How To Test The Engine Compression (1998-2001 2.5L Ford Ranger)

Engine compression problems can create a misfire condition. Engine compression problems can also cause the engine to not start.

In this tutorial, I'm going to explain the engine compression test and more importantly, I'm going to explain how to interpret your test results to find out if there's a problem.

This tutorial covers the engine compression test on the 1998, 1999, 2000, and 2001 2.5L Ford Ranger (Mazda B2500).

You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (1998-2001 2.5L Ford Ranger) (at: autotecnico-online.com).

Symptoms Of Engine Compression Problems

Engine compression problems usually fall into two major categories:

• Cylinder misfire: The engine runs but with a misfire (or a rough idle).
• Engine no-start: The engine cranks, but doesn't start.

Misfire symptoms:

• Usually caused by one 'dead' cylinder with low or zero compression.
• Sets misfire-related trouble codes such as P0300, P0301, P0302, P0303, or P0304.

No-start symptoms:

• A broken timing belt/chain can keep the engine from building compression.
• A blown head gasket can allow compression to escape.
• Severe internal damage (like a thrown connecting rod) will also keep the engine from starting.

In my experience, a compression test is one of the most overlooked checks when diagnosing a tough misfire or a "cranks but won't start" problem. Yet it's one of the quickest ways to rule out (or confirm) mechanical issues.

So whether you're chasing down a cylinder misfire or a no-start issue on your 2.5L Ford Ranger (Mazda B2500), this tutorial will help you confirm if low engine compression is the culprit.

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

As you're already aware, your Ford Ranger's 2.5L 4 cylinder engine has eight spark plugs.

You've got 4 spark plugs on the exhaust manifold side of the engine. You have 4 on the intake manifold side of the engine.

To test the engine compression, you're gonna' need to remove only 4 of them. These 4 spark plugs will be the ones on the exhaust manifold side of the engine. The 4 spark plugs on the intake manifold side will stay in their place.

OK, these are the test steps:

1. 1

   Disconnect the spark plug wires from the spark plugs.
   
   NOTE: Label each spark plug wire with the number of the cylinder it belongs to. This will prevent losing the firing order of the spark plug wires.

2. 2

   Remove the spark plugs from the exhaust manifold side of your Ranger's engine.
   
   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 plug's porcelain insulator crack (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.
   
   Now, record on paper the value at which the needle stopped and the number of the engine cylinder on a piece of paper. Release the pressure on the gauge and repeat this step one more time.
   
   Repeat this test step on the remaining cylinders.

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

CASE 1: Two or more cylinders have 0 PSI compression. This test result lets you know that the engine has severe internal/mechanical problems.

CASE 2: All cylinders have 0 PSI compression. This test result usually means that the timing belt is busted.

CASE 3: All cylinders have compression, but the values vary between them. This test result needs to be investigated a bit more. Your next step is to do a simple calculation to see if the lowest compression reading is causing a misfire problem. Go to: Interpreting The Compression Test Results.

Interpreting The Compression Test Results

As the engine ages, up to a certain point it's normal for the cylinders to wear out unevenly.

In many cases, this variation in the compression values of each cylinder doesn't cause a problem.

But when uneven compression does cause a problem, it's usually because the lowest compression value varies more than 15% of the highest value.

How do you figure this out? You can find out by using my online low compression calculator here: Online Low Engine Compression Calculator or by calculating this 15% difference manually.

To manually find out if the lowest compression value is causing a problem, you'll need multiply the highest compression value by 0.15. You'll then subtract the result of this calculation from the highest compression value. The result of this subtraction is the lowest compression value any of the other cylinders can have. Anything lower and that cylinder will cause a misfire.

For example, let's say that my Ford Ranger gave me the following compression test results:

• Cylinder #1 175 PSI.
• Cylinder #2 165 PSI.
• Cylinder #3 160 PSI.
• Cylinder #4 120 PSI.

The next step is to do the following math:

• Multiply 0.15 (15%) by the highest value: 175 x 0.15. This gives us 26.25, but we'll round it out to 26.
• Next, we subtract 26 from 175: 175 - 26 = 144.
• So now we know that the lowest possible compression value is: 144 PSI.

This means that cylinder #4, which has a compression value of 120 PSI, is the one causing the misfire (P0304 trouble code) because it's below the 144 PSI minimum.

Once we'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 Compression Test.

TEST 2: Wet Compression Test

Low or 0 PSI compression in a cylinder is usually due to one of two things:

• The dead cylinder's piston rings are severely worn out.
• The dead cylinder's cylinder head valves are severely worn-out or damaged.

The cool thing is that we can find out without having to tear the engine apart. And we can accomplish this by doing a simple wet engine compression test.

This test simply involves adding a small amount of engine oil to the dead cylinder and retesting its compression. If the compression value shoots up, then we can conclude that the low compression value is due to worn out piston rings.

If the compression value does not shoot up, we can conclude that the problem is in the dead cylinder's cylinder head valves.

Let's get testing:

1. 1

   Add a tablespoon (or two) of engine oil in the cylinder you'll retest. I suggest using a small and long funnel so that the oil will reach the inside of the cylinder.

2. 2

   Install the compression gauge and tighten it hand-tight only.

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.
   
   2.) The needle will not move at all or stay at the same number you recorded earlier.
   
   Whatever value your compression tester reads, write it down again.

5. 5

   Repeat steps 1 thru' 4 on any other cylinder you need to retest.

Let's analyze your compression test result(s):

CASE 1: The engine compression value shot up. This test result let you know that the dead or misfiring cylinder's compression problem is being caused by worn-out piston rings.

CASE 2: The engine compression value DID NOT shoot up. This test result lets you know that the dead cylinder's compression problem is being caused by worn-out or damaged cylinder head valves.

More 2.5L Ford Ranger Tutorials

You can find a complete list of tutorials here: Ford 2.5L Index Of Articles.

Here's a sample of the tutorials you'll find there:

• Ignition System Circuit Diagram (1998-2001 2.5L Ford Ranger).
• Oxygen Sensor Circuit Diagram (1998-2001 2.5L Ford Ranger).
• How To Test The Front O2 Sensor Heater (1998-2001 2.5L Ford Ranger).
• Rear O2 Sensor Heater Test (1998-2001 2.5L Ford Ranger).