Closed loop / open loop bogging down...
 

My little Nissan 1.5 has just enough power for most situations.
But when I'm giving it more than about 3/4 throttle I can tell when it goes into open loop because there's a dramatic drop in engine power.
If I ease off so that it drops back into closed loop, the power comes back.
I can't do much about the transition to open loop (although I'm investigating that zener diode mod for the TPS) but what could be causing that sudden drop off in power, apart from general age of the engine and components?

I would guess an overly-rich mixture that the O2 sensor is compensating for, at least to some extent. You could confirm that with a Wide-Band O2 meter on a decent dyno, or potentially if you have access to a WBO2 that you can temporarily install in the car.

It could be some other cause, and not related to going open-loop. There are other things that depend on load, like ignition timing and so on. (Depends on what sort of engine management/fuel system/spark system is on the car, as well.)

-soD

The diode mod on the TPS didn't do anything. This engine must calculate when to go into open loop by RPM and airflow.
I'm wondering if a bad fuel pressure regulator could cause this?
eg if fuel pressure is low, during closed loop the ECU can compensate but in open loop it's unable to do so, hence a noticeable lack of power?
I don't have a fuel pressure gauge but I have a spare regulator I can try.

Measuring is almost always better than parts swapping. I'd seriously consider checking with a gauge rather than throwing a new reg at it and seeing what happens.

-soD