79 Silverado Chevy Dual Tank Charcoal Canister Plumbing Guide

Are you struggling with rough idling, poor fuel economy, or a persistent check engine light on your classic truck? You are not alone; many owners of vintage GM trucks face confusion when dealing with the complex vacuum lines of the 79 Silverado Chevy dual tank charcoal canister plumbing. Getting this system right is crucial not just for passing emissions tests, but for ensuring your engine runs smoothly and efficiently. In this guide, we will demystify the routing, explain the function of each component, and provide a clear path to restoring your Silverado’s performance.

Understanding the Role of the Charcoal Canister

Before diving into the hoses and clamps, it is essential to understand why this system exists. The charcoal canister is a critical part of the Evaporative Emission Control (EVAP) system. Its primary job is to capture fuel vapors from the gas tanks that would otherwise escape into the atmosphere.

In 1979, environmental regulations were tightening, and General Motors implemented more sophisticated systems to meet these standards. For trucks equipped with dual fuel tanks, this complexity increases because the system must manage vapors from two separate sources while preventing fuel sloshing from affecting the engine’s vacuum balance.

According to historical automotive data, proper EVAP maintenance can improve fuel efficiency by up to 5% in older vehicles by ensuring the engine management system receives accurate vacuum readings. Ignoring leaks or blockages in this system can lead to a rich fuel mixture, causing black smoke and wasted gasoline.

Key Components of the Dual Tank System

To correctly plumb your 79 Silverado Chevy dual tank charcoal canister plumbing, you need to identify the following key components. Missing even one valve can cause the entire system to fail.

• Charcoal Canister: The central unit filled with activated carbon that absorbs fuel vapors.
• Dual Tank Selector Valve: A solenoid-operated valve that switches between the front and rear tanks.
• Vapor Block Valve (or Rollover Valve): Prevents liquid fuel from entering the vapor lines during sharp turns or accidents.
• Purge Valve: Controlled by the engine computer or thermal switch, this allows stored vapors to be drawn into the engine intake manifold to be burned.
• Vent Valve: Allows fresh air to enter the canister during purging.

Quick Comparison: Single vs. Dual Tank Systems

Step-by-Step Plumbing Guide for the 1979 Silveradoi

Restoring the correct vacuum routing is a meticulous process. Follow these steps carefully to ensure your 79 Silverado Chevy dual tank charcoal canister plumbing is configured correctly.

Step 1: Identify the Tank Selector Valve Location

On most 1979 Silverados with dual tanks, the selector valve is located near the frame rail, often close to the driver’s side rear wheel well. This valve has electrical connections and multiple vacuum ports. Ensure the electrical connector is clean and free of corrosion. If the solenoid clicks when you switch tanks from the dashboard, the electrical side is likely functional.

Step 2: Connect the Vapor Lines from the Tanks

Each fuel tank (front and rear) will have a vapor port at the top.

1. Run a dedicated vapor line from the Front Tank to the input side of the Selector Valve.
2. Run a separate vapor line from the Rear Tank to the other input side of the Selector Valve.
3. Crucial Tip: Use high-quality fuel-resistant rubber hose (SAE J30 R9 rated). Standard vacuum hose will degrade quickly when exposed to raw fuel vapors.

Step 3: Route from Selector Valve to Canister

The output of the Selector Valve should connect to the inlet port of the charcoal canister. This line carries vapors from whichever tank is currently selected. Ensure this line is as short and direct as possible to minimize vapor condensation.

Step 4: Install the Purge Line

The purge port on the charcoal canister must be connected to the engine’s intake manifold.

1. Locate the designated purge port on your carburetor or throttle body injection unit.
2. Connect a vacuum line from the canister’s “Purge” port to this engine source.
3. This line should only allow flow when the engine is running and under specific conditions (often controlled by a thermal vacuum switch).

Step 5: Connect the Fresh Air Vent

The canister needs to breathe. Connect the “Vent” or “Air” port on the canister to the atmosphere. Some models use a simple open port with a filter, while others route it through a vent valve. Ensure this port is not blocked by dirt or debris, as a blocked vent will prevent the canister from purging effectively.

For more detailed technical specifications on emission control systems from this era, you can refer to general automotive engineering principles documented on Wikipedia’s page on Evaporative Emission Control Systems.

Common Mistakes to Avoid

Even experienced mechanics can stumble when dealing with vintage GM wiring and vacuum diagrams. Here are the most frequent errors:

• Swapping Purge and Vent Lines: This is the most common mistake. If you swap these, the canister will not purge, leading to a buildup of pressure and potential fuel leaks. Always double-check the labels on the canister.
• Ignoring the Rollover Valves: Many restorers bypass the vapor block valves in the tank lines. This is dangerous and illegal in many states. These valves prevent liquid fuel from siphoning out in a crash.
• Using Incorrect Hose Diameter: The vapor lines are typically larger in diameter than standard vacuum lines. Using narrow tubing will restrict flow and cause poor performance.

Troubleshooting Your Plumbing Setup

If you have completed the plumbing but still experience issues, consider these troubleshooting steps.

Problem: Strong Gasoline Smell

• Cause: Likely a leak in the vapor lines or a saturated canister.
• Solution: Inspect all connections for cracks. If the canister is old, the charcoal may be exhausted. Replace the canister if it feels heavy or smells strongly of fuel even when disconnected.

Problem: Rough Idle After Refueling

• Cause: The purge valve may be stuck open, allowing too much vapor into the engine at once.
• Solution: Test the purge valve for proper operation. It should be closed when the engine is cold and open only when warmed up.

Problem: Difficulty Switching Tanks

• Cause: Faulty selector valve or clogged lines.
• Solution: Check the electrical signal to the selector valve. If the valve clicks but no fuel flows, the internal diaphragm may be torn.

FAQ Section

Q1: Can I delete the charcoal canister on my 1979 Silverado?

A: While physically possible, it is highly discouraged. Deleting the canister will cause your truck to fail emissions inspections in most states. Furthermore, it can lead to poor drivability and increased fuel consumption due to the loss of the EVAP system’s regulation of fuel vapors.

Q2: How often should I replace the charcoal canister?

A: There is no fixed mileage interval, but most experts recommend inspecting it every 50,000 miles. If you notice a strong fuel smell or poor performance after refueling, it may be time for a replacement. In harsh environments, salt and moisture can degrade the canister faster.

Q3: What type of hose should I use for the vapor lines?

A: You must use fuel-vapor resistant hose. Look for hoses rated SAE J30 R9 or higher. Standard black vacuum tubing will swell and crack when exposed to gasoline vapors, leading to leaks and potential fire hazards.

Q4: Why does my truck run rich after installing new plumbing?

A: This often indicates that the purge line is connected incorrectly or the purge valve is stuck open. When the engine draws in too much fuel vapor, the air-fuel mixture becomes too rich. Check the purge valve operation and ensure the line is routed to the correct port on the intake.

Q5: Is the dual tank selector valve vacuum-operated or electric?

A: On the 1979 Silverado, the selector valve is typically electrically operated via a solenoid, but it uses vacuum to actuate the internal diaphragm that switches the fuel source. Ensure both the electrical connection and the vacuum supply to the valve are intact.

Q6: Can I use a universal charcoal canister?

A: Yes, universal canisters are available, but you must ensure they have the correct number of ports (usually three: inlet, purge, and vent). You may need to fabricate brackets to mount it securely, as OEM mounting points vary by truck configuration.

Conclusion

Mastering the 79 Silverado Chevy dual tank charcoal canister plumbing is a rewarding project that pays off in smoother performance and compliance with emissions standards. By understanding the role of each component—from the selector valve to the purge line—you can confidently diagnose and fix issues that plague many vintage truck owners. Remember, patience and attention to detail are key. Use the right materials, follow the routing diagrams closely, and don’t hesitate to consult community forums if you hit a snag.

If you found this guide helpful, please share it with your fellow classic truck enthusiasts on social media. Helping others keep their 1979 Silverados on the road preserves automotive history and keeps our streets cleaner. Happy wrenching!