Part Two: Understanding the Air Injection System on 1981-1987 XJ6 and V12 XJS Jaguars

In Part One, we discussed the emissions check valve and its role in the air injection system. Now, let’s take a step back and explore how the entire air injection system works on the 1981-1987 XJ6 Jaguars and V12 XJS models. Understanding this system will help you troubleshoot and maintain it effectively.

Why Does the Air Injection System Exist?

When you start your Jaguar with a cold engine, the fuel injection computer (ECU) delivers extra fuel to keep the engine running smoothly during the warm-up period. This process is similar to how old carbureted cars used a manual choke. However, this extra fuel results in increased unburned hydrocarbons in the exhaust, which negatively impacts emissions.

To address this, Jaguar designed the air injection system to reduce those emissions by injecting fresh air into the exhaust stream. The additional oxygen allows unburned hydrocarbons to finish burning before they exit the tailpipe.

Once the engine warms up, the ECU determines that air injection is no longer needed. At this point, the system shuts off the airflow to the air injection rail.

Key Components of the Air Injection System

The air injection system has four main components that work together:

1. The Air Pump
   • Driven by a belt, the air pump supplies a constant flow of fresh air.
   • Since the pump runs continuously, the system relies on valves to control where the air goes.
2. The Diverter Valve
   • Once the engine warms up and the air injection system is no longer needed, the diverter valve redirects the air from the pump to the air filter housing.
3. The Check Valve
   • Prevents exhaust gases from flowing backward into the air injection system when the air pump is not actively injecting air into the exhaust stream.
   • Acts as a critical safeguard to protect the pump and other components from heat and damage.
4. The Air Injection Rail(s)
   
   • These are the pipes that run along the top of the cylinder head, injecting fresh air into the exhaust stream during cold starts.

How the System Works

Here’s the airflow sequence:

1. Cold Start:
   • The ECU commands extra fuel to keep the engine running.
   • The air pump delivers fresh air into the air injection rails, providing oxygen to burn off the unburned hydrocarbons in the exhaust stream.
2. Warm Engine:
   • Once the engine reaches operating temperature, the air injection system shuts off.
   • The diverter valve redirects the pumped air to the air filter housing instead of the air injection rails.
3. Check Valve’s Role:
   • Prevents exhaust gases from forcing their way back into the system and damaging the pump when the air injection system is inactive.

Final Notes

The air injection system may seem complex, but it’s brilliantly designed to improve emissions without compromising engine performance. If any part of the system fails—whether it’s the air pump, diverter valve, check valve, or air injection rails—it can lead to poor emissions performance, excess exhaust noise, or even damage to the system.

With this overview, you should now have a solid understanding of how the air injection system works. If you missed Part One, revisit it for details on replacing the check valve, a common failure point in this system.

Stay tuned for more DIY Jaguar tips, and happy tinkering!