Swapping a B20VTEC engine into your Honda can deliver a thrilling boost in performance. However, integrating this powerful engine with your existing ECU and wiring can sometimes present challenges. One common hurdle faced by enthusiasts is getting VTEC to engage correctly, especially when using a P28 ECU with an OBD2 vehicle. This article delves into the potential issues and troubleshooting steps for those experiencing VTEC problems with a B20VTEC setup and a P28 ECU in an OBD2 car.
Understanding the B20VTEC and ECU Compatibility
The B20VTEC engine is a popular choice for Honda enthusiasts seeking more displacement and VTEC performance. It typically involves pairing a B20B or B20Z block with a VTEC cylinder head, often from a B16A or B18C series engine. When it comes to engine management, the P28 ECU is a frequently used option, particularly for its tuning capabilities. The P28 is an OBD1 ECU, which means it’s designed for older Honda models. When installing a B20VTEC into an OBD2 chassis (like a 1996+ Honda Civic or Integra), you’ll often need to use conversion harnesses to make the older OBD1 ECU work with the newer OBD2 wiring.
The VTEC Engagement Problem with P28 and OBD2
One user encountered a frustrating situation: VTEC was not engaging on their B20VTEC swapped 1997 Integra when using a P28 OBD1 ECU. They had a GSR B18C1 head on a B20B block, a common and potent combination. They noted that while other ECU functions like launch control, shift light, and rev limiter were operational, the crucial VTEC engagement was absent. Interestingly, when they switched to the original P72 OBD2 ECU (which is designed for OBD2 GSR engines and thus more natively compatible with their car’s wiring), VTEC worked. This immediately points to a potential issue related to the P28 ECU setup in their OBD2 vehicle.
Potential Causes and Troubleshooting Steps
If you’re facing a similar VTEC issue with your B20VTEC and P28 ECU in an OBD2 Honda, here’s a systematic approach to troubleshooting:
1. Verify Basic VTEC Requirements
Before diving into ECU specifics, ensure the fundamental conditions for VTEC engagement are met:
- Oil Pressure: VTEC relies on oil pressure to actuate the rocker arms. A faulty VTEC oil pressure switch or low oil pressure can prevent VTEC from engaging. Check the oil level and consider testing the VTEC oil pressure switch.
- Coolant Temperature: The ECU typically requires the engine to be at a certain operating temperature before VTEC will activate. Ensure your coolant temperature sensor is functioning correctly and the engine is reaching normal temperature.
- Wiring and Connections: Double-check all wiring related to the VTEC solenoid and VTEC pressure switch. With an OBD1 P28 ECU in an OBD2 car, wiring conversion issues are a prime suspect. Carefully inspect the OBD2 to OBD1 harness adapter for any loose connections, damaged wires, or incorrect pinouts. Refer to wiring diagrams for both OBD1 and OBD2 systems to verify correct connections.
2. ECU Configuration and Tuning
The P28 ECU, especially when chipped for tuning, needs to be properly configured to enable VTEC.
- VTEC Enable in ECU Software: Using tuning software like Crome, Hondata, or eCtune, confirm that VTEC is actually enabled in the ECU map. It’s possible that VTEC was inadvertently disabled during a previous tuning session.
- VTEC Crossover Point: Even if VTEC is enabled, the crossover point (RPM at which VTEC engages) might be set too high or outside the normal operating range. Review and adjust the VTEC crossover RPM in your tuning software to a suitable value (typically between 4500-5500 RPM for B20VTEC engines, but consult tuning guides for your specific setup).
3. ECU and Harness Compatibility Issues
The core of the problem might lie in the compatibility between the OBD1 P28 ECU, the OBD2 car wiring, and the B20VTEC engine setup.
- OBD2 to OBD1 Conversion Harness: While conversion harnesses are designed to bridge the gap, they are not always perfect. Poor quality harnesses or incorrect wiring within the harness can lead to signal issues. Consider inspecting the harness thoroughly or even trying a different, reputable brand.
- P28 ECU Functionality: While the user tested a “new chipped ECU,” it’s still possible that there’s an issue with the ECU itself, or the chipping process. If possible, try a known good, stock P28 ECU to rule out any problems with the chipped unit. However, a stock P28 will require a base map suitable for a B20VTEC, or at least for a similar engine to avoid running lean or rich.
- OBD2 ECU (P72) Test: The fact that VTEC worked with the P72 OBD2 ECU strongly suggests that the VTEC solenoid and wiring to the engine side are likely functional. This further isolates the problem to the P28 ECU setup and its interface with the OBD2 car.
4. Sensor Malfunctions
Although less likely given the P72 ECU test, faulty sensors can still play a role.
- VTEC Pressure Switch: Test the VTEC pressure switch to ensure it’s opening and closing correctly. A faulty switch can send an incorrect signal to the ECU, preventing VTEC activation.
- Coolant Temperature Sensor (CTS): A malfunctioning CTS can provide incorrect temperature readings to the ECU, potentially preventing VTEC engagement as a safety measure. Use a scan tool to check the CTS readings and compare them to actual engine temperature.
Conclusion: Systematic Troubleshooting is Key
Diagnosing VTEC issues with a B20VTEC OBD2 swap and a P28 ECU requires a methodical approach. Start by verifying the basic VTEC requirements, then move to ECU configuration and compatibility, and finally consider sensor malfunctions. The user’s experience highlights the importance of considering the OBD2 to OBD1 conversion and the potential for wiring or ECU-related problems when VTEC fails to engage. By systematically checking each potential cause, you can effectively pinpoint and resolve the VTEC issue and unlock the full potential of your B20VTEC engine.
