In this tutorial, I'll show you how to use a multimeter to check the internal resistance of the fuel injectors on the 2000-2005 3.1L Buick Century.
Although all six fuel injectors are located underneath the intake manifold plenum, you don't have to remove the plenum to test them. Instead, we'll perform all of our resistance measurements through the 16-pin fuel injector wiring harness connector located behind the alternator.
By the end of these tests, you'll be able to determine whether all six fuel injectors are electrically OK or if one (or more) has an internal fault that requires further testing or replacement.
Contents of this tutorial:
APPLIES TO: This tutorial applies to the following vehicles:
- 3.1L V6 Buick Century: 1994, 1995, 1996, 1997, 1998, 1999.
FUEL INJECTOR CIRCUIT WIRING DIAGRAMS:
1994-1999 FUEL INJECTOR TESTS:
Symptoms Of A Bad Fuel Injector
Every cylinder in your Buick Century's 3.1L V6 engine needs three things to produce power: good compression, the correct amount of fuel, and a strong spark.
Each of the six fuel injectors is responsible for supplying fuel to its cylinder. If one injector develops an internal electrical problem or stops delivering fuel, that cylinder will misfire and the engine won't run as smoothly as it should.
Since the 2000-2005 Buick Century is equipped with OBD II diagnostics, an injector circuit problem may trigger one or more of the following diagnostic trouble codes (DTCs):
- P0201: Injector 1 Control Circuit.
- P0202: Injector 2 Control Circuit.
- P0203: Injector 3 Control Circuit.
- P0204: Injector 4 Control Circuit.
- P0205: Injector 5 Control Circuit.
- P0206: Injector 6 Control Circuit.
More often than not, however, the PCM will detect the problem as a cylinder misfire instead of an injector circuit fault. When that happens, you'll usually see one or more of these misfire DTCs:
- P0300: Random/Multiple 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.
Besides setting a trouble code, a bad fuel injector can also cause one or more of the following symptoms:
- Rough or unstable idle.
- Reduced fuel economy.
- A noticeable fuel smell from the exhaust.
- Poor engine performance, especially during acceleration or when the engine is under load.
Location Of The Fuel Injector Harness Connector
Because all six fuel injectors are located underneath the intake manifold plenum, you can't measure their resistance directly without removing the plenum. Fortunately, that's not necessary for the tests in this tutorial.
We'll check the resistance of each fuel injector through the 16-pin fuel injector wiring harness connector. Photo 1 of 2 in the image viewer above identifies each terminal of this connector.
On all 2000-2005 3.1L Buick Century models, the 16-pin fuel injector wiring harness connector is located behind the alternator (see photo 2 of 2).
All of our resistance measurements will be made on the fuel injector wiring harness side of the connector, which is the half with the male spade terminals.
Testing The Fuel Injector Resistance
The factory specification for the fuel injectors is 11.4 to 12.6 Ohms when measured at an ambient temperature of 50-95° F (10-35° C). This specification applies when measuring the injector directly across its two terminals after it's been disconnected from the wiring harness.
Since we'll be checking each injector through the 16-pin fuel injector wiring harness connector, don't be surprised if your readings are slightly higher than the factory specification. The additional wiring between the connector and the injector can add a small amount of resistance.
For that reason, the most important thing isn't the exact resistance value of each injector. What you're looking for is that all six injectors have very similar resistance readings. If one injector measures noticeably higher or lower than the rest, it's a strong indication that it has an internal electrical problem.
If one injector stands out from the others, the next step is to remove the intake manifold plenum and measure that injector directly across its two terminals. This will let you confirm whether its resistance is within the factory specification.
NOTE: Need a multimeter or need to upgrade yours? Check out my recommendation here: Tekpower TP8268 AC/DC Auto/Manual Range Digital Multimeter (Amazon affiliate link).
With that out of the way, let's get started:
- 1
Set your multimeter to the Ohms (Ω) function.
- 2
Disconnect the 16-pin fuel injector wiring harness connector.
NOTE: All resistance measurements are made on the fuel injector wiring harness side of the connector (the half with the male spade terminals). - 3
Measure the resistance between terminals A and C to test fuel injector #1.
- 4
Measure the resistance between terminals A and D to test fuel injector #2.
- 5
Measure the resistance between terminals A and E to test fuel injector #3.
- 6
Measure the resistance between terminals A and F to test fuel injector #4.
- 7
Measure the resistance between terminals A and G to test fuel injector #5.
- 8
Measure the resistance between terminals A and H to test fuel injector #6.
Let's interpret your test results:
CASE 1: All six injectors have very similar resistance readings. This is the expected result and indicates that none of the injectors has an obvious internal electrical problem.
If you still suspect a fuel injector issue, the next section will show you how to continue diagnosing it: How To Find The Bad Or Clogged Fuel Injector.
CASE 2: One injector has a resistance reading that's noticeably different from the others. This is usually a sign that the injector is internally defective.
Remove the intake manifold plenum and measure that injector directly across its two terminals. If its resistance isn't within the factory specification of 11.4 to 12.6 Ohms, you've confirmed that the injector is bad and needs to be replaced.
Removing The Intake Manifold Plenum
If your test results indicate that you need to remove the intake manifold plenum to inspect or verify a fuel injector, keep these tips in mind before you begin:
TIP 1: Cover the lower intake manifold runners with clean shop towels immediately after removing the plenum. This simple step helps keep screws, bolts, washers, tools, and any other foreign objects from accidentally dropping into the engine.
If something falls into one of the open intake runners and the engine is started, it can cause extensive internal engine damage. Spending a few seconds to cover the openings is cheap insurance against a very costly repair.
TIP 2: Install the intake manifold plenum gaskets without using any sealant. Don't apply RTV silicone or any other gasket sealer. These gaskets are designed to seal properly when installed clean and dry.
TIP 3: Tighten the intake manifold plenum bolts to 11 ft-lbs (15 N·m) using a torque wrench. This helps ensure the plenum seals evenly while preventing damage caused by over-tightening the bolts.
TIP 4: Disconnect the negative (-) battery cable before removing the intake manifold plenum. This extra safety step prevents the engine from being accidentally cranked while you're working around the fuel injectors and the exposed intake manifold.
How To Find The Bad Or Clogged Fuel Injector
When a cylinder misfire shows up, it's tempting to point the finger at the fuel injector right away. But before we condemn it, we need to remember that a misfire can be caused by a lack of spark, a lack of compression, or a lack of fuel.
That's why the best way to find a bad or clogged fuel injector is to follow a simple diagnostic path. We eliminate the most common causes first, and then we zero in on the injector.
This is the step-by-step process I follow:
- Identify the cylinder that's misfiring.
- Since the 2000-2005 Buick Century is an OBD II vehicle, a scan tool or code reader will usually identify the misfiring cylinder with a P0301 through P0306 trouble code.
- If you only have a P0300 random misfire code, then you'll need to identify the dead cylinder with a cylinder balance test or another diagnostic method.
- Make sure the misfiring cylinder is getting spark.
- A bad spark plug, bad spark plug wire, or ignition coil problem can make the engine act just like it has a bad fuel injector.
- So before we blame the injector, we need to make sure that cylinder is getting a good, strong spark.
- This tutorial will help you with that test: How To Test The Ignition Coil Packs (1994-2005 3.1L V6 Buick Century).
- Make sure the misfiring cylinder has good compression.
- A cylinder with low or zero compression will misfire no matter how good the spark or fuel injector is.
- This is one of those tests that's easy to skip, but it's too important to leave out.
- You can find the compression test here: How To Test Engine Compression (1994-2005 3.1L V6 Buick Century).
- Check the fuel injector's internal resistance.
- Once we know the cylinder has spark and compression, our next step is to check the injector's resistance with the tests in this tutorial.
- If one injector's resistance reading is very different from the others, remove the intake manifold plenum and test that injector directly at its two terminals. If it's outside the factory specification, you've found the cause of the misfire.
- Check whether the injector is actually delivering fuel.
- A fuel injector can have a good resistance reading and still be clogged or restricted.
- If the injector passes the resistance test, the next step is to remove it and check its spray pattern and fuel delivery.
- This Ford Taurus tutorial explains how to use a DIY tool to check injector flow: Fuel Injector Spray Test (1990-2007 3.0L V6 Ford Taurus, Mercury Sable).
The key to this diagnostic approach is that each step tells us where to go next. Once we've identified the misfiring cylinder, we check for the three things that cylinder needs to work: spark, compression, and fuel.
If spark and compression are good, then the fuel injector becomes our main suspect. If the injector passes the resistance test and the spray pattern test, then we can rule it out and keep looking for the real cause of the misfire.
This step-by-step process keeps us from guessing and replacing parts blindly. It helps you find the actual problem before spending money on parts your Buick Century doesn't need.
More 3.1L V6 Buick Century Diagnostic Tutorials
You can find a complete list of 3.1L V6 Buick Century diagnostic tutorials and wiring diagrams here:
Here's a list of articles you'll find there:
- How To Test The Fuel Pump (1994-2005 3.1L V6 Buick Century).
- How To Test The Ignition Coil Packs (1994-2005 3.1L V6 Buick Century).
- How To Test The TPS With A Multimeter (1997-2005 3.1L V6 Buick Century).
- How To Test Engine Compression (1994-2005 3.1L V6 Buick Century).
If this info saved the day, buy me a beer!
