How To Test Engine Compression (Ford F150, F250, F350)

This tutorial will help you to test the engine compression on your Ford F-150, F250, and F-350 pickups.

This tutorial will also help you interpret your compression test results to find out if a compression problem is causing an engine performance or driveability issues.

APPLIES TO: This tutorial applies to the following vehicles:

• Ford Bronco 4.9L, 5.0L, 5.8L: 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996.
• Ford F150 4.9L, 5.0L, 5.8L: 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996.
• Ford F250 4.9L, 5.0L, 5.8L: 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996.
• Ford F350 4.9L, 5.8L: 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997.

Symptoms Of Low Or No Engine Cylinder Compression

Engine compression problems usually cause 1 of 2 types of problems:

• The compression problem will cause the engine to not start.
• The compression problem allows the engine to start but the engine runs with a misfire problem or idles very rough.

If your Ford F-Series pickup starts and runs, an engine compression problem will cause one or more of the following symptoms:

• Bad gas mileage.
• A heavier exhaust smell coming out of its tailpipe.
• Engine is not as peppy as it was once.
• Rough idle that goes away as soon as you accelerate the engine.

Important Compression Test Tips

TIP 1: Label the spark plug wires with the cylinder # that they belong to before removing the spark plugs. This will help you to not lose the spark plug wires' firing order.

TIP 2: Let the engine cool down completely before removing the spark plugs. If the engine has been running for any length of time, let it cool down before removing the spark plugs. Removing the spark plugs from a hot engine can cause the spark plug hole threads to strip in the cylinder head. This is a nightmare you want to avoid!

TIP 3: Have your helper wait outside of the vehicle while you set up the compression tester in the cylinder. This is an important safety precaution that will keep your helper from mistakenly cranking the engine while you set up the test.

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: Finding The Dead Cylinders

You're going to need a compression tester to be able to perform the engine compression test. If you don't have one, you can run down to your local auto parts store (AutoZone, O'Reilly Auto Parts) and borrow one from them (for a small deposit which they'll return once you return the tool).

If you'd like to buy your own and save a few bucks in the process, check out my recommendations here: Which Compression Tester Should I Buy?

IMPORTANT: If the engine has been running for any length of time, let it cool down completely before removing the spark plugs.

OK, 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. Remember, the engine can not be hot!
   
   When removing the spark plugs, be careful not to drop any on the floor, or you run the risk of having the spark plug's 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.
   
   Release the pressure on the gauge and repeat this step one more time.

6. 6

   Repeat test steps 3 - 5 on the remaining cylinders.

Let's examine your test results:

CASE 1: One or two cylinders had a much lower compression value than the others. Up to a certain range, this could be normal.

To further interpret these test results go to: Interpreting The Compression Test Results.

CASE 2: Three or more cylinders had almost no compression or 0 PSI compression. Having three or more cylinders with almost no compression or 0 PSI compression will cause the engine your Ford F150 (F250, F350) to not start.

Having a compression value close to 0 PSI or 0 PSI is usually due to a problem in the cylinder head valves or piston rings of the affected cylinders.

We can find out which of the two it is by doing a wet compression test. For this test go to: TEST 2: Wet Engine Compression Test.

CASE 3: The compression value of all six cylinders was similar and above 120 PSI. This test result lets you know that an engine compression problem is not behind the misfire condition and/or engine no-start problem you're troubleshooting.

I'll explain why: If the engine had a compression problem causing a misfire or a no-start condition, then your test results would have indicated one or more cylinders with low or very close to 0 PSI compression.

Since your test results indicate that all cylinders have very similar compression values, you can rule out engine compression as a source of the misfire or engine no-start problem you're trying to diagnose.

Interpreting The Compression Test Results

Depending on the age and wear and tear of your Ford pickup's engine, more than likely you're going to see a variation in the compression test values you got in TEST 1.

Up to a certain point, this variation will not cause any engine performance or driveability issues.

But if the compression value variations are too great, then you're definitely going to see the engine suffering a misfire problem or a rough idle problem.

Finding out if the low compression values (you got from TEST 1) are causing a problem is not difficult. In this section, I'll explain how to do just that.

The rule of thumb is that the lowest compression value can not vary more than 15% of the highest value (that you wrote down in TEST 1). If any value is lower by more than 15%, then that engine cylinder is going to misfire. This misfire will cause your Ford F150 (F250, F350)'s engine to idle rough.

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 understand how to figure out this 15% thing manually, I'll use the following compression test results:

• Cylinder #1 175 PSI.
• Cylinder #2 165 PSI.
• Cylinder #3 160 PSI.
• Cylinder #4 120 PSI.
• Cylinder #5 160 PSI.
• Cylinder #6 170 PSI.

The next step is to do the following math:

• Multiply .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 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 Engine Compression Test.

TEST 2: Wet Engine Compression Test

It's not difficult to figure out what's causing the low or 0 PSI compression reading you got in TEST 1.

In most cases, the low or 0 PSI compression test result is caused by the piston rings or cylinder head valves of the affected cylinder.

Thankfully, we don't need to disassemble the engine to find out where the problem lies. Since performing a simple wet compression test (on the affected cylinder) to find out where the problem lies.

In case you're wondering, a wet compression test simply involves adding 1 to 2 tablespoons of engine oil to the cylinder that has low or 0 PSI compression and then re-testing its compression.

If the compression value increases, then you can conclude that the problem lies in the affected cylinder's piston rings.

If the compression value remains the same, then you can conclude that the affected cylinder's intake/exhaust valves are worn or damaged.

OK, let's get started:

1. 1

   Add 1 or 2 tablespoon of engine oil in the cylinder you need to 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 on the cylinder you just added oil to.

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.
   
   What ever value your compression tester reads, write it down again.

5. 5

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

Let's examine your test results:

CASE 1: The compression value of the cylinder you added oil to increased. This test result tells you that the low or near 0 PSI compression value is due to worn out piston rings of that specific cylinder.

CASE 2: The compression value of the cylinder you added oil to DID NOT increase. This test result confirms that the cylinder head valves of that cylinder are worn-out or damaged.

More Ford F150, F250, F350 Diagnostic Tutorials

You can find more tutorials Ford F150, F250, and F350 in the following index:

• Ford 4.9L, 5.0L, 5.8L Index Of Articles.

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

• How To Test The Ford Ignition Control Module (Distributor Mounted).
• How To Test The Ford Ignition Control Module.
• Throttle Position Sensor (TPS) Test (Ford 5.0L, 5.8L).
• How To Test The Ford Mass Air Flow (MAF) Sensor.