Recently I have been getting less than normal fuel economy and I suspect I may be running rich, but not so much as to throw codes or the car to run poorly.

The only things I did recently was install a MSD coil-pack, newer plug wires and 55 TB

I have not changed the plugs, but I was considering doing so if that may help.

I want to say I've seen an avg of 16mpg and I do get on it ever so often, so I'm not sure if that is the problem.

my main annoyance of recent is that sometimes I'll be in what I think is 4th gear and I try and floor it sometimes it doesn't want to downshift.. darn autos :-( and as such I guess it may be using more gas during this than normal.

How old is the O2 sensor? O2 sensors can go bad as they age but not bad enough to throw a code.

Another issue could be leaking injectors, vacuum leaks, incorrect spark plugs, fuel regulator malfunctioning, incorrect coolant temp reading at the sensor and bad air intake temp sensor.

All these things can read inaccurate but not necassarily throw an engine code. Look in the manual for testproceedures and start working with each sensor one at a time.

If the front o2 is more than 4 years old I would replace it.

Originally posted by Star Turbo Talon
How old is the O2 sensor? O2 sensors can go bad as they age but not bad enough to throw a code. Another issue could be leaking injectors, vacuum leaks, incorrect spark plugs, fuel regulator malfunctioning, incorrect coolant temp reading at the sensor and bad air intake temp sensor. All these things can read inaccurate but not necassarily throw an engine code. Look in the manual for testproceedures and start working with each sensor one at a time. If the front o2 is more than 4 years old I would replace it.

I'm going to guess the o2 sensors are original and thus 13-years old.

Also to add in some other symptoms: The car sometimes "stalls" while parked, and sometimes it will try to do the same while driving and I can feel the car "kick", but no back-fire noises just a very brief either engine or battery light.. cant be sure as it happens so fast.

I'm wondering if I can detect anything with an obd2 scanner/software.

Also the TPS and IAC are "new" and came with with the 55tb, So i guess I may have to start going through the other sensors to see if any of them are faulting.

I replaced my O2 sensor at 10 years, it was still working appropriately.

What were the Short term Fuel trims with the old sensor and what were the Short term fuel trims with the new sensor? If you cant tell me that you cant honestly expect me to believe the 13year old sensor was operating as accurace as a new sensor Erik.

In fact its pretty much impossible for a 13year old sensor to accurately measure fuel trims like a new sensor due to heat degradation, carbon build up and corrosion of the internal element.

So I guess I ought to look at replacing my 02 sensors then. I think there are 2 of these correct?

Originally posted by iamnotwhoiam
So I guess I ought to look at replacing my 02 sensors then. I think there are 2 of these correct?

Yes, but only the front sensor is used for adjusting fuel trims. The rear is to check cat efficiency, and as such can be old, slow, and shitty and the ECU doesn't care, as long as it passes check.

Originally posted by DarkOne

Originally posted by iamnotwhoiam So I guess I ought to look at replacing my 02 sensors then. I think there are 2 of these correct?

Yes, but only the front sensor is used for adjusting fuel trims. The rear is to check cat efficiency, and as such can be old, slow, and shitty and the ECU doesn't care, as long as it passes check.

sweet. that will save some $ then.

Think this is my primary culprit or could it be other sensors?

Originally posted by Star Turbo Talon
What were the Short term Fuel trims with the old sensor and what were the Short term fuel trims with the new sensor? If you cant tell me that you cant honestly expect me to believe the 13year old sensor was operating as accurace as a new sensor Erik. In fact its pretty much impossible for a 13year old sensor to accurately measure fuel trims like a new sensor due to heat degradation, carbon build up and corrosion of the internal element.

I did not see any difference with STFT and LTFTs between the two sensors, as measured on my scanguage. I also did not see much of change when comparing multiple runs between the old sensor and new sensor (no other variables changed) on a smog dyno in cali (long story there!) However, I do know that over time, mileage eventually decreases - I actually showed that with some statistical tests in a thread here back in 2004-2006 or so. That was actually a good data set - as I logged every tank of gas and corresponding trip odo readings gathered over 100,000 miles. Though I can't say the decrease in MPG was strictly from the o2 sensor aging, the car aging, or the driver aging and getting more hostile :)

I would argue that my 10 year old sensor was probably as accurate as the new one while cruising. However, it probably has an increased lag to get to the proper reading.