cause one of two headaches: a cylinder misfire issue or an engine no-start problem.

And usually, an engine compression test is the last thing on anyone's mind when diagnosing either of those two issues.

In this tutorial, I'm going to explain how to do the engine compression test (which isn't difficult at all), and I'm also going to show you how to interpret the results so you can tell if you've got internal mechanical engine problems or not.

APPLIES TO: This tutorial applies to the following vehicles:

• 2.0L SOHC Ford Focus: 2000, 2001, 2002, 2003, 2004.
• 2.0L DOHC Ford Focus: 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010.

Symptoms Of Low Or No Cylinder Compression

Generally, engine compression problems can be divided into two categories:

• Rough idle: Low compression in one or more cylinders can cause a rough idle that never set s a specific misfire code (it if does, it'll be a P0300 DTC).
• Cylinder misfire: Low compression in one or more cylinders will definitely cause a cylinder misfire in the affected cylinder with low compression when the engine is operating.
• Engine no-start problem: Zero compression on all cylinders causing the engine to crank but not start.

If the engine starts and runs, you'll also see one or more of the following misfire trouble codes:

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

You'll also see one or more of the following symptoms:

• Hard start: The engine may struggle to start, especially when cold, or it may not start at all.
• Poor engine performance: The engine may feel sluggish, lacking power and responsiveness, particularly during acceleration.
• Excessive oil consumption: Oil may leak into the combustion chamber due to worn piston rings or valve seals, causing increased oil consumption and possibly blue smoke from the exhaust.

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

What makes the engine compression test pretty easy on your 2.0L Ford Focus is that the spark plugs are simple to get to and remove —unlike many other vehicles where you've got to take off a bunch of stuff just to reach them.

Once they're out, we'll check the compression of each cylinder and then go over your test results below.

Depending on what those results show, we may also run a wet compression test to see what's causing any low compression value you got (if you got one).

As you disconnect the spark plug wires and remove the plugs, take a moment to check if the wire boots or the spark plugs are soaked in engine oil. This is very common on the 2.0L Focus. If you do find oil in there, you've already uncovered a problem —the valve cover gaskets need to be replaced.

CAUTION: Be careful and stay alert at all times since you'll be working around a cranking engine. Keep your hands and any loose clothing away from moving parts.

NOTE: If your Focus' engine is hot, let it cool down before pulling the spark plugs. Removing them from a hot engine can damage the plug threads in the aluminum cylinder head.

Alright, these are the test steps:

1. 1

   Disable the ignition system by unplugging the ignition coil pack from its electrical connector. This keeps the coil from sparking during the test.

2. 2

   Remove the spark plugs.
   
   NOTE: The engine must be cold to safely remove the spark plugs.

3. 3

   Thread the compression gauge into the spark plug hole of cylinder #1.
   
   Hand tighten the gauge only —don't use a tool to snug it down.

4. 4

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

5. 5

   Write down the reading and the cylinder number on paper.

6. 6

   Release the pressure on the gauge and retest the same cylinder to confirm the result.

7. 7

   Remove the gauge and repeat the test on the remaining 3 cylinders.

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

CASE 1: 0 PSI compression in 2 or all 4 cylinders. This result points to one of the following problems:

• Timing belt failure.
• Blown head gasket.
• Severe engine damage.

Any compression value under 100 PSI (even if it isn't 0 PSI) means you've got internal mechanical engine trouble.

CASE 2: Low compression in one or more cylinders. Up to a certain point, this can be normal —especially on high-mileage engines.

But if a cylinder's compression is more than 15% lower than the highest reading you got, that cylinder will cause a misfire.

You can quickly figure out if the compression values are within range by doing some simple math. Go to: Interpreting Your Compression Test Results.

CASE 3: 0 PSI compression in one or more cylinders. Any cylinder with 0 PSI is considered "dead" and will cause either a misfire or a no-start problem.

The next step is to do a wet compression test to see if the issue is caused by worn piston rings or bad cylinder head valves. Go to: TEST 2: Wet Compression Test.

CASE 4: Compression is similar across all 4 cylinders. This is the result you want to see. It tells you the engine has good compression and that an internal mechanical problem is not the cause of the issue.

If you're troubleshooting a cylinder misfire or an engine no-start problem, this rules out compression as the culprit. You'll need to continue troubleshooting the fuel system, ignition system, or other areas to find the real cause.

Interpreting Your Compression Test Results

On a high mileage 2.0L Ford Focus, it's pretty common to see one or more cylinders with a slightly lower compression reading.

If the difference between cylinders is small, you usually won't feel it —the engine will run just fine with no noticeable problems.

But when the gap is too wide, that's when you'll run into issues like a rough idle or a misfire.

The way to know for sure if the low reading is a problem is to check whether it's more than 15% lower than the highest reading you got.

You can figure this out in one of two ways: either do the math yourself with pen and paper, or use my Online Low Engine Compression Calculator over at easyautodiagnostics.com.

If you'd rather calculate it by hand, here's how:

• STEP 1: Take the highest compression reading and multiply it by 0.15 (this equals 15%).
• STEP 2: Round the number to the nearest whole number (for example, 25.6 rounds to 26).
• STEP 3: Subtract that rounded number from the highest compression value.
• ANSWER: The result is the lowest compression a cylinder can have before it's considered a problem.

Here's a quick example: Let's say the compression readings came back as follows:

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

The next thing to do is run the calculation like this:

• STEP 1:  175 × 0.15 = 26.25.
• STEP 2:  Round 26.25 to the nearest whole number → 26.
• STEP 3:  175 − 26 = 149.
• ANSWER:  149 PSI. Any cylinder that shows this value (or less) will cause a misfire.

Since cylinder #2 only measured 95 PSI, I can say for sure it's "dead" and responsible for the misfire.

To figure out if your own lowest compression reading is acceptable, you'll need to do the same calculation —but using the highest reading from your test, not the example shown here.

Once you've confirmed which cylinder is "dead", the next step is to pinpoint why the compression is low. For that, go to: TEST 2: Wet Compression Test.

TEST 2: Wet Compression Test

When you see a cylinder with low or even 0 PSI on a compression test, it usually comes down to one of two problems:

• Badly worn or damaged cylinder head valves.
• Badly worn or damaged piston rings.

So if your compression gauge showed low compression in one (or more) of the 4 cylinders on your Ford Focus, the next step is to run the test again after squirting a little oil into that cylinder. This second check is called a "wet" compression test.

If worn piston rings are the cause, the cylinder's compression will jump up after the oil is added. If the problem is in the valves, the reading will stay about the same.

Here's how to do the test:

1. 1

   Pour about a tablespoon (or two) of engine oil into the cylinder you're going to retest.
   
   NOTE: A long, narrow funnel works best to make sure the oil actually gets down into the cylinder.

2. 2

   After adding the oil, reinstall the compression gauge and hand tighten it just like before. Do not use a tool to snug it down.

3. 3

   Have your helper crank the engine until the gauge needle stops moving upward.

4. 4

   One of two things will happen:
   
   1.) The needle climbs higher than the reading you wrote down earlier for that same cylinder.
   
   2.) The needle doesn't budge or it stays right at the previous number.
   
   Whatever the new reading is, make sure to write it down.

5. 5

   Repeat steps 1–4 on any other cylinder showing low or 0 PSI compression.

Now, let's break down what the results mean:

CASE 1: The compression reading went up after adding oil and retesting. This confirms the low compression is caused by worn piston rings.

Here's why: The oil you poured in helped the piston rings seal better. With the rings sealing the compression inside the cylinder, the gauge showed a higher reading. This type of result only happens when the problem is in the rings.

CASE 2: The compression reading did not change after adding oil and retesting. This means the low reading you got in the dry test is caused by leaking or damaged cylinder head valves.

More 2.0L Ford Focus Diagnostic Tutorials

You can find a complete list of tutorials and wiring diagrams for the 2.0L Ford Focus in this index:

• 2.0L Ford Focus Index Of Articles.