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2003 chevy trailblazer_lt
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Discussion Starter #1
He guys and gals I own a 2003 Trailblazer LT 4x4, with 87000 miles. I change the oil every 2000 miles, and the tires are perfectly inflated. I fill up at Chevron (Regular), and I drive like there is an egg under the gas peddle. I do some freeway driving but the majority is just around town. To date I have flushed the transmission, changed the fan clutch (regular not electric), changed the radiator (overheating while towing to the 3/4 mark), flushed the radiator 4 times, changed the thermostat (hoping it would fix the over heating issue, but did not), and installed a True Flow Air intake kit. I have the 3.73/GT4 gear ratio.
Everything works and drives as it should but I am getting 9-10 MPG sometimes lower and almost never higher!!! This is averaging over a 5 month span. If I can't get that number up then I will be forced to sell, I really can't afford to fill this thing up every week or for that matter twice a week. I live in Beaverton, Oregon so the climate is pretty mild.
Please help I can't go back to a car!
Thank You
Shaun
 

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Try getting a PCM4less tune, use fuel injector cleaner that you put in the gas tank make sure you clean the air filter.

That's all I can think of doing I mean I have an 03' and with 104,000 on the clock and I still get 13-19 mpg depending on where I drive and thats always a constant ever since I bought the thing. Sorry for the bad MPG your getting
 

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2007 chevy trailblazer_ls
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346 Posts
Id suggest doing a complete tune-up. Check your plug-wires, change your plugs, clean your MAF (IMPORTANT!!), clean your air filter, change your PCV valve, run some injector cleaner for a tank, and when you are done, be sure you reset your PCM (unplug battery for 20-30 minutes OR unplug it and hit the brakes a few times with the car on, instantly drains the remaining juice in the system).

If none of that helps, there might be another major issue.
 

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2003 chevy trailblazer_lt
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Discussion Starter #4 (Edited)
Things I forgot and Thank you for the help

I ran fuel injector cleaner in August and the Air filter is brand new. I bought the intake kit to help the MPG but no luck. I have also unhooked the battery for several hours to reset things ( I have done this several times).
This week I have some down time so I was going to clean the Throttle Body and replace the spark plugs.
I have considered the PCMforless tune, but I need it down tuned for normal driving and a good solid tune for towing the boat (fishing and crabbing season).
Any help is greatly appreciated.
Thanks again
Shaun
 

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2002 olds bravada
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2,369 Posts
one of your oxygen sensors (most likely the one before the cat) may be faulty..
 

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2004 gmc
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26,181 Posts
...Check your plug-wires...clean your MAF...change your PCV valve...
Not sure I've ever read so much hooey in one post that wasn't a joke. I6 engines have no plug wires or PCV valves. And in 2003, they didn't have a MAF sensor. I'll give you the benefit of the doubt on the MAF sensor that you are assuming all older vehicles are like yours.

But to suggest checking plug wires and PCV valves shows that you've never worked on your own engine, or read enough posts on trailvoy to be spouting them back as advice. :nono:

I agree with the idea to look at the raw O2 sensor data and see if you have a lazy one before the cat. Or possibly a clogged cat. A decent shop can check backpressure and diagnose that. And definitely a new set of plugs - Delco 41-103 are highly recommended here.
 

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2007 chevy trailblazer_ls
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346 Posts
Calm yourself roadie, not the end of the world. Made a mistake. Been thinking about what I need to do to get rid of the CEL on my brothers Grand Prix all morning as Ive already done half of whats said above (yes, it was scanned, came up MAF but it already got replaced :( ).

Just kinda mixed the two up a bit. I honestly didnt know we didnt have a PCV valve, but I did know coils, just mixed thoughts :( And no, didnt look at his year, my mistake.
 

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2003 gmc envoy_slt
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7,190 Posts
Calling PCMforless is your best bet. I believe the biggest difference between a HiPo tune and a towing tune is the amount of torque management that is removed. You could also save some money if you stop wasting it on 2k oil changes.
 

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2007 chevy trailblazer_ls
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A tune isnt going to help mpg @ 9-10. Honestly, I didnt see any improvement in mileage. Im sure some people do, and I wont debate that, but you wont see gains of 5-6 mpg to get him to around what he should be. Theres a bigger problem.
 

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2004 chevy trailblazer_lt
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1,614 Posts
with that kind of gas mileage there is definatley something wrong majorly. You've already replaced most of the things that would cause the problems, so all i can say is whats been mentioned here:

Clogged cat
O2 Sensor gone bad
Bad Coilpacks
Fuel filter

Other possible
Timing
Fuel Management system
basically all computer controlled items that could be solved with a tune.

I'm lifted with 33's and I still get 14 at minimum on average, up to 19 on the highway. And since you do more around town driving, the 3.73's actually help your mpg's a bit.
 

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actually the gear ratio you have will hurt your MPG compared to the 3.42's i have the lower the ratio the more MPG youll get
 

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2003 chevy trailblazer_lt
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Discussion Starter #13
Thanks for the speedy replys

It looks like I've got my week packed and it has not even started!!
I plan to start with the Plugs, work my way to a new fuel filter, and have the cat checked. I don't currently have any CEL but I do get the occasional P0440 because the gas guys didn't tighten it enough. Would it be throwing any codes if the back pressure (with the cat) was messed up and the O2 sensor was faulty?
Thank You
Shaun
 

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2003 gmc envoy_slt
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7,190 Posts
actually the gear ratio you have will hurt your MPG compared to the 3.42's i have the lower the ratio the more MPG youll get
On the highway, yes. In the city with mostly stop and go driving the 4.10 gears are best for mileage. They make it easier to get the truck moving from a stop. The 3.73's are good for combined driving.:m2:
 

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2003 chevy trailblazer_lt
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Discussion Starter #15
monday update

Today I replaced the spark plugs AC Delco 41-103. They seem to be doing fine, those rear two are a real B to get to. I bought a fuel filter and air filter cleaner. I plan to do this tomorrow.
I need to find a muffler shop to check the back pressure of the cat, does anyone have any recommendations. Then I was told to replace the o2 sensor before the cat. I looked through the forums and wasn't able to find a part number.
Thank You again for the help
I hope to gain so decent mpg numbers.
 

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2004 chevy trailblazer_lt
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I talked to my friend thats a mechanic today and typically crazy mpg loose is associated with the lose or inproper functioning of a o2 sensor or clogged cat.

Hope it all goes well for you, at some point one of these things you replace is gonna make a huge difference and you'll learn to love it again... and oh a tune really helps!
 

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2004 chevy trailblazer_lt
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I agree that this is way lower than normal.

Unless you are hauling around a ton of extra weight, if the vehicle seems to be running okay and not throwing codes, then it is throttle body, plugs, cat, O2 sensor or fuel filter.

Usually when plugs go seriously bad, they throw a misfire code, and when a fuel filter gets clogged enough to affect mileage, it also causes drivability problems. My money is on the O2 sensor or the cat. (Or both; as the O2 sensor gets lazy and plugs misfire, the cat can go south in a hurry.)

To test your cat, get the exhaust backpressure tested. (They simply put in a compression-type pressure gauge where the front O2 sensor fits and see what kind of pressure there is at idle and at high throttle. You should have almost 0 psi at idle and no more than 1 or 2 psi at high throttle. Bad cats will often show over 4 psi.)

We are finding more and more reports of bad cats as these vehicles age.

We are also seeing a lot of O2 sensors, cats and plugs deteriorate enough to affect performance and mileage but not enough yet to throw a code.

After you clean the front and back of the throttle body with the battery disconnected, replace your plugs and O2 sensor (most of us routinely replace the front O2 sensor every time we do the plugs,) replace the fuel filter and have your cat tested, here is another way to save a ton of money on maintenance: don't change the oil so often.

You are wasting a lot of money on unneeded oil changes. Trust the oil life monitor, and change the oil when it tells you. Don't forget that we take 7 litres of oil, which is way more than most engines, and drivers have gone over 12,000 miles or more between changes. Add up the cost of all those unnecessary oil changes and your overall operating costs just went down a ton.

Good luck and let us know!
 

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2003 chevy trailblazer_ls
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The one thing that I haven't heard you (or almost anyone else) mention as yet was hooking up a decent engine management scanner to actually read what is going on with your system...

That is the place to start (and I'm not talking a free Auto Zone code read -- that is WORTHLESS, I repeat, WORTHLESS).

I'm on the bandwagon for the PCM for Less tune -- it REALLY helps, but you should be getting MUCH better mileage than 9-10, even with a stock setup.

Find someone that has a Genesys or a Snap On Modis, Versus, or Solus and get the car scanned real-time to SEE if your O2 is bad, to SEE the fuel trim curve, and to SEE spark patterns and timing, then go from there. One additional thought -- a stuck open thermostat. Running cold will also give the poor fuel economy you are seeing, but it could be a whole host of issues, from sticking e-brakes to bad wheel bearings (front or rear), slipping transmission, engine tune, fuel filter, oil choice, plugged cat, etc., etc., etc.

BTW, I'm running an 03 LS 4x4 w/True Flow, resonator delete, new Delco plugs, Amsoil, and a PCM 4 L tune w/3/73 gears and 255 tires and pulling down 15+ in AGGRESSIVE city driving (I rarely run under 75 on the freeway and I don't waste time getting there either) and up to 23 on the highway on longer trips. Your vehicle is capable of doing that mileage, and you'd REALLY have to be pounding on it to do worse than I do, driving wise, and if economy is your issue, I doubt that you are doing that.

Edit -- adding this: I even talked to my wife this morning on the way in to work (we get to ride together every day), mentioning that we could get a Toyota Corolla and only have to fill up once every 2 weeks (as I laid the throttle on the floor and went around a couple of dump trucks, just to hear the True Flow sing...). She said, yup, we could. Nah... I like the Trailblazer. :)
 

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2007 chevy trailblazer_ls
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3,700 Posts
You've almost certainly got an O2 sensor or cat problem...

BTW, you said you drive mostly city... how long is your normal commute? If you say 1.5 miles, I'm going to say that 10mpg is normal.

Mike
 

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2002 chevy trailblazer_ltz
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2,762 Posts
Yikes bad info! One person said plug wires and PVC and one said 7 liters of oil.(1st) no PVC valves,(2nd)plug wires are part of the coil packs and (3rd),7 QUARTS not liters.
 
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