So I have had my 06 H3 since July of 2006. I have an Airaid CAI and Gibson cat-back system. With both of those I was averaging around 18 mpg. About a year ago I blew the exhaust manifold and had it replaced. My mileage plummeted to about 13 mpg and I made four seperate trips to the dealer to get it fixed. They never found anything.

Finally, last spring, following the recommendation of the customer service dept at Hummer I tried a different dealer in Illinois. They said there was a computer issue and after the "fix" the milage sky rocketed to just over 16. WooHoo!!! Like I said, that was in the spring when the weather here in WI finally started to warm up. Now that temps are hitting the low 40s to upper 30s my mileage is slipping away yet again. I have never had a truck get noticeably worse mileage in the winter than in the summer, but I'm beginning to think that might be my problem.

My question is, what could be wrong with the truck to account for a 13% mpg loss when the temps fall? Any ideas from anyone would be appreciate. If I have to go through another winter of 12 mpg my dislike of this H3 is going to grow into full fledged hatred.

Thanks.

 

Talk to importkiller, he is getting a lot less mpg than you. In cold weather, startup strategies from your PCM run the fuel mixture extremely rich, so if you keep your foot in it before up to full temp you are killing whatever mileage you have. Put the factory intake box lid back on (during the coldest 3 months of the year) and see what happens.

Also, an improper repair of the exhaust manifold is not going to help, if something wasn't done right?

 

The only way the exhaust manifold could be part of the problem would be if it was improperly installed and was leaking causing a lack of back pressure on one or more cylinders un-evenly. Back pressure has to be held fairly equal to keep all cylinders on the same plane and doing the same amount of work. Cold weather "may"contirbute tothis situation because metal shrinks when cold and will cause leaks where there normally are none until the engine heats up. I had an intake manifold do this to me once making me crazy trying to find the problem. This might not be a fix to your problem but maybe it will get someone thinking....

 

"My question is, what could be wrong with the truck to account for a 13% mpg loss when the temps fall"?

This is out there a bit, but since your MPG decrease occurs in cold wether......

Per Goodyear:

Goodyear notes in a media advisory yesterday, this is a cold time of year for most of North America, and lower temperatures can cause tire pressure to drop by a pound or two for every ten degrees the mercury drops outside.

Blah, blah, blah......and.............

Keeping your vehicle's tires inflated to the correct pressure is one of the simplest ways to make sure you're not wasting fuel unnecessarily as you drive.

You running stock tires?

 

also running synthetic fluids wherever possible will make your ride run more efficiently cold stiff oil and lube really make your ride work harder to move also where do you live altitude could have a factor as well .

 

synthetics and some good plugs,keep that baby runing more efficient

 

1.During the original exhaust manifold repair, maybe some gasket material got into the the pipe, causing a problem with the converter?
2. Definatly check tire pressure.
3. Definatly go back to the stock intake, and
4. Did you really think you would get good MPG with a Hummer?