1995 Park Ave early upshift, poor mileage?

My 95 park ave (not 97) seems to upshift too early and sometimes it won't go into 4th. Had a scantool on it and no tranny codes but while driving it clearly showed it didn't go into 4th. Shut it off to reset, then it worked. I also am running lean and got a code for O2 sensor. I only get 20 mpg on the highway but lately I've gone as low as 14. I'm relplacing the O2 sensor and already did plugs, wires, etc tuneup. Could the early upshift cause the poor mileage? Can the codes be reset on the tranny? You would never know there is something wrong generally because it runs beautifully. It also has a long crank time but I'm guessing that's related to the O2 sensor. The only other thing I noticed is that once in a while it shift hard, but not often.

It also has a long crank time but I'm guessing that's related to the O2 sensor. Wrong guess here, the 02 sensor has nothing to do with starting, either hot or cold!

Did you have the trans scanned with a trans capable scanner? This sounds like you may have a sticking throttle valve, solenoid, or modulator problem. It could also be the PCM! You need some scan time with someone who knows how to interrupt the readings. (Not the typical AutoZone parts goober)! If the trans is not going into 4 th (Overdrive) at highway speeds, gas mileage will be adversely affected!

The only code pulled, including tranny, was for the O2 sensor running lean. There was no back and forth, number stayed at 126, 128. Would only run lean. MAF was running on the low side as well, barely within parameters. Sometimes it shifts into 4th, sometimes it doesn't. You don't always need to shut the car off to fix it. Yesterday it was shifting into 4th around 46 mph, dropping the rpm's to around 1200. Really lugging when that happened. Used professional scantool, not autozone or other parts shop. Person using it knows how to use it, but frustrated with no other codes and majority of numbers running within their range. Getting between 14 and 20, last fillup was 17. Bought used with 39K and never got the 27 EPA states it should.

As for the long crank, replaced pump and filter, lines, etc. Still takes 5 seconds or so of cranking to get going. Sometimes needs a few tries.

Your 02 sensor controls fuel trim and when it get’s lazy, it is slow to respond to rich/lean conditions. It appears that your 02 is more than “lazy” so obviously, your 02 is nearly dead!

When the 02 sensor drops to around 300mv, the PCM commands the fuel system to “go rich”. When the 02 output reaches 600mv, the PCM commands the fuel system to “go lean”. If you observed this function on a scanner, you should see a rapid flash back & forth between rich and lean. O2 "counts" is how many jumps back and forth it makes. Change the 02 first and then take a look at several scanner data readings while the engine is warm & in closed loop.

First, observe the new 02 sensor data to see if it is shifting rapidly from rich to lean. Next, check the TPS reading @ idle and just off idle. Then, look @ STFT & LTFT data. Short Term Fuel Trim (STFT) is the control of the Injection Pulse Time Open over the mid to upper range of engine operation. Long Term Fuel Trim (LTFT) is the control of Injection Pulse Time Open (ti) (also called Injection Pulse Width) over the entire range of engine operation. It is primarily calculated at idle, or low-load mid-range engine operation and is averaged over time.

Finally, you need to check the fuel pressure using the following procedure:
1) Ignition ON - Prime pressure =
2) Start engine - idle pressure =
3) Remove FPR vacuum line - pressure =
4) Reinstall vacuum line and measure quick/rapid acceleration pressure =

See what data you can provide and post back with your findings.

There is no cross count for the O2 so it's being replaced. Will use the other info and work on it this weekend, although I believe the fuel pressure was recently checked and it was right on but I'll have to check the records. Thanks for your help.

Regarding the shifting, make sure the vacuum hose to the pressure modulator on the transaxle is intact. Yes, it still has a vacuum modulator in 1995 - Right on the front of the transaxle case.

Okay, took a while but I have a few readings. No short term fuel, only long term. Starts at 0, then jumps to 149 and stays there. Replaced MAF and it ran better. O2 not replaced yet but clearly bad. If the car idles for more than a few minutes, the check engine light goes on as well as traction off light and then it goes into open loop and stays there and the O2 runs lean with no cross counts. Rest of the stats:
P0131 Ho25 High voltage
P074 Torque converter clutch failure
P0171 Fuel trim lean

Fuel trim 182 at idle, 128 after 5 minute when check engine light comes on.
TPS voltage .46
Trim cell 0
LT trim 0, then 149 with check engine light on
iac position 51
MAF g/sec 1
CCP duty cycle 13
CCP solenoid on

Have not check fuel pressure. Not enough time at the end of the day.

Fuel trim 182 at idle, 128 after 5 minute when check engine light comes on.
TPS voltage .46
Trim cell 0
LT trim 0, then 149 with check engine light on
iac position 51
MAF g/sec 1
CCP duty cycle 13
CCP solenoid on
LTFT is obviously off the map! You could change the 02 sensor and it may help some, however, whatever the problem is that started all this, more than likely is what killed the 02 sensor. Your cat will be next, if not already. If the ECU drops into Open Loop for what ever reason, you will notice that the long term fuel trim adaptation value will show 0.0 ms. This is because the ECU is no longer looking at the O2 sensor, and therefore can't make any adjustments to the fuel delivery, it must rely only on the fuel curve that has been programmed into the drive map. Adaptation values that are between +10% and -10% of the base injection pulse width are an indication that the ECU is maintaining proper fuel control, anything above or below these thresholds, means the fuel curve is out of control! Obviously, if your LTFT is 182, the STFT has reached the upper threshold many times and when it does, it bumps the LTFT up another notch. All of your symptoms point to the classic PCM failure, which is not that uncommon on the 94-95 models.