I recently replaced the two precat o2 sensors on my boss's 1995 Thunderbird because for one, my engine analyzer was showing that all four of the sensors were reading a constant lean regardless of throttle response and two, because the car has around 150,000 miles on it and was due to be replaced anyhow. I noticed that instead of seeing a dark sooty color on the end, I instead found a white powder residue. He told me that he has been running seafoam through it. After I replaced them, I cleared the codes and then ran the analyzer again as the engine was running and found that there was still no change. The new sensors were also reading a constant lean. I noticed that the o2 sensor heaters were continually ON the whole time I had the car running with idle temp up around 205 degrees and an outside temp of 86 degrees. When I would goose the throttle, I noticed that there was a slight hesitation before the engine picked up. It would also break up a bit as the RPMs would race up. Each time I would let off the throttle before it would get past 5,000 rpm. Tomorrow, I'm going to check the fuel pressure to see if it's low. I read somewhere that when the engine is in open loop mode, that the sensors are ignored and will not fluctuate. It didn't show on my analyzer if the engine was in open loop or not. My analyzer is a Matco Determinator version 3.0 if that helps any.

 

I understand that this isn't a truck. But I didn't think it mattered since I am dealing with the engine. He says that he replaced the MAF sensor a year ago. He says that the fuel filter hasn't ever been replaced that he can remember and he bought the car new. I checked the fuel pressure at the fuel rail and found that at idle, it has around 20-25 PSI. It increases to around 35 psi when I rev it. I was expecting for the pressure to be lower then what I read since the filter has never been replaced. We are going to replace the fuel filter tomorrow. As I mentioned in the original post, the o2 sensors had a white powder on the sensor instead of the normal sooty color. Could a lean condition cause this white powder to form?

 

Some of the early Tbird PCM software has issues with reporting the actual O2 sensor information on code scanners/datalogging devices. I've seen this many times-especially with stuff like Snap-on, Matco, and other generic scan tools. The '95 Tbird was one of the "beta" testers for OBD-II, and are not fully compliant with the standards used today.
I'll bet there was nothing wrong with the original O2 sensors that you removed. PCM's in that year also has some issues with fuel injector driver hardware. Check the plugs, wires and coil packs for the hesitation you're describing, or it could be a bad MAF.
JL

 

Maybe you really are lean and the O2s are reporting correctly. Do the sensors switch above and below 450mVdc as they should? The frequency and amplitude of the post cat 02s should mirror the pre cat ones, but on a much reduced basis if the cats are doing their job. Are you feeding your cats the right cat food? Apparently NOT! Cats respond well to the right cat food which is NOT lean cat food. The cat food has to be at 14.7 to 1 to keep the cats happy.

 

I replaced the precat 02 sensors but there was no change. I ended up unplugging the MAF sensor and found that it made a big difference in throttle response and wouldn't bog like before when at full throttle. He said that he replaced the sensor last year but it didn't solve the problem then either. Anyways, I had him remove the sensor and clean it with MAF cleaner. He says that it seemed to make an improvement. I still don't understand why if he did replace the MAF sensor, that it still had the same problem, yet when unplugging it, the car would run much better. He also says that he has been running seafoam in the tank.