My rebuilt 2056 is back in the car and running, but it runs lean as soon as it warms up. There doesn't seem to be anything wrong with the fifth injector, which is the one that shuts down when the engine warms up, and we can't find an air leak anywhere, although we haven't given up on that search. Anything else we should be looking at?

Check the Aux air regulator. It may be stuck open.

Ok, how do you know if its running lean? Do you have wideband? stock cam? Is it lean at idle? cruise? wide open throttle? more info please.

The fifth injector, also known as the Cold Start Valve, can only be open while the starter is actually cranking. It gets its power from the same yellow wire the starter does. It gets its ground from the Thermo-Time Switch (in pre-75 cars, it's just a thermo-switch but still has the same name) and only when the switch is below about 32F, depending on the switch.

So the CSV will basically never affect the mixture of the car when it is running!

The Auxiliary Air Regulator feeds extra air in, but the Manifold Pressure Sensor "sees" that air as higher pressure, so it adds fuel to compensate. The AAR will never change your mixture either. (The 1.8's L-jet doesn't have an MPS, but the AAR is only fed metered air in that system anyway.)

Assuming you have D-jet, here are a few things to look at:
- Injector seals. These are far enough from the plenum that they actually can affect the mixture, unlike most vacc leaks.
- Head temp sensor. This affects the mixture during warm-up, and should theoretically no longer affect anything by the time the motor is warm.
- Adjustment knob on the ECU. This should adjust the idle mixture only. CW would be richer, CCW would be leaner.
- Fuel pressure. Should be 29 PSI; higher is richer and lower is leaner.

It's possible that there is something else going on that makes the mixture too rich when the engine is cold, and the whole thing has been adjusted to lean it out. Which would mean that when warmed up it would be too lean.

--DD

Not Hi-jacking but I found this to be really helpful in evaluating lean vs rich.