Swapping a B18C Type R engine into an older Honda Civic, like an EG model, is a popular upgrade for enthusiasts seeking more power. However, after completing the swap, some owners encounter a frustrating issue: the engine cranks but refuses to start, especially when using a P73 ECU, which is known to be the OBD2 ECU for the B18C Type R. This guide will help you diagnose and resolve this common problem, ensuring your B18c Type R Obd2 swap fires up successfully.
Understanding the Immobilizer Problem with P73 ECUs
The P73 ECU, often found with B18C Type R engines, is an OBD2 computer. If your B18C engine comes from a Japanese Domestic Market (JDM) vehicle, the P73 ECU might not have an immobilizer. However, ECUs from other markets like UK, US, or Australia (UKDM/USDM/AUDM) often include an immobilizer system for theft prevention. This immobilizer is embedded within the ECU and requires a matching signal from the car’s immobilizer system to allow the engine to start.
When you swap a B18C engine with a P73 ECU into an older Civic that doesn’t have a factory immobilizer, like many EG models, a mismatch occurs. The P73 ECU expects an immobilizer signal that the older car simply doesn’t provide, resulting in a no-start condition. The engine will crank normally because the starter system is functional, but the ECU prevents fuel delivery and ignition, effectively stopping the engine from “kicking over.”
Symptoms and Initial Diagnosis
The primary symptom is straightforward: your engine cranks strongly when you turn the key, but it doesn’t start or even sputter. A key diagnostic step to confirm this immobilizer issue is to revert to your old ECU, such as a P28 (often used with D-series engines). If the engine starts and runs fine with the old ECU, but fails to start with the P73, the immobilizer is highly likely to be the culprit. You might also want to check if there is spark when cranking, as mentioned in the original forum, to further isolate the issue to fuel delivery, which is often blocked by the immobilizer.
Solutions to Resolve the No-Start Issue
Fortunately, there are effective solutions to overcome this immobilizer problem and get your B18C Type R OBD2 swap running smoothly with the P73 ECU.
1. Immobilizer Removal or Bypass from the P73 ECU
The most direct solution is to have the immobilizer function removed or bypassed directly from your P73 ECU. This requires expertise in ECU modification and is often performed by specialized tuning shops or ECU repair services. They can reprogram or chip the ECU to disable the immobilizer, allowing it to function in vehicles without the matching immobilizer system. This is the solution that was successfully applied in the original forum discussion.
2. Consider a JDM P72 ECU (If Applicable)
While the discussion mentions the P72 ECU in relation to intake manifold air bypass (IAB) control, it’s worth noting that JDM P72 ECUs (from JDM B18C engines, like the SiR-G) are known to typically not have immobilizers. If you have access to a JDM P72 ECU that is compatible with your B18C engine setup, this could be an alternative to using a P73 with immobilizer issues. However, ensure compatibility and consider that the P72 is designed for a slightly different engine variant (though often functionally very similar).
3. Intake Air Bypass (IAB) Considerations (Secondary Issue)
While not directly related to the no-start issue caused by the immobilizer, the forum also mentions that the P73 ECU might not optimally control the dual-stage intake manifold (IABs) present on some B18C engines. The P72 ECU is often cited as being better suited for controlling these IABs. However, for the immediate problem of getting the engine to start, resolving the immobilizer issue with the P73 ECU is the priority. IAB functionality can be addressed later if you want to optimize intake manifold performance, possibly through ECU tuning or using a different ECU.
Conclusion
If you’re facing a no-start condition after a B18C Type R OBD2 swap using a P73 ECU, especially in an older Honda like an EG Civic, the immobilizer is the most likely culprit. By understanding the immobilizer issue and implementing solutions like immobilizer removal from the P73 ECU, you can successfully get your engine running and enjoy the enhanced performance of your B18C Type R engine. Remember to consult with experienced tuners or mechanics for ECU modifications to ensure the process is done correctly and safely.
