H3 gas mileage SUCKS

[wannabeH3]

i really enjoy the H3 but i fully agree the mileage is pathetic and no where near the advertised 20 mpg i typically get between 13-15 mpg

[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.

[oneworld]

alomost 2000 miles and the truck is averaging about 15 > 17 > most of my driving is 70 to 80 on the highway and around town softer not working the brakes hard at every stop >so I guess I am not pushing the gas around town. It takes a bit to stop the momentum of the wieght on the H3 so my local driving is low key not ziping around town.

[HummerNewbie]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by f5fstop:
but I am well known for jack rabbit starts, </div></BLOCKQUOTE>

My dog thinks I do that as well

[trading13]

LOL thats cute. I hope a saint bernard fits ok in an h3.

[Desert Dan]

for a 4700 pound brick shaped SUV I think the mileage people are reporting is pretty good!!!

I wish my Jeep did as well.

GM didn't claim 19-20 mpg in the city!

[DWPC]

Driving a 4700 pound 4WD brick with a big pipe rack and lights bolted to the front at 80 MPH and complaining about mileage takes chutzpah. I don't know of any vehicle that doesn't take a major mileage hit at speeds over 70 vs. EPA figures, which we all know are optimistic anyway.

The H3 ain't nevergonna be a SoCal freeway cruiser (unless it gets a diesel), but I bet if you cut your speed to 65-70, you'd see a large mileage improvement.

I don't mean to be a wise-a##, but buying an H3 to commute 90 miles a day!! What were you thinking?

[sharkman]

I appreciate your candidness. The reason I am complaining is that the new TrailBlazer SS has a 395HP LS2 engine (the same as the new Corvette). The Blazer weighs the same as the H3 AND it is rated with the same gas mileage (16/19). I asked HUMMER and Chevrolet how this can be possible: how 2 similar weighing vehicles (outside of aerodynamics) can have 2 totally different engines (a V8 vs an inline 5) and get the same gas milegae! Hell, if I know about the SS, I may have purchased it over the H3 (you get power AND great gas mileage). The answer given to me was, "I know it is frustrating, but I don't know how to explain it."

[H3OH]

Makes you wonder why there are new Trailblazers just piling up at used car places such as Carmax. Do a search on their site and you'll see plenty of 2005 Trailblazers up there. It's scary.

[DWPC]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">How 2 similar weighing vehicles (outside of aerodynamics) can have 2 totally different engines (a V8 vs an inline 5) and get the same gas milegae! </div></BLOCKQUOTE>

It IS the aerodynamics! Drag goes up with a cube factor. The H3 is a box (actually a bunch of boxes stuck together). Compared to the H3, the TB is downright streamlined. Going from 70 to 80 increases your H3's already high drag dramatically.

[jp's-h3]

Could altitide affect mpg?
I live in slc,ut. and get pretty good mpg (16-17 city)
I'm not sure what I'm getting for hwy mpg. will have to check soon. I LOVE MY H3!

[ChevyHighPerformance]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by 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. </div></BLOCKQUOTE>

In part throttle while the engnie is warm, the PCM reads the pre-cat O2 and adjusts the injector pulse width to maintain ~14.7:1 A/F. The PCM uses Strims and Ltrims together to adjust the A/F during part throttle. Strims react fast and Ltrims are slowly adjusted to bring the Strims hovering about zero. The PCM adjusts very quickly during part throttle. The issue is when you go to WOT. During WOT the PCM enters power enrichment (open loop). The PCM zeros the Strims and uses the data learned in the last cell (before entering WOT) as a calibration for WOT. No learning is done during WOT. So if you don't give the PCM enough time to learn the Ltrims for the cells, you can have undesired WOT performance. If you understand the cell arrangement, learning can be done in about 10 - 15 miles. Otherwise you may need 50 - 100 miles to be sure the "used" cells have learned.

This is an issue with dyno testing immediately having made a modification. If you look at WhiteH3's dyno from K&N, you'll see that only 3 miles were added beteeen the stock air box and K&N's. This is too short for a learning process and with a better breathing air box (like the K&N) results in a temporary lean WOT running which is shown on the A/F plot (11.13 stock and 11.92 with the K&N). (After the PCM has learned, the WOT A/F with the K&N will return to the 11.13.) My guess is that this test was also run with the hood up which helps prevent the K&N intake from drawing in hot underhood temperatures.

[f5fstop]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by sharkman:
I appreciate your candidness. The reason I am complaining is that the new TrailBlazer SS has a 395HP LS2 engine (the same as the new Corvette). The Blazer weighs the same as the H3 AND it is rated with the same gas mileage (16/19). I asked HUMMER and Chevrolet how this can be possible: how 2 similar weighing vehicles (outside of aerodynamics) can have 2 totally different engines (a V8 vs an inline 5) and get the same gas milegae! Hell, if I know about the SS, I may have purchased it over the H3 (you get power AND great gas mileage). The answer given to me was, "I know it is frustrating, but I don't know how to explain it." </div></BLOCKQUOTE>

it takes less throttle input to move a vehicle with a large V8 with torque to spare, versus a five cylinder that does not have torque to spare. Yes, a V8 with less throttle input is using more fuel than the five cylinder using the same throttle input, but the five cylinder would require a great throttle opening. This would only affect starts from a stop, unless you were passing.
Also, even though the trailblazer is not considered aerodynamic, it is extremely aerodynamic when compared to the Hummer H3, which is similar to moving a wall down the road.
I'm also guessing, but I would guess the trailblazer weighs in at about 500 pounds or less than the H3. The H3 has a heavier frame, extra cross members, and heavier front/rear axles and transfer case.
As I have said earlier, the fuel mileage figures given are based upon a Fed Regulation on how to determine fuel mileage. These tests are performed under ideal condition, and really do not reflect actual driving done by you or me.
It does sound like you not getting as good of mileage as others, but it could be due to your driving habits versus others, the wind, the altitude, etc.
I know from what I have seen so far with my mileage, I have no complaints.

[sharkman]

Thanks for the input. I am going to drive 2 full tanks @ no faster than 70mph highway and see what happens to my mileage.

[bparker]

Have you checked to see if you have any Hondas or Toyotas stuck under your bumpers? You might have one adding weight to your vehice thus poorer milage. /shrug

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by sharkman:
Thanks for the input. I am going to drive 2 full tanks @ no faster than 70mph highway and see what happens to my mileage. </div></BLOCKQUOTE>