Converting your 1996 and newer Honda Civic to OBD1 for tuning often presents a challenge with the Idle Air Control Valve (IACV). The IACV is crucial for maintaining a smooth idle by regulating the airflow that bypasses the throttle body. OBD1 systems utilize a 2-wire linear IACV, which is incompatible with the 3-wire rotary IACV found in OBD2 Civics. To overcome this, you can either replace your intake manifold and throttle body with OBD1 versions or, as detailed in this guide, modify your existing OBD2 intake manifold to accommodate a 2-wire IACV. This modification, while requiring some mechanical skill, allows you to retain your OBD2 intake manifold while utilizing an OBD1 ECU for tuning purposes.
Understanding the IACV Difference
The primary difference lies in the operation and wiring of the OBD1 and OBD2 IACVs. OBD1 systems use a 2-wire IACV that operates linearly, controlling airflow with a simple open and close mechanism. OBD2 systems, conversely, employ a 3-wire rotary IACV, which offers more precise idle control but is electronically incompatible with OBD1 ECUs. This incompatibility necessitates physical modification when performing an OBD1 conversion on an OBD2 Civic. While the focus here is on the mechanical mounting, understanding this electrical difference is key to why this modification is necessary for a successful OBD1 conversion.
Mounting the OBD-1 IACV: A Step-by-Step Guide
This modification requires basic mechanical skills and tools, including a drill, tap, and grinder. It’s crucial to proceed carefully to avoid damaging your intake manifold.
Preparing the Intake Manifold
First, inspect your OBD2 intake manifold to confirm it has the cast bosses for mounting the 2-wire OBD1 IACV. These bosses are pre-cast areas on the manifold designed to accommodate either IACV type. The throttle body itself is generally compatible with both IACV styles, simplifying the process. The modification is primarily focused on the intake manifold.
Drilling and Tapping
You will need a 2-wire IACV from an OBD1 VTEC engine. Position the OBD1 IACV onto the intake manifold using the existing pre-drilled bolt hole as a guide. This single hole will allow you to align the IACV correctly for drilling the second hole. Once positioned, use the IACV as a template to mark and drill the location for the second mounting hole. After drilling, tap this hole. A 5/16-18 tap and bolt are commonly used and are suitable for this application. It’s important to control the drilling and tapping depth; approximately 1/2 inch depth is sufficient to secure the IACV without risking leaks into the intake manifold passages.
Throttle Body Modification and Air Passage Creation
The throttle body has two bypass holes designed for IACV functionality. On the intake manifold where the throttle body mounts, you’ll notice a square pattern on the upper left side. This area is where you need to focus your drilling to connect the throttle body passages to the OBD1 IACV. The goal is to align the throttle body’s bypass holes with the ports on the 2-wire IACV.
Specifically, locate the bypass hole on the intake manifold that is closer to the throttle body mounting surface. You need to drill from this point to meet with the closer port on the newly mounted OBD1 IACV. Crucially, do not drill through into the main intake tract. This passage should only connect the throttle body bypass to the IACV.
The second bypass hole on the intake manifold, the one further from the throttle body mounting surface, needs to be drilled through into the main intake tract. This creates the necessary air passage for the IACV to regulate idle air. [Imagine an image here showing the intake manifold with arrows indicating drill points and air passage directions].
Block Off Plate Fabrication
With the OBD1 IACV mounted, you now need to address the original 3-wire IACV location on the throttle body. Since the 3-wire IACV is no longer in use, you must create a block-off plate to seal its mounting point on the underside of the throttle body. You can use a piece of sheet metal or, as in the original example, carbon fiber if available. Use the 3-wire IACV as a template to create the block-off plate, drilling holes that match the mounting points on the throttle body.
It may also be necessary to grind away a small portion of the intake manifold casting to ensure clearance for the OBD1 IACV electrical connector. [Imagine an image here showing the block off plate installed and the OBD1 IACV mounted].
Final Installation
Once the drilling, tapping, and block-off plate are complete, you are ready to install the modified intake manifold with the 2-wire OBD1 IACV onto your OBD2 Civic. Ensure all connections are secure and properly sealed.
Conclusion
Adapting a 2-wire OBD1 IACV to an OBD2 Civic intake manifold is a viable solution for those performing OBD1 conversions. While it requires careful drilling and tapping, this modification allows you to maintain compatibility with OBD1 engine management systems for tuning while retaining your existing intake manifold and throttle body. This guide provides a detailed process for this conversion, enabling enthusiasts to successfully complete their OBD1 swap projects.
