Randy L

Hello all,
Please excuse my LONG post, but I need to fully describe my condition in hopes someone may be able to help me out.

First off, I just bought a fully stock '97 Z28 with Auto trans, and 69K miles on it. SO far, I have changed the oil, tranny fluid, air filter, and the Bank 2 Sensor 2 Heated O2 sensor (got a code). I have spark plugs ready to install this weekend.
The car ran extremely smooth, never hesitated, always idled rock steady at 800RPM and no smoke or fluids dropping. WHat threw the code for that O2 sensor was one WOT push onto the freeway to catch up with traffic. Immediately after that, I got the code for that sensor, accompanied with the code for random misfiring (P0300). After I changed the sensor, it all went right back to normal.
Last weekend, I tested the 0-60 acceleration of the car and immediately afterwards, it ran VERY rich and idled badly. Black smoke coming from the exhausts and definate engine miss. You can also smell the gas really bad. I also got a couple codes which (after retrieving them with my scanner) read as:
1. (P0152) Bank 2 Heated O2 Sensor 1 faulty
2. (P0172) Fuel system running rich on Bank 1
3. (P0300) Misfire

I have 2 pre-cat O2 sensors coming in the mail..might as well change both since they do have 69K miles on them.

Now what I'm trying to understand is that (after some lengthy reading), I have interpreted (I may be wrong so please correct me!) that the O2 sensors need to be heated to 600 deg. F in order to function. Until that happens (as well as during heavy load pulling and WOT conditions) the PCM goes into a "Open Loop" condition meaning that it doesn't recognize the signals from the O2 sensors. Here's where I get a little lost....
I have read and heard that O2 sensors (the pre-cat ones) are critical for any fuel injected engine to run properly. But if during an "Open Loop" condition (like the engine runing cold and warming up) the sensors are ignored, so a incorrect signal from a faulty sensor (P0152) wouldn't be acknowledged right?

Therefore, until the PCM goes into "Closed Loop" mode, will a bad O2 sensor STILL make a cold engine run bad? ANother thing that confuses me is that the passenger side O2 sensor is showing bad...and the driver side fuel bank is running rich. Related?

I'm trying to figure things out while waiting for my sensors to arrive since there may be further problems that need fixing...possibly some solenoid making the injectors dump too much fuel into the engine.

Randy

indieaz

There are probably others who can give you much more information than I but I'll tell you what I do know. While in Open loop you are right - there is no feed back from the O2 sensors. However, your PCM will store long term data from the O2 sensors to make fuel corrections which will affect the mixture in open loop as well. Also, the code for the O2 sensor is stored in the PCM, so when you start the car up it's going to give you an SES light right away. While you are in open loop the O2 sensors will not affect what are called "short term" fuel trims since feedback from them are ignored. But once your long terms are affected enough this won't matter too much. I would still expect the car to run slightly better in open loop - you might want to try resetting the PCM (unplug battery, turn ingition to "run") and then start the car up and see if it runs better for the first couple minutes. If that helps you could also unplug your O2 sensors entirely after resetting the PCM, this will force the car to run in open loop which may be a temporary solution until you get the replacement O2s in.