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97 Thunderbird 4.6, 98 Mark VIII LSC
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The STFTs represent the commanded lambda to achieve perfect fuel burn, so if they're staying around 1 then the O2 sensors are doing their job, at least, getting the mixture dialed in. If you have a wideband O2 to do a reality check to ensure it's actually running at 14.08:1 then you can rule out the O2s being bad.

If the original MAF was acting up and was off when the car was tuned, and the new MAF sensor is within spec, that could explain it. It's also possibly a vacuum leak, though all the heater circuit codes is puzzling. The front O2s run through the engine harness and the rear O2s run through the transmission harness so a common harness issue is less likely. I would not be surprised if the HEGO fuse is blown, or possibly the MOFSETs in the PCM are acting up?
 

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Super Moderator
97 Thunderbird 4.6, 98 Mark VIII LSC
Joined
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12,409 Posts
What MAF sensor and injectors was the canned tune written for?

So to give some context, lambda is the term used to describe the mixture independent of the fuel type. Lambda 1 is always perfect combustion and can represent 14.64:1 with E0 gasoline, 14.08:1 E10 gasoline, 9.76:1 for E85 etc. To convert to AFR from lambda you multiply the stoich AFR by the lambda: E10 lambda .85 (a good lambda for WOT) is 11.97:1 AFR; lambda less than 1 is a rich mixture and over 1 is a lean mixture.

STFT is showing you the PCM's commanded lambda. In closed loop it indicates the lambda commanded to get an actual lambda of 1 as reported by the O2s. So if you see an STFT of .90 (or -10 in some software) it means there the PCM is having to enrich the mixture to get to an actual lambda of 1.0 - e.g. a lean condition.

Where it gets complicated is with adaptive fueling. The PCM keeps track of higher-than-normal STFTs (beyond 3-4% either way) and stores that correction factor as LTFT. This correction factor is applied to injector duty cycle - so an LTFT is opposite of STFT. An LTFT of 10% means 10% more fuel than what the lambda or STFT shows. In a tune this can be turned off to only need to look at STFT to see whether you're running rich or lean. That information ultimately gets used to dial in the airflow characteristics of whatever MAF sensor is on the car.

Anyway...

The PCM switches the mixture from slightly lean of lambda 1 to slightly rich of lambda 1 during closed loop cruise. In a perfect scenario it would go from about .98 to 1.02 but as I said before, ±4% is typically acceptable. The switching cycle is only a few seconds, nowhere near 15-20 seconds. As long as you're seeing that switching between rich and lean, it means the PCM has achieved closed loop "perfect" fueling and is maintaining it. A voltage of about .45 on a narrow band O2 sensor is lambda 1. The PCM is adjusts the STFT looking for the O2 voltages to go lean (~.20 volts) then rich (~.6 volts) and repeats that cycle.

Your O2 voltages seem OK for the most part, though the switching period is a little irregular.

Your LTFTs indicate about 15% additional fueling is required at idle. Either the MAF sensor is not correct in the tune, is acting up (not likely given your troubleshooting), or there is a vacuum leak. It's possible all 4 O2s are junk, but I don't consider that the first thing I'd run to and check. Look at your HEGO fuse under the steering wheel. If that's good do some testing of the harness for shorts on the heater circuits.
 
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