Combining upstream oxygen sensors from a dual-bank exhaust system into a single sensor for use with an older cylinder head can be achieved in a few ways. This is a common modification when swapping engines in Jeep XJs, often involving replacing the stock 0331 head with a pre-2000 0630 or 7120 head. The challenge arises because the newer engine has two upstream and two downstream O2 sensors, while the older head and exhaust manifold are designed for a single upstream sensor.
Two Main Approaches to Bypass O2 Sensors
There are two primary methods to address this incompatibility and effectively bypass the extra O2 sensors:
Method 1: Simulate Downstream Sensors and Modify Header
This method involves using O2 sensor simulators for the downstream sensors and modifying a pre-2000 header to accommodate the upstream sensors. The process is as follows:
- O2 Simulators: Install O2 simulators in place of the downstream sensors. These simulators trick the engine control unit (ECU) into thinking the catalytic converters are functioning correctly.
- Header Modification: Obtain a header designed for a pre-2000 engine (0630 or 7120 head). This header will typically have two separate exhaust pipes, one for each bank of cylinders (1-3 and 4-6). Weld two O2 sensor bungs onto the header, one on each pipe at the junction of the cylinders (e.g., where cylinders 1, 2, and 3 merge into one pipe).
- Sensor Installation: Install the upstream O2 sensors into the newly welded bungs. This allows the ECU to monitor the exhaust gases from both banks.
- Exhaust Modification: Cut the downpipe after the pre-catalytic converters and have an exhaust shop connect it to the modified header.
Method 2: Using an Aftermarket Header
This method involves attempting to use an aftermarket header designed for the 0331 head (the newer engine) on the older 0630 or 7120 head.
- Header Compatibility: The theory is that some aftermarket headers for the 0331 head may bolt onto the older heads and already have the necessary O2 sensor bung locations. This would eliminate the need for welding and potentially simplify the installation.
- Verification Required: While this method has been reported to work anecdotally, it’s crucial to verify compatibility before attempting it. Not all aftermarket headers will fit, and incorrect fitment can lead to exhaust leaks or other issues.
Conclusion: Choosing the Right Method
Both methods offer solutions to bypass extra O2 sensors when swapping to an older cylinder head in a Jeep XJ. The first method, while requiring more fabrication, provides a more reliable solution by directly addressing the difference in sensor locations. The second method offers potential simplicity but requires careful research and verification of header compatibility. Ultimately, the best approach depends on individual skills, resources, and the specific components being used. Always consult with a qualified mechanic if you are unsure about any part of the process.
