For BMW enthusiasts looking to enhance the performance of their OBD2 vehicles, a common topic of discussion is the OBD2 to OBD1 conversion. Often, parts from a 94 325i are mentioned in these conversions. But Is A 94 325i Obd2? The answer is no. The 1994 BMW 325i is an OBD1 vehicle. This distinction is crucial, especially when considering modifying your later model OBD2 BMW. This article, brought to you by the experts at carparteu.com, will delve into the intricacies of OBD2 to OBD1 conversions, drawing insights from a detailed forum discussion and expanding upon it to provide a comprehensive guide for those considering this modification.
Many BMW owners, particularly those with models originally equipped with OBD2 systems, explore the idea of converting back to OBD1. This isn’t about whether a 94 325i is OBD2, but rather leveraging the simpler, more easily tunable OBD1 system found in models like the 1994 325i. The original post we’re referencing details such a conversion on a ’99 M3, highlighting the parts and processes involved. While the example car is a ’99 M3, the principles and many of the parts discussed are relevant to a broader range of BMWs, including the 325i and similar models from that era.
Let’s break down the conversion process, drawing from the experience shared and expanding upon it with expert insights.
Essential Components for an OBD2 to OBD1 Conversion
The notion that an OBD2 to OBD1 conversion is a simple “electronics swap” is quickly dispelled once you delve into the details. It’s a comprehensive undertaking that requires careful planning and execution. Based on the original forum post and our expert knowledge at carparteu.com, here’s a list of essential OBD1 components you’ll need, many of which can be sourced from a car like a 94 325i:
- OBD1 Engine Harness: This is the backbone of the conversion, providing the necessary connections for the OBD1 sensors and ECU.
- OBD1 ECU (413 “red label”): The “413 red label” ECU is highly sought after as it typically comes from non-EWS (immobilizer) vehicles, simplifying the conversion process.
- OBD1 Chip: An aftermarket performance chip, like the one from Active Autowerkes mentioned, is often used to optimize engine performance with the OBD1 setup.
- OBD1 Intake Manifold: The M50 intake manifold, commonly found on OBD1 BMWs, is known for its superior flow compared to OBD2 manifolds and is a key component for performance gains in this conversion.
- OBD1 Fuel Rail: OBD1 and OBD2 fuel systems differ significantly, necessitating the use of an OBD1 fuel rail.
- OBD1 Crank Position Sensor: The location and type of crank position sensor differ between OBD1 and OBD2, making this a necessary swap.
- OBD1 Cam Position Sensor: Similar to the crank sensor, the cam position sensor is also OBD1 specific.
- OBD1 Knock Sensors (x2): OBD1 systems utilize different knock sensors.
- OBD1 Oxygen Sensor: OBD1 oxygen sensors are required for compatibility with the OBD1 ECU.
- OBD1 HFM (Hot Film Mass Air Meter): The mass air flow sensor needs to be OBD1 compatible.
- OBD1 Main Engine Coolant Hose: Differences in the cooling system necessitate using the OBD1 coolant hose.
- OBD1 Throttle Boot: The connection between the MAF and throttle body differs, requiring an OBD1 throttle boot.
- OBD1 Throttle Body (Optional): While the OBD2 throttle body can be adapted, using an OBD1 throttle body simplifies the installation.
- OBD1 Valve Cover and Coil Packs (Optional): While not strictly necessary, using an OBD1 valve cover can streamline coil pack wiring.
It’s crucial to understand that sourcing parts, possibly including from a 94 325i (which, to reiterate, is not OBD2), is just the beginning. The conversion involves numerous intricate steps.
Navigating the Conversion: Key Areas of Focus
Let’s delve into some of the critical areas highlighted in the original post and expand on them with our expertise:
1. Valve Cover and Coil Packs: Routing the Wiring
The valve cover difference isn’t merely cosmetic. OBD2 valve covers don’t have the provisions for OBD1 coil pack wiring routing because OBD2 wiring enters from the passenger side, while OBD1 enters from the driver’s side. You have choices here:
- Option 1: Retain OBD2 Valve Cover: As the original poster did, you can keep your OBD2 valve cover and use a Dremel or similar tool to carefully create routing channels for the OBD1 wiring. This allows you to retain your OBD2 coil packs, which are compatible with the OBD1 harness.
- Option 2: Switch to OBD1 Valve Cover: Using an OBD1 valve cover simplifies wiring routing but necessitates switching to OBD1 coil packs. Both OBD1 and OBD2 coil packs are electrically compatible with the OBD1 harness, so the choice is largely based on preference and desired aesthetics.
Here’s a visual comparison to illustrate the difference:
OBD2 coil setup showcasing the wiring and valve cover configuration.
OBD1 coil setup with different valve cover and wiring arrangement.
2. Coolant System Adaptations
The coolant system presents another area requiring modification. The main coolant pipe from the timing cover differs between OBD1 and OBD2. OBD2 uses a fixed metal pipe, while OBD1 uses a rubber hose connected to an aluminum neck. Solutions include:
- OBD1 Timing Cover Swap: Replacing the OBD2 timing cover with an OBD1 version ($100 approx.) provides the correct outlet for the OBD1 coolant hose.
- Coolant Pipe Adapter: A more cost-effective solution is a coolant pipe adapter ($20 approx.) from aftermarket suppliers like Bimmerworld, AA, or Turner Motorsport. This adapter, secured with JB Weld, fits into the OBD2 timing cover, allowing you to connect the OBD1 coolant hose using a hose clamp.
Location of the coolant pipe adapter (aluminum pipe below oil filter housing) in the cooling system.
3. Intake Manifold: The Performance Advantage
The OBD1 intake manifold, specifically the M50 manifold, is often cited as a source of performance gains in this conversion. Its superior flow characteristics compared to OBD2 manifolds (M52/S52) are well-documented. Key features of the OBD1 manifold include an air temperature sensor and a vacuum port for the fuel pressure regulator on the underside. Importantly, the M50 intake manifold is a direct bolt-on to M52/S52 cylinder heads, simplifying this aspect of the conversion.
4. Throttle Body Considerations
You have options when it comes to the throttle body:
- OBD2 Throttle Body with Adapter: You can retain your OBD2 throttle body but will need an adapter to address the gasket seal difference. OBD1 throttle bodies have a flat mating surface, while OBD2 throttle bodies have a recessed gasket. Adapter plates ($20 approx.) or extended gaskets ($15 approx.) are available to bridge this difference.
- OBD1 Throttle Body: Using an OBD1 throttle body eliminates the need for adapters and provides a direct fit with the OBD1 intake manifold.
5. Coolant Temperature Sender Wiring
OBD1 and OBD2 systems use different coolant temperature sender configurations. OBD2 uses a single sender, while OBD1 uses two. Solutions include:
- Wiring Splice: You can splice the OBD1 harness wiring and use the OBD2 connector to connect to your single OBD2 temp sender.
- Plug-and-Play Adapter: For a cleaner solution, coolant temperature sender wiring adapters ($50 approx.) from suppliers like Turner Motorsport offer a plug-and-play connection.
6. Crank Position Sensor Relocation
The crank position sensor location is different between OBD1 and OBD2. OBD2 sensors are on the engine block, while OBD1 sensors are on the timing cover. You must use an OBD1 crank position sensor and install it in the timing cover location. The OBD2 sensor can be left in place to plug the hole.
7. Fuel System Overhaul
The fuel delivery system is significantly different. OBD2 fuel rails have both lines at the rear, and the fuel pressure regulator is under the car. OBD1 fuel rails have the supply line at the front and the return line at the rear, with the fuel pressure regulator on the rail itself. This necessitates:
- OBD1 Fuel Rail Installation: You must use an OBD1 fuel rail.
- Fuel Line Modification: Remove the OBD2 fuel pressure regulator from under the car and run new 8mm fuel lines to connect to the OBD1 fuel rail. Bridge the gap left by the removed regulator with new fuel line, connecting the feed from the fuel filter to the front of the OBD1 fuel rail and the return line from the rear of the OBD1 fuel rail to the return line under the car.
- Vacuum Line Connection: Connect the OBD1 fuel pressure regulator vacuum line to the one-way valve on the underside of the OBD1 intake manifold.
8. PCV System Adaptation
The Positive Crankcase Ventilation (PCV) system also differs. Options depend on your valve cover choice:
- OBD2 Valve Cover: If retaining the OBD2 valve cover, you can try to mount the OBD2 breather valve under the intake manifold.
- OBD1 Valve Cover: With an OBD1 valve cover, use the OBD1 breather valve, which clips onto the crankcase vent port and connects to the ICV and dipstick.
- Breather Catch Can: Another option is to use a breather catch can.
- Hybrid Approach: The original poster used a hybrid approach, connecting the OBD2 valve cover to the OBD1 breather valve using a rubber hose and a barbed connector for the oil drain line to the dipstick.
Example of an OBD1 PCV system setup.
Conclusion: Is an OBD2 to OBD1 Conversion Right for You?
Converting your OBD2 BMW to OBD1 is not a trivial undertaking. It requires a significant amount of research, parts sourcing (potentially including from a 94 325i or similar OBD1 model), and mechanical expertise. However, for those seeking greater tuning flexibility and potentially enhanced performance from the M50 intake manifold, it can be a worthwhile modification.
This guide, based on expert insights and real-world experience, provides a comprehensive overview of the process. Remember to prioritize safety, double-check all connections, and consider consulting with a BMW specialist if you are unsure about any aspect of this conversion. At carparteu.com, we are dedicated to providing you with the information and resources you need to maintain and modify your BMW. Explore our site for more guides, parts, and expert advice to keep your BMW running at its best.
