How to do a Cylinder Balance Test (Ford 5.0L, 5.8L)

This tutorial will help you to do a cylinder balance test on your 5.0L engine (or 5.8L engine).

The cylinder balance test is one of the most important diagnostic techniques that will help you find a ‘dead’ cylinder that's causing engine miss (also known as a misfire) or a rough idle (without having to spend time and money replacing things left and right to see if they'll solve the problem).

Here are the contents of this tutorial at a glance:

  1. Cylinder Balance Test Basics.
  2. Cylinder Balance Test.
  3. Misfire Troubleshooting Tutorials.

You can find this tutorial in Spanish here: Cómo Hacer una Prueba Balance de Cilindros (Ford 5.0L, 5.8L) (at: autotecnico-online.com).

Cylinder Balance Test Basics

In the majority of vehicles, you're able to ‘short out’ the engine cylinders, one at a time, by disconnecting and reconnecting the fuel injectors to see if this will affect the engine's Idle... after all, if the cylinder is ‘dead’ to begin with... unplugging the fuel injector will have NO effect on the engine's RPMs.

But... but in the Ford 5.0L engine (and 5.8L engine), disconnecting and reconnecting all 8 fuel injectors (to do a cylinder balance test) is not an option. Why? Well, because as you may already know... 4 of those 8 are underneath the intake manifold's plenum.

The other most popular method, of doing a cylinder balance test is pulling the spark plug wire off of the spark plug while the engine is running. But this method can damage the ignition coil (because it forces the spark to find a path to ground on or inside the ignition coil itself and in the process it gets fried).

Well, the cylinder balance test I'm gonna' show you in this tutorial doesn't involve disconnecting or connecting the fuel injectors or yanking on the spark plug wires while the engine is running. It involves shorting out each spark plug wire using a 12 Volt automotive test light and some small pieces of vacuum hose.

Cylinder Balance Test

How to do a Cylinder Balance Test (Ford 5.0L, 5.8L)

OK, so now you know that you'll be ‘shorting out’ one cylinder at a time on your Ford 5.0L engine (5.8L engine)...

...And you're gonna' safely accomplish by connecting a small piece of vacuum hose between the spark plug wire and the distributor tower it connects to.

Once the vacuum hoses are installed, you're gonna' ‘short out’ the cylinder by simply touching the vacuum hose with your 12 Volt automotive test light.

In the photos I've provided (in the image viewer), you can see exactly how this works.

OK, this is what you're gonna' do (in more detail):

  1. 1

    Install a small piece of vacuum hose between the spark plug wire and the distributor cap tower.

    1. See the 3 photos in the image viewer to get a clear idea of how this is done.
    2. NOTE: Although in the photos I only have one spark plug wire set up this way... I always set up 2 Plug Wires at a time and I suggest you do the same (it'll cut down the testing time and will give you 2 cylinders to compare test results).
  2. 2

    Start the engine and let the idle stabilize.

    1. If the small pieces of vacuum hose have been installed correctly, they won't affect the idle.
    2. How will you know? Because the idle won't be worse than before installing them.
  3. 3

    Start the engine and let the idle stabilize.

    1. If the small pieces of vacuum hose have been installed correctly, they won't affect the idle.
    2. How will you know? Because the idle won't be worse than before installing them.
    3. NOTE: Take all necessary safety precautions while working around a running engine.
  4. 4

    Short out the one spark plug wire by touching the vacuum hose with the tip of your 12 Volt automotive non-powered test light..

    1. NOTE: The 12 V test light needs to be grounded on a good clean metal spot on the engine or better yet, on the battery negative terminal.
    2. When you touch the vacuum hose with the tip of your 12 V test light, you should hear and feel a drop in the engine's idle (RPMs).
    3. If you also have a vacuum gauge connected, you'll see the gauge's needle drop.
    4. If the cylinder was ‘dead’ to begin with for any reason (not getting spark, low compression, etc.), you won't hear/feel a drop in the engine's idle (RPMs) or see the vacuum gauge's needle react.
  5. 5

    Repeat the test on the next cylinder.

    1. Testing 2 cylinders in the same go will help you have a comparison.
  6. 6

    Turn off the engine and repeat the set up and test on the next 2 spark plug wires

    1. You're gonna' perform this set up and test on all 8 cylinders.
  7. 7

    Interpret the results. After testing all cylinders, now you need to interpret the results...

CASE 1: Touching the vacuum hose w/ the 12 V test light DID NOT cause a drop in the RPMs this tells you that the specific cylinder you tested is ‘dead’.

The cylinder could be ‘dead’ for a number of reasons. For example:

  1. BAD spark plug (broken, severely worn out, carbon tracks, etc.)
  2. BAD spark plug wire.
  3. BAD distributor cap.
  4. Low engine compression.
  5. BAD fuel injector.

Don't let this big list of ‘possibles’ worry you... because one of the most important things you've accomplished with the cylinder balance test, is to find out which cylinder is the ‘dead’ one and this narrows down and focuses your troubleshooting efforts.

You'll find a list of articles that will further help you track down the problem that's causing that specific cylinder to misfire here: Misfire Troubleshooting Tutorials.

CASE 2: Touching the vacuum hose w/ the 12 V test light caused a drop in the RPMs this tells you that the specific cylinder you tested is OK and not the cause of the misfire (miss or ‘dead’ condition).

If you tested all 8 cylinders, and you did not see a change/drop in the RPMs, then the cause of the rough idle or miss is affecting all cylinders evenly.

Although it's beyond the scope of this article to test such a condition, I can tell you that the most likely culprit behind this type of problem is a leaking intake manifold causing the air/fuel mixture to lean out. Among other things:

  1. EGR valve stuck open.
  2. Fuel pump that's outputting insufficient fuel volume/pressure.
  3. Major vacuum leak.
  4. Low engine compression across all 8 cylinders.