How To Test Engine Compression (1997-2010 4.6L, 5.4L Ford F150, F250, F350)

In this tutorial, I'll explain how to test the engine compression on the 4.6L and 5.4L V8 engine in the Ford F150, F250, and F350 pickup.

I'm also gonna explain how to interpret your test results to see if any of the compression reading you got are causing an engine performance problem or an engine no-start problem.

APPLIES TO: This tutorial applies to the following vehicles:

• 4.6L Ford F150: 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010.
• 5.4L Ford F250 Light Duty: 1997, 1998, 1999, 2000.
• 5.4L Ford F250 Super Duty: 1997, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010.
• 5.4L Ford F350 Super Duty: 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010.

E-SERIES VANS: The engine compression test tutorial for the V8 Ford E-Series vans can be found here:

• How To Test Engine Compression (1997-2007 4.6L, 5.4L V8 Ford E150, E250, E350) (at: troubleshootmyvehicle.com).

Important Tips And Suggestions

TIP 1: Don't remove the spark plugs with a hot engine. Removing the spark plugs from a hot engine can/will damage the spark plug hole threads in the aluminum cylinder head.

TIP 2: Perform the engine compression test with a cold engine. The two main benefits of doing this are:

• Preventing damage to the spark plug threads in the cylinder heads.
• Reducing the risk of burns from hot engine components.

TIP 3: Take all necessary safety precautions as you work around a cranking engine. Think safety all of the time.

Symptoms Of Low Or No Cylinder Compression

An engine compression issue will usually cause one of two problems:

• An Engine No-start Problem: This engine no-start problem is the result of more than three or more cylinders not producing any compression.
• A Cylinder Misfire Problem: This occurs when one or more cylinders produce a low compression pressure that's less than 15% of the highest compression value produced by the strongest cylinder.

Here are some other specific symptoms you may see with low cylinder compression:

• Blue smoke coming out of the tailpipe.
• Rough idle (engine misfires).
• Bad gas mileage.
• Engine 'misses' at idle but 'miss' disappears as you accelerate.
• Check engine light is illuminated with a MAP sensor trouble code (even tho' the MAP sensor is good).
• Misfire trouble codes:
  • P0300: Random Cylinder Misfire.
  • P0301: Cylinder #1 Misfire.
  • P0302: Cylinder #2 Misfire.
  • P0303: Cylinder #3 Misfire.
  • P0304: Cylinder #4 Misfire.
  • P0305: Cylinder #5 Misfire.
  • P0306: Cylinder #6 Misfire.
  • P0307: Cylinder #7 Misfire.
  • P0308: Cylinder #8 Misfire.

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

If your pickup is equipped with a 4.6L engine with ignition coil packs, I recommend two very important things:

• Label the spark plug wires with the cylinder number they belong to.
• Use a spark plug wire puller tool to unplug the spark plug wire from the spark plug.

Why use a spark plug wire puller? Cause pulling on the wire boot by hand can cause the wire's metal terminal to pull off and stay stuck on the spark plug. The following tutorial offers you more info: How To Use A Spark Plug Wire Puller And Where To Buy One.

OK, let's begin:

1. 1

   Disable the ignition system (4.6L V8 only) by disconnecting the ignition coil packs from their electrical connectors.
   
   NOTE: This is a safety precaution that will prevent the ignition coil from sparking during the test.

2. 2

   Disable the fuel system by disconnecting the fuel injectors.
   
   NOTE: Disabling the fuel system will prevent fuel from being injected into each cylinder when the test is performed.

3. 3

   Remove the spark plugs. Remember, the engine can not be hot!
   
   NOTE: 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.

4. 4

   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.

5. 5

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

6. 6

   Record the value at which the needle stopped and the number of the engine cylinder on a piece of paper.

7. 7

   Release the pressure on the gauge and check the cylinder's compression one more time.

8. 8

   Repeat test steps 4 - 7 on the remaining cylinders.

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

CASE 1: One or more 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: All six cylinders had almost no compression or 0 PSI compression. This low or zero compression on all cylinders will cause the engine to not start.

Having a compression value close to 0 PSI or 0 PSI is usually due one of the following problems:

• A busted timing chain.
• Blown head gasket.
• The engine threw a rod.

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.

Interpreting The Compression Test Results

There's a good chance that some of the compression values you got in TEST 1 are lower than the majority of the other values.

Within a certain range, differences in your engine's compression values won't cause any engine performance issues.

However, if the low compression values differ by more than 15% from the strongest cylinder, you're gonna have engine performance problems.

By doing some easy math, which I'll explain how to do in this test section, you can find out if these low compression values are causing an issue.

As a general rule, the lowest compression value shouldn't be more than 15% lower than the highest value you noted in TEST 1. If it's off by more than that, the engine cylinder will misfire. This misfire will make your Ford pickup's V8 engine 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 #3 170 PSI.
• Cylinder #4 120 PSI.
• Cylinder #5 170 PSI.
• Cylinder #6 170 PSI.
• Cylinder #7 170 PSI.
• Cylinder #8 90 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 cylinders #4 and #8, which have compression values of 120 PSI and 90 PSI respectively, are causing the misfire issue because they're below the minimum of 144 PSI.

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

If your results from TEST 1 show a cylinder or cylinders with low compression, the next step is to perform a 'wet' engine compression test.

To do this, add a small amount of motor oil to the affected cylinder and check its compression again.

If the compression value increases (compared to what you recorded in TEST 1), it tells you that the compression rings are causing the low compression issue.

If the compression value does not increase, you can conclude the problem is caused by worn/damaged cylinder head valves or valve seats in that cylinder.

Here's what you'll need to do next:

1. 1

   Add a tablespoon (or two) 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

   Once you've added the oil, install the compression gauge.
   
   As before just hand tighten the gauge.

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 cylinders you need to test.

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

CASE 1: The compression value shot up for the low compression cylinder. This confirms that the low compression value registered in this cylinder in the dry test is due to worn piston rings.

The reason the compression value shot up is due to the fact that the motor oil you just added helped the piston rings to create a tighter seal. This type of test result only happens when the problem is due to worn piston rings.

CASE 2: Your compression value DID NOT shoot up (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.

More 4.6L, 5.4L Ford F150, F250, And F350 Tutorials

There are quite a few 4.6L, 5.4L V8 F-Series pickup 'how to' tutorials and wiring diagrams you can find here:

• 4.6L, 5.4L Ford F150, F250, And F350 Index of Articles.

E-Series Vans: You can find all of the available tutorials and wiring diagrams for the 4.6L and 5.4L V8 E-Series vans in this index:

• 4.6L, 5.4L Ford E150, E250, And E350 Index of Articles (at: troubleshootmyvehicle.com).

Crown Vic/Grand Marquis: You can find all of the available tutorials and wiring diagrams for the 4.6L V8 Ford Crown Vic and Mercury Grand Marquis in this index:

• 4.6L Crown Victoria And Grand Marquis Index of Articles (at: troubleshootmyvehicle.com).