How To Do A Cylinder Balance Test (Ford 5.0L, 5.8L)

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

The cylinder balance test is one of the most important diagnostic techniques that'll help you identify a 'dead' cylinder causing an engine miss (aka misfire) or a rough idle without having to spend time and money swapping things left and right to see if they solve the problem.

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

For most vehicles, you can 'short out' the engine cylinders one at a time by disconnecting and reconnecting the fuel injectors to see if that affects the engine's idle RPM. If any cylinder is 'dead' to begin with, unplugging its fuel injector will have NO effect on the engine's RPM.

On the Ford 5.0L (and 5.8L) engine, disconnecting and reconnecting all 8 injectors (to do a cylinder balance test) is not an option. Why? Well, as you may already know, 4 of those 8 injectors are located under the intake manifold plenum.

The other most popular way to perform a cylinder balance test is to disconnect the spark plug wire from 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 in the ignition coil itself.

Well, the cylinder balance test I'm gonna show you in this tutorial doesn't involve disconnecting or connecting the fuel injectors or pulling the spark plug wires (from the spark plugs) while the engine is running. My test involves shorting each spark plug wire with a 12 volt automotive test light and some small pieces of vacuum hose.

Cylinder Balance Test

OK, so now you know you're gonna 'short' one cylinder at a time on your Ford 5.0L (5.8L) engine.

The key to safely 'shorting out' the cylinder is 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'll 'short out' the cylinder by simply touching the vacuum hose with your 12 volt automotive test light.

In the photo above, you can see exactly how this is set up and how it gonna work.

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

1. 1

   Disconnect two spark plug wires from the distributor cap.

2. 2

   Install a small piece of vacuum hose between the spark plug wires and the distributor cap towers.
   
   NOTE: Although in the photo above I only have one spark plug wire set up this way, I always set up two spark 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).

3. 3

   Start the engine and let the idle stabilize.
   
   If the small pieces of vacuum hose have been installed correctly, they won't affect the engine's idle. If the engine's idle is worse than before installing them, you'll need to figure out what went wrong.
   
   NOTE: Take all necessary safety precautions while working around a running engine.

4. 4

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

5. 5

   You should hear and feel a drop in the engine's idle (RPMs) when you touch the vacuum hose with the tip of your 12 V test light.
   
   If you also have a vacuum gauge connected, you'll see the gauge's needle drop.
   
   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.

6. 6

   Repeat the 'shorting out' of each of the two spark plugs wires to make sure of your test result.

7. 7

   Turn off the engine and repeat the steps 1-6 on the next 2 spark plug wires.
   
   You're gonna' perform this set up and test on all 8 cylinders.

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

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:

• Bad spark plug (broken, severely worn out, carbon tracks, etc.)
• Bad spark plug wire.
• Bad distributor cap.
• Low engine compression.
• 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 finding 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 with the 12V 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 rough idle 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 misfire is affecting all cylinders equally.

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

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

Misfire Troubleshooting Tutorials

OK, so you've found the dead cylinder on your Ford 5.0L or 5.8L engine, what's next? Well, the next step is to find out what's causing the misfire (rough idle) condition.

There are several problem areas that you need to test and these areas are:

1. Ignition System. For example:
   • Bad distributor cap.
   • Bad spark plug.
   • Bad spark plug wires.
2. Fuel System. For example:
   • Bad fuel injector.
   • Fuel injector not being activate by PCM or due to a short in the wiring.
3. Engine Mechanical. For example:
   • Low engine compression.

Don't get discouraged by this list of problem areas, because the most important thing you've done (to narrow down the rough idle or cylinder misfire) is to know which cylinder is the 'dead' one.

I'm gonna offer you my diagnostic strategy that'll further assist you in finding the root cause of the misfire on your Ford 5.0L or 5.8L engine.

Here are my suggestions (and the diagnostic strategy I use):

1. Test the ignition system first.
   • The ignition system is usually the culprit behind most of the misfire ('dead' cylinder) conditions.
   • You can find an in-depth tutorial on how to test the distributor cap and spark plug wires here: How To Test The Ford Ignition Control Module (at: easyautodiagnostics.com).
2. Test the engine compression second.
   • After making sure that the ignition system is providing spark to the 'dead' cylinder, the next step would be to check that cylinder's compression.
   • You can find an in-depth tutorial on how to test the engine compression here: How To Test Engine Compression (4.9L, 5.0L, 5.8L).
3. Test the fuel injectors last.
   • After eliminating the ignition system and the engine compression as the cause of the misfire, the next step is to make sure that the fuel injector is not fried internally.
   • You can find an in-depth tutorial on how to do a fuel injector resistance test here: How To Test The Fuel Injectors (5.0L Engine, 5.8L Engine).

The list of tests above can be modified to suit your specific diagnostic and troubleshooting needs (since there really isn't a specific cookie cutter way of diagnosing this type of issue).