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Discussion Starter · #1 ·
:confused: My 05 TB with 40k, I6. New and properly inflated tires. I am getting around 8 -9 mpg on the interstate. I filled up drove 32 miles and used 1/3 of a tank of fuel. I drive to and from work, 32 miles one way, literally all the way on the interstate at 70 mph. I do not drive the truck hard by any means. I have checked the xfer case fluid level, which is full. I cannot figure out what would be causing me to have such crappy fuel mileage. I have used Seafoam, and Lucas Fuel injector cleaner as well. I have tried using different octanes with no luck. Someone please give me any ideas.
 

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2003 chevy trailblazer_ls
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First off welcome to the site.
How long have you had the TB?
When did the mileage drop?
What did you do just prior to the mileage dropping?
How many miles on the odo?
Have you done all the proper maintance? (air filter, oil changes, fluid changes per the manual, etc?)
 

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2005 chevy trailblazer_ls
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Do you carry a large payload?
Are they OEM size tires?
Any Mods done to your truck?
Anything on your roof or to your vehicle to effect your aerodynamics?

I see about 9 around town and 12 on the highway
However, its winter gas, im lifted, i live in a busy city and have slightly bigger tires but theyre mud-terrains.
 

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Discussion Starter · #4 · (Edited)
Info

I have had the TB for almost a year. The mileage has gotten progressively worse since I bought the thing. I have right at 42k on the odometer. I am big on maintenance so everything has been changed/checked/clean properly. I didn't do anything before the mileage drop except drive. Like I said before, the mileage has gotten progressivley worse but I have noticed a big change within the past 2-3 months. I have only added emergency lights and a siren, but that would not affect the fuel mileage. All of my lights are inside, and I have not added anything to the outside of the vehicle. OEM size tires, and I do not pull/haul anything.
 

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2005 chevy trailblazer_ls
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When did you last change the oil?
Have you checked/cleaned your air filter lately?
How high of RPMs do you see on acceleration?
On highway, do you see alot of hills?
Do you use cruise control? :undecided
 

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2003 chevy trailblazer_ls
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I have had the TB for almost a year. The mileage has gotten progressively worse since I bought the thing. I have right at 42k on the odometer. I am big on maintenance so everything has been changed/checked/clean properly. I didn't do anything before the mileage drop except drive. Like I said before, the mileage has gotten progressivley worse but I have noticed a big change within the past 2-3 months. I have only added emergency lights and a siren, but that would not affect the fuel mileage. All of my lights are inside, and I have not added anything to the outside of the vehicle. OEM size tires, and I do not pull/haul anything.
A slow change over a year. You did the seafoam and lucas and that is good. Do you know how to check an O2 sensor? I would look at that and also a plugged cat. But @ 42k hmmm, Are you burning cheap gas or Top Tier fuel? Do you have access to a drive ability scanner with real time?
 

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Discussion Starter · #10 · (Edited)
A slow change over a year. You did the seafoam and lucas and that is good. Do you know how to check an O2 sensor? I would look at that and also a plugged cat. But @ 42k hmmm, Are you burning cheap gas or Top Tier fuel? Do you have access to a drive ability scanner with real time?
I use top tier fuel, and I do not have access to a scanner. How can I check the O2 sensor, and I do not and have not had any SES lights come on
 

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Thats all left that i can think of...cause the 05 doesnt have the MAF sensor yet

plugged cat he would feel no power down low end and would take him and hour to merge so it wouldnt be that

cruise control can hurt your mpg alot, cause its not rpm control its speed control so going to be all over the place to keep your speed consistant.

What gears do you have?
 

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Discussion Starter · #12 ·
Thats all left that i can think of...cause the 05 doesnt have the MAF sensor yet

plugged cat he would feel no power down low end and would take him and hour to merge so it wouldnt be that

cruise control can hurt your mpg alot, cause its not rpm control its speed control so going to be all over the place to keep your speed consistant.

What gears do you have?
That's the only thing I am not sure of. How can I find out what gears I do have?
 

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If you have a DVM you can check your O2 sensor and get a good idea if it is working correctly. The PCM sets the dwell to the injectors by what the O2 sensor says. If the O2 sensor is giving bad numbers the PCM will dump in too much fuel.
http://mr2.com/TEXT/O2_Sensor.html
Read that and maybe it will help.
Gearing = Top of page, RPO codes under resources
Everybody please keep in mind that this is not a sudden change, it has been getting worse for a year.
What was the MPG when you first bought the truck?
 

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2003 gmc envoy_slt
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I think almost everyones mileage has dropped the last few months. The only other thing I can think of is your thermostat. Is the temp gauge reading straight up at 210? If it is reading slightly less than that you may be running cold, but not cold enough to throw a code.:m2:
 

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I changed the oil less than 1k miles ago. Brand new air filter, No more than
3k on acceleration, and thats when merging onto the interstate. No hills, and I do use cruise control
There's a good clue. If your cat is going bad, it could get progressively worse without you noticing a significant drop in acceleration. Cats usually signal they are going bad by a failure to accelerate up a hill, and then a failure to accelerate properly when you get above 5000 RPMs. You just may not be driving in those conditions much.

It is always better to try the easiest, cheapest and most common fixes first.

So, in order:

- Clean your throttle body, inside and out. (Good instructions on this site, and it will take a screwdriver, a rag, $5 worth of throttle body cleaner and less than an hour under the hood.)

- Get your own good scan tool that will also do live readouts. First, check your coolant temperature. If it is under 200 degrees, then your thermostat is bad. (This causes a HUGE hit in mileage because the computer thinks the engine is cold the whole time it is running.)

- On the scan gauge, check the readouts for the oxygen sensor. On the front sensor, the voltage should be jumping VERY quickly between 200 mV and 800 mV. (if the front sensor is jumping slowly between readings, then your front sensor is bad.) In addition, your STFT (short term fuel trim) should vary above and below 0 quickly as well. It should read + or - 2% to 8% and will also jump around a lot.

- On the scan gauge, the back sensor should be pretty solid, and should hardly be moving at all. (If the back sensor jumps around almost as much as the front, then your cat is cooked.)

- Get someone to listen to your engine and tell you if it sounds like a jet engine on takeoff. (If it does, your fan clutch needs replacing.) We get attuned to small changes in sounds over time, so it sometimes takes a third party to tell us. The I6 engine should be almost silent at idle and so quiet you can hear the injectors tick. (GM had to include a starter interlock because people were accidentally trying to start an already-running engine.)

- Finally, take it to a good muffler shop and ask for an exhaust backpressure test. They will insert a compression gauge into the front O2 sensor hole and measure the backpressure. (It should read 0 PSI at idle and no more than 4 or 5 psi at higher throttles. If it reads over this, then the cat is cooked.)


All of the above are easy and cheap to do. Be careful because there is an initial tendency to want to throw parts at the problem but we are much more elegant than that in here; stick with us, let us know what you find, and we'll see if we can help. Scan tools are amazing diagnostic devices if you know what to look for.

And, by the way, when you get it running better ... get that sucker out on the highway and open it up when you merge. As long as you drive within legal and prudent parameters, these engines are NOT designed to be idled around town. They are designed to be used and enjoyed to their fullest. (Think about it; it wasn't that many years ago that 4-valve-per-cylinder, double-overhead-cam inline six engines were found only in BMWs and Formula One racers!)

Good luck and let us know what those scan readings are!
 

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Thats all left that i can think of...cause the 05 doesnt have the MAF sensor yet

plugged cat he would feel no power down low end and would take him and hour to merge so it wouldnt be that

cruise control can hurt your mpg alot, cause its not rpm control its speed control so going to be all over the place to keep your speed consistant.

What gears do you have?
check the vvt solenoid put
 
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