Introduction

This page deals with a problem which is increasingly common with engine swaps using early VTEC engines, with the PW0 or PR3 ECU.

Symptoms

Poor running at partial throttle, which the engine 'missing' and feeling 'flat'. If the throttle is opened to 25% or more the engine will run much better, but still may be down in power at full throttle.

Diagnosis

Unplug both oxygen sensors, and reset the ECU by removing the power in order to clear the long term closed loop mixture adjustments. The ECU will show an oxygen sensor error code but the engine should run much better.

Explanation

With the PW0/PR3 ECUs there are two oxygen sensors. A common problem is to wire one oxygen sensor into both pins in the ECU, or reverse the wiring of the primary and secondary oxygen sensors. In both cases the car will run badly under light throttle until sufficient load is placed on the engine for it to run out of closed loop.

The ECU uses each oxygen sensor to tune the mixture in one pair of cylinders. Normally the ECU will slowly lean the mixture for one cylinder pair, and look for the oxygen sensor showing the mixture going lean. Once this happens the ECU will start to enrichen the mixture until the oxygen sensor shows a rich mixture. The effect of this is to swing the mixture over a narrow range close to stoichiometric, typically lambda 0.98 to 1.02, which keeps the mixture close to the ideal range for the catalytic converter. The ECU will tune each cylinder pair independently of each other.

If the oxygen sensor wires are swapped around then the ECU will still try to see a response from each cylinder pair. The ECU will start to lean out one cylinder pair, but will look at the oxygen sensor for the other cylinder pair, so will keep leaning the mixture out. The ECU will see that the other cylinder pair is lean, so will enrichen the mixture for that cylinder pair. The effect usually is that the ECU will run one cylinder pair about 20% lean, and the other 20% rich. The result is that the engine will run flat, without much response and may miss badly. A lambda sensor placed in the exhaust after the secondary pipes join will show that the mixture is roughly correct. Once the load on the engine is increased to a point where the ECU will stop running in closed loop then the engine will run much better, but will still run one cylinder pair lean and the other rich thanks to the long term closed loop adjustment.

Wiring one oxygen sensor into the two ECU pins will produce a similar problem, except the ECU will alternate between running each cylinder pair rich and lean.

Correcting the problem

If the oxygen sensors are wired into the wrong pins in the ECU the solution is to swap the wires around. On JDM cars the oxygen sensor plugs are different to prevent the sensors from being inadvertently swapped.

If one oxygen sensor is wired into both ECU pins, then another oxygen sensor must be added, or the ECU updated so that it runs only in open loop (Hondata ROMs can be made to do this). If another oxygen sensor is added then it is important that each oxygen sensor only sees exhaust gas from one cylinder pair.