Horrible gas mileage.

[Rgorman11709]

So I just acquired a 2005 Trailblazer LS with a 4.2. The vehicle runs beautiful smooth idle and seems to run great all around. I have been getting 11 to 12 miles per gallon at Best. I know all the fluids where flushed out some point last year but other than that I'm not sure what other maintenance had been done. It has 121k miles on it and I'm at a loss. It also under hard acceleration, basically 3k and over rpm it sounds like it's struggling and gets louder and has an odd vibration. Not sure if there is a known clutch fan issue or if this is even the problem or a cause for the mileage but I'm at a loss. I got this and was excited to see more than 14 miles per gallon that my last vehicle got. Any info and help would be appreciated. Thank you.

 

[gmcman]

Welcome to the club! Do you have a 4x4 model?

Good news and bad news.

Good news is you may have a partially clogged cat and/or very dirty air filter, may need new plugs. Any trouble codes? Do you have access to a scan tool with live data?

Bad news is these aren't meant for good gas mileage. These are heavy SUV's and a good average is 15-17 city, 19-22 hwy and 22 is more often seen with the 3.73/3.42 rears in which yours has. Some get higher but low 20's is about it on the HWY if you have a tail wind.

 

[triz]

Do you have the roaring jet engine sound constantly. If so that can be your fan clutch. Not sure what kind of driving your doing city vs hwy. Some things that may help that don't cost much would be to clean your throttle body, air filter and a bottle of Techron or Amsoil PI performance improver.

Also you might want to look and see if the 02 sensors were ever changed. They start to get lazy and they will also affect your mileage. Lastly check your tires. Make sure they are at the proper PSI.

 

[gmcman]

Also check your coolant temp, when hot the needle should be straight up and not more than 1 line to the left of straight up.

Bad fan clutch, worn plugs, low coolant temp, old O2 sensor will kill MPG's

 

[Playsinsnow]

For starters...

Check condition of ac delco 41-103 plugs.

Clean throttle body. There is a how-to on this site.

Check tire psi (if it was cold and low 12mpg is not unheard of in the city). This is just a reminder to check tire psi.

And check the coolant temp as stated. Mine ran a tick to the left a bit before the CEL came on.


How much idle time do you let it run? Are these hard fillup mpg #s or DIC?

 

[Rgorman11709]

It is a 4x4 model. The temp needle stays straight up. Not sure what Axle ratio it has. I know it has a tow package not sure if that helps. I have a receipt from a relative who bought it brand new and at 110k it had the powers tearing fluid, coolant, both differentials and the tcase flushed. I asked about a plug change out and she had no idea. I also haven't checked the air filter or even much else yet. It's not a constant jet fan sound just just over 3k rpm

 

[gmcman]

Good point on DIC or actual gallons filled. To reset the DIC if you have one, select the average MPG and hold down the arrow until it resets, LS trim may not have it.

As far as transfer case is concerned, unless the dealer performed the drain, I would still pull the fill plug and check to see if the fluid is blue, very important to ensure that the GM Stabilitrak fluid is used.

 

[Rgorman11709]

Good point on DIC or actual gallons filled. To reset the DIC if you have one, select the average MPG and hold down the arrow until it resets, LS trim may not have it.

As far as transfer case is concerned, unless the dealer performed the drain, I would still pull the fill plug and check to see if the fluid is blue, very important to ensure that the GM Stabilitrak fluid is used.

I do that calculations myself. Your correct the LS does not come with it. I had the oil changed and they guy that checked the level did indicate that it was blue.

 

[IllogicTC]

-Air Filter
-Spark Plugs
-Throttle Body Cleanliness
-Final Drive ratio (check your glove box for a code on the sticker, GU6 would have best mileage, GT4 is midgrade and GT5 would be worst)
-Fluid condition. It appears you've already done this part.
-Low ECT, or a constantly-rich condition. Check with a live data scan tool.
-O2 sensor (upstream)
-Slight intake leak (enough to cause a call for more fuel, but not enough to make the system mad and throw a code)
-Tire pressure and tire condition. Follow the door sticker for pressure recommendations.
-Failed fan clutch
-CPAS wearing out or getting dirty (works well enough not to throw code, but is just riding the edges of the PCM's tolerance for actual vs. desired position)
-Top Tier Gas. These engines seem prone to carbon buildup with no-name gasoline brands. Top Tier generally helps to minimize further buildup and can sometimes help to alleviate current buildups over time, and may provide a little better mileage. Consider a bottle of Techron if you've been running non-TT.
-Exhaust manifold leak. Pretty common to get cracked, a small crack may mess with the O2 readings a little without actually throwing any sort of code.
-Partially-restricted exhaust. Maybe not enough to make the PCM mad yet, but could be an issue which is coming up. This can happen if your engine is constantly running rich, or if you have leaky injectors (which would also cause excess fuel use)

Really, there's just tons of stuff to check, and we can start narrowing a ton of it down with a live data scan tool.

 

[CaptainXL]

Check compression first to see if it is within spec. A higher compression engine gets the best possible mileage. A worn out engine gets bad gas mileage. Mainly due to the fact that you need to press the accelerator pedal down further to get any respectable amount of power out of the engine. Such as in highway merging, passing and city stop and go traffic.

Sometimes people are unaware that the engine overheated at least once with the previous owner/s. This has been known to stick and/or coke the rings and affect the compression. Compression should optimally be 200 psi and above . Mine is around 150 psi on all cylinders and I still get 18 highway. But don't ask it to get up and go. It will guzzle gas if I try and ask it to. For me, 12 mpg in the city is not uncommon in the summer. Don't even ask what I get in the winter.

It's either that or you have an exhaust leak more than likely pre-cat. That's if you checked all the basics as the previous members suggested to check.

 

[IllogicTC]

Check compression first to see if it is within spec. A higher compression engine gets the best possible mileage. A worn out engine gets bad gas mileage. Mainly due to the fact that you need to press the accelerator pedal down further to get any respectable amount of power out of the engine. Such as in highway merging, passing and city stop and go traffic.

Sometimes people are unaware that the engine overheated at least once with the previous owner/s. This has been known to stick and/or coke the rings and affect the compression. Compression should optimally be 200 psi and above . Mine is around 150 psi on all cylinders and I still get 18 highway. But don't ask it to get up and go. It will guzzle gas if I try and ask it to. For me, 12 mpg in the city is not uncommon in the summer. Don't even ask what I get in the winter.

It's either that or you have an exhaust leak more than likely pre-cat. That's if you checked all the basics as the previous members suggested to check.

Good thinking with compression. You aren't gonna get the power created by the combustion if it's all just going right past the rings. Winter gasoline isn't exactly the optimal blend for checking your maximum possible mileage, either, now that I think about it.

 

[dla442]

Well. I have a k&n air filter that maintained regularly. I have experienced a consistent 12-13 mpg. I recently put a fram paper back in while I cleaned, dried and oiled the k&n. While the fram was in the mpg went to almost 16 mpg! I just put the k&n back in to see the 400 mile experiment in the difference between them. Either I had oiled the k&n to heavily restricting flow(which the closed off end of the filter was distorted inwards from the engine sucking air) or the k@n just sucks. Will find out and post my results on mpg between the two. I lightly oiled the clean kn filter and time will tell. If my mpg falls again, I trashing the kn filter and staying with the paper. I have always read that kn won't hurt performance vs the paper filter. I am starting to wonder now. Hear so many different stories on this topic.

 

[jsheahawk]

Well. I have a k&n air filter that maintained regularly. I have experienced a consistent 12-13 mpg. I recently put a fram paper back in while I cleaned, dried and oiled the k&n. While the fram was in the mpg went to almost 16 mpg! I just put the k&n back in to see the 400 mile experiment in the difference between them. Either I had oiled the k&n to heavily restricting flow(which the closed off end of the filter was distorted inwards from the engine sucking air) or the k@n just sucks. Will find out and post my results on mpg between the two. I lightly oiled the clean kn filter and time will tell. If my mpg falls again, I trashing the kn filter and staying with the paper. I have always read that kn won't hurt performance vs the paper filter. I am starting to wonder now. Hear so many different stories on this topic.

Roadie swears that we should stick with paper.

Jared

 

[Playsinsnow]

Well. I have a k&n air filter that maintained regularly. I have experienced a consistent 12-13 mpg. I recently put a fram paper back in while I cleaned, dried and oiled the k&n. While the fram was in the mpg went to almost 16 mpg! I just put the k&n back in to see the 400 mile experiment in the difference between them. Either I had oiled the k&n to heavily restricting flow(which the closed off end of the filter was distorted inwards from the engine sucking air) or the k@n just sucks. Will find out and post my results on mpg between the two. I lightly oiled the clean kn filter and time will tell. If my mpg falls again, I trashing the kn filter and staying with the paper. I have always read that kn won't hurt performance vs the paper filter. I am starting to wonder now. Hear so many different stories on this topic.

I regularly, annual, clean my K&N. 118k miles of use later I still get oem mpgs at the least and my intake tube is K&N oil free because I do not rush the drying of the oil. (I read the thread and am going Green next time)

I get 12-13 in winter with 4wd or hauling @ss with a trailer.

Do you have k&n oil on top of your washer fluid reservoir or in the intake tube?

There are so many factors to poor mpgs.
It's been 9 months (maybe 12k miles tops) since I even looked at my K&N and I still got over 20mpgs driving 190some miles this weekend. 192miles topped with 9.25 gallons. I was loaded down the first half of the trip.

 

[DocBrown]

So I just acquired a 2005 Trailblazer LS with a 4.2. The vehicle runs beautiful smooth idle and seems to run great all around. I have been getting 11 to 12 miles per gallon at Best. I know all the fluids where flushed out some point last year but other than that I'm not sure what other maintenance had been done. It has 121k miles on it and I'm at a loss. It also under hard acceleration, basically 3k and over rpm it sounds like it's struggling and gets louder and has an odd vibration. Not sure if there is a known clutch fan issue or if this is even the problem or a cause for the mileage but I'm at a loss. I got this and was excited to see more than 14 miles per gallon that my last vehicle got. Any info and help would be appreciated. Thank you.

You have perfectly described the symptoms of a clogged CAT. It's pretty common for these to clog up at 80-120k. They don't always throw a code when they are clogged either. Mine didn't. The bad news that if you can't DIY, its expensive. Even if you can DIY, its a real pain to change out. The good news is that once its changed out it'll run and behave like new.

 

[IllogicTC]

You have perfectly described the symptoms of a clogged CAT. It's pretty common for these to clog up at 80-120k. They don't always throw a code when they are clogged either. Mine didn't. The bad news that if you can't DIY, its expensive. Even if you can DIY, its a real pain to change out. The good news is that once its changed out it'll run and behave like new.

And even better news: the backpressure test can be comfortably done by someone with any knowledge of repair above "righty-tighty, lefty-loosey"

 

[Mark20]

Also check your coolant temp, when hot the needle should be straight up and not more than 1 line to the left of straight up.

Bad fan clutch, worn plugs, low coolant temp, old O2 sensor will kill MPG's

Welcome to the Nation.

The temp needle should be straight up or one tick to the right. If one tick to the left it is running cool and will eventually throw a code. I missed my gauge indication and was surprised by the check engine light.

 

[dla442]

I regularly, annual, clean my K&N. 118k miles of use later I still get oem mpgs at the least and my intake tube is K&N oil free because I do not rush the drying of the oil. (I read the thread and am going Green next time)
I get 12-13 in winter with 4wd or hauling @ss with a trailer.
Do you have k&n oil on top of your washer fluid reservoir or in the intake tube?
There are so many factors to poor mpgs.
It's been 9 months (maybe 12k miles tops) since I even looked at my K&N and I still got over 20mpgs driving 190some miles this weekend. 192miles topped with 9.25 gallons. I was loaded down the first half of the trip.

The last oiling I think it did over do it...there was a collection on the bottom of the air box. This time was a light spray of even oil. Pretty much one pass of spray on each untreated area. Will see in a while the comparison

 

[Sparky]

Welcome to the Nation.

The temp needle should be straight up or one tick to the right. If one tick to the left it is running cool and will eventually throw a code. I missed my gauge indication and was surprised by the check engine light.

I disagree. GM wasn't that consistent lol. One tick to the left CAN be just fine and my truck is an example of that. Always has been one tick to the left, it runs at around 195, everything A-OK, no pending codes ever and my stock cat wasn't anywhere near clogged at 190K miles and my MPG has always been fine.

If it is 2 ticks or more to the left then you have issues, no exceptions. Well, unless your gauge stepper motor went out and it spun to some random position lol.

 

[HARDTRAILZ]

Check plugs. Only ac delco gives decent results in these motors.

 

[Mark20]

I disagree. GM wasn't that consistent lol. One tick to the left CAN be just fine and my truck is an example of that. Always has been one tick to the left, it runs at around 195, everything A-OK, no pending codes ever and my stock cat wasn't anywhere near clogged at 190K miles and my MPG has always been fine.

If it is 2 ticks or more to the left then you have issues, no exceptions. Well, unless your gauge stepper motor went out and it spun to some random position lol.

I wonder if your gauge's zero point is off a bit compared - at least - to mine.