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

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

This tutorial will also help you to 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 two recommendations to you:

1) Which one to buy:  The engine compression tester that I have always used is the Actron CP7827 Compression Tester Kit. My only complaint about this engine compression tester is that it does not come with a case to store it in.

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.