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