I have a 87 TPI unit in my 62 Nova. I have a 5.7 setup chip and injectors, I have a set of Sanderson shortie headers and my O2 sensor is down about 8 to 10 inches from the collector. I've been having some trouble with a code 33 setting and have been watching it with my Tech I scanner. This code will set just about anytime. Today I had the scanner set to record if it see's a code 33 and sure enough it set a code 33 under part throttle about 2000 rpm's, ecm in Closed loop and temp up to 191. One thing I noticed was the O2 sensor was reading 0. So I watched it all the way home and noticed on desell the O2 sensor would go to 0 and under excell it would bounce around 300 to 800. I just put a new O2 sensor and have not put it out on the road, but my question is is this normal. I dont remember seeing an O2 sensor reading 0 unless it was bad..

Mitch

 

Code 33 is for: High voltage (low vacuum) at mass air flow sensor (or MAP sensor).

What you're describing about your O2 sounds normal. On decel, the computer leans out the fuel/air mixture, which means the O2 sensor will show a very lean exhaust- displaying voltages around zero. On acceleration, the O2 is bouncing around from lean to rich, meaning the ECM is continually correcting the fuel/air ratio back to 14.7 (except when in power enrichment mode).

It sounds like your O2 circuit is fine. I'd start looking at the mass air flow sensor circuit. I'm no expert on MAF cars, but I'm sure others here are.


EDIT: One thing I would be concerned about is the location of the O2. Typically with headers, the O2 sensor can cool off to the point where it drops out of closed loop. The thin steel of the headers doesn't retain heat like cast iron exhaust manifolds. With my headers, I ended up running a three-wire heated O2 sensor. The signal wire on the sensor connects to the signal wire from the ECM. The other two wires on the sensor connect to (+) and (-) from the switched 12V circuit. The heater keeps the sensor hot at all times so the ECM remains in closed loop.

 

YEP - heated O2 sensor is usually the best route if you have headers. Easy to do, too. Even I could do it!!!!