H3 gas mileage SUCKS

[f5fstop]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by bparker:
Yes, those are the short term fuel trims and they are changed for driving conditions - but they will not be correct until the long term fuel trim tables are updated after 500 miles - then and only then will the short term fuel trim tables be accurate and optimized.

Brent

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by f5fstop:
Air/fuel misture on the modern engines is almost instantaneous. The PCM constantly reads the O2 sensors, as well as the MAF sensor, air temp sensor, ect., it then computes the most optimum air/fuel ratio, and constantly changes this as you drive. Chaning the air filter or cat back exhaust will be sensed almost instantly by the computer. Besides, there is litte to no affect with these items in regard to air/fuel mixture.
The transmission has a learning capability, called adaptive strategy, that will adjust with driving over a period of time, usually within 50 miles. </div></BLOCKQUOTE> </div></BLOCKQUOTE>

In short (no pun intended) the STFT is a PCM erasable memory register and has direct results on fuel mileage during closed loop operation. The PCM changes the injector pulses per this information.
The LTFT is a matrix arranged by the PCM and MAP, and as the engine operating conditions change over time the LTFT is changed within the PCM.
Your PCM on the H3 will try its best to keep a air/fuel misture of 14.7:1, while the engine is in closed loop operation, no matter what the LTFT tables are showing as compared to the STFT.
Basically the LTFT is used primarily for engine warm up and WOT conditions; primarily open loop operation, and the O2 sensors are not warm enough to signal, and the coolant is still not at operating temp as signaled by the coolant sensor. STFT is the instantaneous correction value determined from the O2 sensor readings (after they have warmed and started their signals to the PCM and the coolant is at operating temp) and is functional during closed loop operation.

If you are trying to obtain fuel mileage during warm up and WOT operations, then I would agree LTFT is important. However, for driving as explained and normally performed by most drivers, STFT is what is important.

In addition, as I said before, the K&N statements are very overrated, and have little effect on air/fuel mixture in real life.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by sharkman:
I drive exactly like you (45 miles each way to work) and a combo of city and freeway. The BEST MPG I have gotten is 14-15. That is why I am so frustrated: the H3 is supposed to get close to 19 and I am not anywhere near that. I keep telling Hummer there is an issue with this truck, but nobody seems to care. </div></BLOCKQUOTE>
This thread started out with you saying you were driving on the freeway, which I took as constant speed. Now you say it is a combo of city and freeway. City/highway is not advertised as 19 mpg; that is a measurement of highway only and is figured as all other vehicles during ideal conditions at a set speed. (Not saying this is right, but it is the way the Feds want fuel mileage figured...at this time.)
My total city mileage appears to be coming in at around 14.5 MPG; 1.5 MPG less than per the ratings, but I am well known for jack rabbit starts, and I hit almost every traffic light on my 11 mile commute each way; so I can't really complain.