Testing the fuel injectors can be a challenge considering that on the 4.0L OHV engine, three of the six fuel injectors are underneath the intake manifold plenum. On the 4.0L SOHC engine, all six are under the intake manifold plenum.
To resistance test the fuel injectors underneath the intake manifold plenum, you'll need to remove it.
This tutorial will explain the fuel injector resistance test. I'm also sharing my own fuel injector diagnostic strategy that'll help you find out if a fuel injector is indeed causing an engine performance problem.
NOTE: You can find the 1997-2000 4.0L Ford Ranger (Mazda B4000) fuel injector test here:
- Fuel Injector Resistance Test (1997-2000 4.0L Ford Ranger And Mazda B4000) (at: easyautodiagnostics.com).
Contents of this tutorial:
APPLIES TO: This tutorial applies to the following vehicles:
- 4.0L V6 Ford Aerostar: 1997.
- 4.0L V6 OHV Ford Explorer: 1997, 1998, 1999.
- 4.0L V6 SOHC Ford Explorer: 1997, 1998, 1999, 2000, 2001.
- 4.0L V6 SOHC Mercury Mountaineer: 1998, 1999, 2000, 2001.
You can find the 1991-1996 4.0L Ford Explorer (Aerostar) fuel injector test here:
Fuel Injector Resistance Specifications
| Aerostar | ||
|---|---|---|
| Year | Engine | Ohms |
| 1997 | 4.0L (OHV) | 11-18 Ohms |
| Explorer | ||
|---|---|---|
| Year | Engine | Ohms |
| 1997-2001 | 4.0L (OHV & SOHC) | 11-18 Ohms |
| Mountaineer | ||
|---|---|---|
| Year | Engine | Ohms |
| 1998-2001 | 4.0L (SOHC) | 11-18 Ohms |
The Fuel Injector Resistance Test
The fuel injector resistance test involves checking the internal resistance of the fuel injector's coil winding.
The individual fuel injector's resistance is then compared to the factory specification.
If the resistance is not within the specification, you can conclude that the fuel injector is bad and needs replacement.
If the resistance is within specification, you can conclude that the fuel injector doesn't have an internal open-circuit or short-circuit problem.
If you haven't already, take a look at the following section:
NOTE: You'll need a multimeter to test the fuel injector resistance. If you don't have one, check out my recommendations here: Buying A Digital Multimeter For Automotive Diagnostic Testing (at: easyautodiagnostics.com).
Let's start the test:
- 1
Removing the intake manifold's plenum.
NOTE: Place clean rags in the open intake runners.
See this section: Important Precautions When Removing The Intake Manifold Plenum. - 2
Disconnect all six fuel injectors from their electrical connectors.
- 3
Place your multimeter in Ohms mode.
- 4
Probe the metal male spade terminals inside the fuel injector with the multimeter test leads.
- 5
Your multimeter should register a resistance between 11-18 Ohms.
- 6
Repeat test steps on the remaining fuel injectors.
Let's analyze your test results:
CASE 1: All resistance values were within specification. This is the correct and expected test result and it confirms that fuel injectors do not have an internal short-circuit or open-circuit problem.
CASE 2: A fuel injector resistance value is not within specification. This test result confirms the fuel injector has suffered an internal short-circuit or open-circuit problem and needs replacement.
Fuel Injector Troubleshooting Strategy
There is a method that you can use to find out if you've got a bad fuel injector on your 4.0L Ford Explorer (Aerostar or Mercury Mountaineer).
I've used this diagnostic strategy many times over the years and it has always pinpointed the exact source of the misfire or rough idle condition.
It has also helped me avoid having to remove the intake manifold plenum when my test results indicated that the problem wasn't due to a bad fuel injector.
PART 1: Identify The Dead Cylinder.
Identifying the 'dead' cylinder is the critical first step of this fuel injector diagnostic strategy.
You can pinpoint the 'dead' cylinder by performing a cylinder balance test.
The biggest benefit of the cylinder balance test is that it'll help determine if the 'dead' cylinder's fuel injector is located underneath the intake manifold plenum.
NOTE: Even if your Ford Explorer (Mercury Mountaineer) is OBD II equipped and you've got a cylinder misfire diagnostic trouble code, do the cylinder balance test to make sure you've identified the correct cylinder.
PART 2: Make Sure The Dead Cylinder Is Getting Spark.
After you've identified the 'dead' cylinder, your next step is to make sure its spark plug wire delivers spark to its spark plug.
You must perform the spark test with a dedicated spark tester. Any spark test method that does not involve a spark tester will only have you chasing ghosts (and wasting time and money).
You'll also need to:
- Check the spark plug for any apparent damage.
- Check the spark plug for excessive wear.
- Check the spark plug electrodes are not blocked with carbon.
- Check the spark plug's air gap.
- Check the spark plug, and the spark plug wire does not have carbon tracks.
- Check the spark plug wire isn't missing the metal connector that connects to the spark plug.
If the 'dead' cylinder isn't getting spark, you've found the source of the misfire (rough idle), you don't need to test the fuel injectors.
If the 'dead' cylinder is getting spark and the spark plug and spark plug wire are OK, the next step is checking the 'dead' cylinder's compression.
PART 3: Make Sure The Dead Cylinder Has Compression.
If the ignition system is not causing the 'dead' cylinder issue, your next step is to make sure the 'dead' cylinder produces enough compression to start and complete the air/fuel mixture's combustion process.
Now, you don't need to test all six cylinders. But for the accuracy of your diagnostic, you should test all six.
You can find the compression test explained in detail here:
If the 'dead' cylinder's compression value is 15% lower than the highest compression reading your compression test has given you, then you've found the cause of the misfire (rough idle) issue, and you don't need to test the fuel injectors.
If the 'dead' cylinder's compression value is within a normal range, your next step is to test the fuel injectors.
PART 4: Test The Fuel Injectors.
If you've reached this point, you have:
- Confirmed the 'dead' cylinder is getting spark.
- Confirmed the 'dead' cylinder's compression is OK.
If the 'dead' cylinder's fuel injector is underneath the intake manifold plenum, you can now make plans to remove it and resistance test the fuel injector.
