Replacing The Fuel Plate: 12-Valve Cummins Engine Maintenance Guide

Changing the fuel plate on a 12-valve Cummins engine can increase horsepower and torque without sacrificing engine reliability. The process involves removing the AFC housing, modifying or replacing the fuel plate, and making adjustments to the governor lever, star wheel, and AFC housing. It is important to note that there are different fuel plates and torque specifications for different models and years of the Cummins engine, so it is crucial to have the correct information for your specific engine before starting the process.

Removing the AFC housing

First, locate the AFC housing. It is on the passenger side of the engine, and you will need to remove the front passenger-side AFC housing break-off screw to access it. This screw is a factory break-off screw with a rounded head and no screwdriver, Allen head, or TORX slot. To remove it, you can use a hand impact tool with a 6-inch long 1/4" extension and a T15 TORX bit. If the center of the screw is difficult to access, you may need to use a center punch to start a hole. Set the hand impact tool to the removal position (counter-clockwise) and use light hammer taps to loosen the screw. Gradually tap harder until the screw loosens, forming a TORX shape that allows you to reinstall it later with the T15 TORX bit. In some cases, you may need to use a T20 TORX bit if the hole in the screw is too large. There is usually a lock washer and a flat washer under these screws, so be sure to keep track of them.

Alternatively, you can use a sharp chisel to remove the break-off screw. Make a notch on the outside diameter of the screw head and apply light blows with a tangential force to the chisel in a counter-clockwise direction to remove the screw.

Once the AFC housing break-off screw is removed, you can lift and remove the AFC housing. With the housing removed, you will now have access to the fuel plate, which is located at the back of the injector pump inside the AFC.

If you have never removed the AFC housing before, there may be a tamper-proof bolt holding it down. Some people suggest hammering the right-size Torx bit into the head of the bolt, but another method is to use a long chisel or punch with a sharp edge. There is no need to remove fuel lines or intakes; simply loosen the clamp holding the fuel lines together. Slide the chisel or punch down through the fuel lines until it reaches the head of the tamper-proof bolt and give it a few firm taps with a hammer. The head of the bolt is softer than it looks, and you should be able to create a decent groove with the chisel or punch. Then, use a flat-blade screwdriver to help remove the bolt easily.

Next, remove the other three bolts on the AFC housing using a socket/extension; these are 8mm. Slightly pull down and away on the solenoid bracket, just enough to create space to lift out the AFC housing. Set the AFC housing aside, and you will now have access to the fuel plate, which is held in place by two screws.

Remember to mark the location of the fuel plate in its stock location before making any adjustments or replacements. You can scribe a mark around it, measure the distance from the body, or use any other suitable method. This reference point will be useful if you ever need to return the fuel plate to its original position.

Now that the AFC housing is removed, you can proceed with adjusting or replacing the fuel plate as needed. Just be sure to put everything back together in the reverse order when you're done.

Modifying the fuel plate

Firstly, locate the fuel plate. It is situated at the back of the injector pump, inside the AFC, under the AFC housing. To access it, you need to remove the AFC housing. This involves dealing with a tamper-proof screw, which can be challenging. One method is to use a long chisel or punch to create a groove in the head of the bolt, allowing it to be removed with a flat-blade screwdriver. Alternatively, you can use a hand impact tool with a T15 TORX bit to remove the screw.

Once the AFC housing is removed, you will see the fuel plate. Mark the location of the fuel plate in its stock position for reference. If you simply want to move the plate forward, loosen the two screws holding it in place, slide it forward, and then tighten the screws.

If you are installing a new fuel plate, or cutting a new profile into the existing plate, remove the two screws holding the plate and lift it out. For a new plate, simply place it in position and reassemble in reverse order. If you are cutting a new profile, use a marker to indicate the desired profile on the plate, and then use an electric angle grinder to cut and shape the plate. Finish by smoothing the profile with a mini angle grinder and a 3M Roloc disc.

After modifying the fuel plate, remember to clean the area with brake cleaner and dry it before reinstalling the fuel plate. You can mount the plate all the way forward or return it to the stock location using your reference marks.

Adjusting the governor lever

First, it's important to understand what the governor lever does. The governor lever controls the fuel delivery at maximum boost, and it needs to be adjusted properly for optimal performance. With the AFC housing removed, you'll be able to see the fuel plate and the governor lever. The lever should hit the cam plate flush at the very bottom edge.

To adjust the governor lever, follow these steps:

• Move the throttle lever until it just touches the cam plate.
• Look down through the opening behind the cam plate and observe where the lever is striking the plate.
• For plates similar to the stock plate, adjust the lever so it is flush with the bottom edge of the plate and does not ride under the plate.
• For cam plates with a taper at the bottom, adjust the lever so it hits at the tip of the bottom nose of the plate. It can be slightly below, but ensure it doesn't ride under the plate.
• Loosen the locknut with a 10mm socket.
• Make small adjustments to the screw, fine-tuning the lever height.
• Tighten the locknut once the desired height is achieved.

Remember, the fuel plate is also known as a full load throttle stop. This means that if you're not running at wide-open throttle or close to it, the plate isn't actively doing anything.

By adjusting the governor lever correctly, you can ensure that your engine is delivering the right amount of fuel at maximum boost, resulting in improved performance and power output.

Cranking the star wheel

Step 1: Accessing the Star Wheel

Start by removing the AFC housing. This will give you access to the star wheel, which is located behind a large plug on top of the AFC housing. Use an 8mm Allen wrench to remove this plug, being careful not to lose the sealing washer underneath.

Step 2: Adjusting the Star Wheel

The star wheel controls the quickness of the fueling response to boost. To adjust it, rotate the top of the star wheel. If you're experiencing weak acceleration with minimal smoke, turn the star wheel towards the engine a few revolutions. On the other hand, if your vehicle produces heavy black smoke on initial acceleration, rotate the star wheel away from the engine. Make small adjustments and test-drive the vehicle after each change until you achieve the desired results.

Step 3: Reinstalling the Plug

Once you're happy with the adjustments, reinstall the large plug on top of the AFC housing. Make sure to use the sealing washer to prevent any leaks.

Troubleshooting:

If you're still encountering heavy smoke even with the star wheel moved completely to the rear position, you may need to adjust the AFC housing. Loosen the AFC housing and slide it rearward by about 1/16 of an inch. Then retighten and test again. Repeat this process until you achieve the desired smoke levels.

Remember, adjusting the star wheel is just one part of the process of changing the fuel plate on your 12-valve Cummins. Make sure to follow a comprehensive guide for the entire process, which includes steps such as removing the AFC housing, modifying the fuel plate, adjusting the governor lever, and sliding the AFC housing forward.

Sliding the AFC housing forward

To slide the AFC housing forward, you must first remove the AFC housing. This can be done by removing the front passenger side AFC housing break-off screw. This screw is a factory break-off screw with a rounded head and no screwdriver, Allen head, or TORX slot. To remove it, you can use a hand impact tool with a 6-inch-long 1/4" extension and a T15 TORX bit. Set the hand impact tool in the removal position (counter-clockwise) and use light hammer taps on the impact tool to loosen the screw. Tap progressively harder until the screw loosens. This method will form a TORX shape into the screw, allowing it to be reinstalled later with the T15 TORX bit.

In some cases, the hole in the top of the screw may be too large for the T15 bit, so you may need to use a T20 TORX bit instead. There is usually a lock washer and a flat washer under each of these screws. Optionally, you can use a sharp chisel to remove the break-off screw by making a notch in the outside diameter of the head and then applying a tangential force with light blows to the chisel counterclockwise to remove the screw.

With the AFC housing removed, you can now see the fuel plate. Sliding the stock plate will increase HP quite a bit. Slide the plate all the way forward and if you still want more power, you can contact a specialist to cut the plate or send them your plate to be cut to your desired profile.

Frequently asked questions