For owners of the 99 Audi A4 1.8T, understanding and potentially modifying your vehicle’s systems is a common pursuit. One system that often comes under scrutiny for removal is the Secondary Air Injection (SAI) system. While designed to reduce emissions during cold starts, many enthusiasts find it to be unnecessary and cumbersome in the long run. Let’s delve into the SAI system on your 99 Audi A4 1.8T, explore its function, and guide you through the process of its removal.
Understanding the Secondary Air Injection (SAI) System in Your Audi A4 1.8T
The SAI system in your 99 Audi A4 1.8T is engineered to operate exclusively during the initial cold start phase of your engine. Its primary objective is to rapidly heat up the catalytic converter, thereby reducing harmful emissions released in the first minutes after ignition. This is achieved by injecting fresh air directly into the exhaust stream. This extra oxygen facilitates an “afterburn” effect, reacting with the rich fuel mixture common during cold starts and significantly raising the temperature of the catalytic converter much faster than would naturally occur.
However, after approximately 60 to 90 seconds of operation post cold start, the SAI system becomes inactive. Beyond this brief period, it essentially serves no functional purpose in the everyday running of your 99 Audi A4 1.8T. In fact, many view it as added complexity under the hood, contributing to potential points of failure and increased maintenance.
Here is a diagram illustrating the factory vacuum system and components of the SAI system:
Key Components of the SAI System on a 99 Audi A4 1.8T (Referencing the Diagram):
- BLUE: Secondary Air Injection Pump: This electric pump is responsible for forcing fresh air into the exhaust system. It’s often a source of noise during cold starts and a common failure point.
- RED: SAI Fresh Air Piping: These hoses and pipes carry the air from the pump to the combi valve and into the exhaust manifold.
- VIOLET: Combi Valve: This valve, also known as a combination valve, is vacuum-operated and controls the flow of air into the exhaust ports on the cylinder head. It prevents exhaust gases from flowing back into the SAI system.
- GREEN: Vacuum Line to the Combi Valve: This small vacuum line actuates the combi valve, opening it when the SAI system is active.
- ORANGE: N112 SAI Solenoid Valve: This solenoid valve, controlled by the engine control unit (ECU), regulates the vacuum signal to the combi valve, activating the SAI system.
- PINK: Path to Constant Vacuum Source for the N112: This vacuum line provides a constant vacuum source needed for the N112 solenoid to operate.
Considerations Before Deleting Your 99 Audi A4 1.8T SAI System
Deleting the SAI system on your 99 Audi A4 1.8T is a modification that requires careful consideration. While it can simplify your engine bay and eliminate potential failure points, there are a few drawbacks to be aware of:
- Diagnostic Trouble Code (DTC): Removing the SAI system will invariably trigger a P0441 code (“SAI incorrect flow”). This is because the ECU expects to see feedback from the system, which will no longer be present.
- Check Engine Light (CEL): The P0441 code will illuminate your check engine light on the dashboard. While the car will run perfectly fine, the light can be bothersome and mask other potential issues.
- Emissions Readiness: The SAI system is part of the vehicle’s emissions control system. Deleting it will cause the SAI emissions monitor to fail the “readiness” test during an OBD2 emissions inspection.
State Emissions Regulations: Depending on your location and local emissions regulations, deleting the SAI system might cause your 99 Audi A4 1.8T to fail an emissions test. Some regions allow for one or two “not ready” monitors to pass inspection, which might permit SAI deletion. However, it’s crucial to research your local regulations before proceeding.
Electrical Connection: A crucial point to remember is that the N112 solenoid valve must remain electrically connected. If it’s completely removed and unplugged, it will set an “open circuit” code, potentially leading to further issues. You can either leave the N112 solenoid plugged in without vacuum lines or replace it with a resistor to mimic its electrical load. Resistors designed for this purpose are available from aftermarket suppliers.
Step-by-Step Guide to Removing the SAI System on Your 99 Audi A4 1.8T
If you’ve weighed the pros and cons and decided to remove the SAI system from your 99 Audi A4 1.8T, here’s a step-by-step guide based on common practices:
Parts to Remove:
- Secondary Air Injection Pump
- Accordion hoses connected to the pump
- Metal hard line mounted to the front of the turbo
- Metal hard line mounted to the valve cover
- Combi valve located at the back of the cylinder head
- Vacuum line to the N112 solenoid valve
- Pump mounting bracket (optional)
Procedure:
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Disconnect the Battery: Always start by disconnecting the negative battery terminal for safety.
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Remove the SAI Pump and Hoses: Locate the SAI pump (BLUE in the diagram) and disconnect its electrical connector. Remove the accordion hoses connected to the pump and then unbolt and remove the pump itself.
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Remove Metal Hard Lines: Carefully unbolt and remove the metal hard lines connected to the turbo and valve cover. These may be held in place by brackets or clips.
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Remove the Combi Valve: Locate the combi valve (VIOLET in the diagram) at the rear of the cylinder head. Disconnect the vacuum line (GREEN) and unbolt the combi valve from the head.
Removing the combi valve from the cylinder head of a 99 Audi A4 1.8T, preparing for block-off plate installation.
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Block Off the Combi Valve Port: Once the combi valve is removed, you’ll need to seal the port on the cylinder head to prevent exhaust leaks. You can purchase a dedicated block-off plate from aftermarket vendors or fabricate one yourself. A common method is to use a square piece of steel, the original metal gasket, and high-temperature silicone sealant to ensure a proper seal.
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Plug the Airbox Hole: After removing the SAI system, you’ll have an open hole in your airbox where the SAI system’s air intake hose was connected. You’ll need to plug this hole to prevent unfiltered air from entering your intake system. A rubber expanding plug, often referred to as a “freeze plug,” in the 1-inch size, is ideal for this purpose. These are readily available at auto parts stores.
A rubber expanding plug, commonly used to seal the airbox hole after SAI system removal on a 99 Audi A4 1.8T.
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Address the N112 Solenoid: Leave the N112 solenoid valve (ORANGE) electrically connected. You can remove the vacuum lines (GREEN and PINK) from it and tuck it away, or secure it in place. Alternatively, install a resistor in place of the solenoid’s electrical connector.
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Reassemble and Test: Double-check all your work, ensure all removed ports are properly blocked off, and reconnect the battery. Start your 99 Audi A4 1.8T and check for exhaust leaks around the combi valve block-off plate. Scan for diagnostic trouble codes – expect the P0441 code to be present.
Conclusion: Simplifying Your 99 Audi A4 1.8T Engine Bay
Deleting the SAI system on your 99 Audi A4 1.8T is a popular modification for simplifying the engine bay and removing potential failure points associated with an emissions system that is only active for a very short period. While it will result in a check engine light and a readiness monitor failure, the engine’s performance and daily drivability are unaffected. Always consider your local emissions regulations before undertaking this modification. For those seeking a cleaner engine bay and are not concerned with SAI-related emissions checks, removing the SAI system on a 99 Audi A4 1.8T can be a worthwhile modification. Remember, if you are unsure about any step, consult with a qualified automotive technician.
