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I got a trailblazer for my Son paid 4K total it had low mileage and looked fairly clean inside. There were a few things I knew about like New shocks, rear trailing arms i had to fix then found it needed valve cover gaskets because there was oil on the spark plug boots, then new front hubs and steering sensors because of abs lights and codes, and a few odds and ends.I fixed all this myself and thought it was good to go I was wrong, he kept going to the Gas station alot. I accused him of idling and listening to the radio goofing with friends you know the normal teen stuff. So I filled it up drove it for a while and Poof I checked the MPG and its under 10 mpg over 3 tanks now no matter who is driving. I purchased a Blue driver tool and pulled 2 logs from a trip to home depot. There are no codes in the system and Im at a loss where to go from here.? I thought Fan but the fan looks like it is functioning properly, Then O2 sensors but im not sure they are bad. Thought termostat but the numbers look right there also. I have attached the logs as a text file but they are exports from bluedriver and csv files. Anyone have any insight. Its an 07 TB 4.2 i6 4wd LS 105K miles. It idles fine no stalling the power seams ok but its hard to compare it to my V8 truck. He loves the truck but sub 10 MPG is not acceptable.
It sounds like a cat converter , power ok and high gas consumption, I drilled a hole just before the bend before the cat from the engine , the size of the straw from a Sea foam spray can for cleaning fuel injectors Corbin and throttle bodies I sprayed it in and ran the trailblazer and it cleaned it up and senses, I saved money and getting a new cat. I put a ribbit In the hole and a clip around it. Never had a problem ever again. Great power and gas for the mile. Ps it will smoke for a few mins.
 

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Cat is not too hard to replace but you do have to pull the transmission cross member to do so, support the tranny of course, and dont forget the new flange/gasket. hour job tops
 

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I still have to flush the antifreeze so that isnt a bad idea at the same time. Im thinking about a new Mass airflow sensor also. Things are so much easier without computer controls I long for the days of old when it was gas+spark = go LOL. I owe you a Virtual Beer. Thanks for the collaboration.
NO mass air sensor unless its the V8
 

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Cat is not too hard to replace but you do have to pull the transmission cross member to do so, support the tranny of course, and dont forget the new flange/gasket. hour job tops
Just clean it, it’s like cleaning a Throttle body. But if it Welds together because of years of not doing anything. Good luck
 

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It sounds like a cat converter , power ok and high gas consumption, I drilled a hole just before the bend before the cat from the engine , the size of the straw from a Sea foam spray can for cleaning fuel injectors Corbin and throttle bodies I sprayed it in and ran the trailblazer and it cleaned it up and senses, I saved money and getting a new cat. I put a ribbit In the hole and a clip around it. Never had a problem ever again. Great power and gas for the mile. Ps it will smoke for a few mins.
I got a trailblazer for my Son paid 4K total it had low mileage and looked fairly clean inside. There were a few things I knew about like New shocks, rear trailing arms i had to fix then found it needed valve cover gaskets because there was oil on the spark plug boots, then new front hubs and steering sensors because of abs lights and codes, and a few odds and ends.I fixed all this myself and thought it was good to go I was wrong, he kept going to the Gas station alot. I accused him of idling and listening to the radio goofing with friends you know the normal teen stuff. So I filled it up drove it for a while and Poof I checked the MPG and its under 10 mpg over 3 tanks now no matter who is driving. I purchased a Blue driver tool and pulled 2 logs from a trip to home depot. There are no codes in the system and Im at a loss where to go from here.? I thought Fan but the fan looks like it is functioning properly, Then O2 sensors but im not sure they are bad. Thought termostat but the numbers look right there also. I have attached the logs as a text file but they are exports from bluedriver and csv files. Anyone have any insight. Its an 07 TB 4.2 i6 4wd LS 105K miles. It idles fine no stalling the power seams ok but its hard to compare it to my V8 truck. He loves the truck but sub 10 MPG is not acceptable.
May I make a suggestion....You have an INCREDIBLE amount of advice here....SIT ON IT FOR A FEW DAYS!!! These people ALL mean well....BUT you could wind up hating this car....and SPENDING a fortune. If you do not have one of those $3,000 testers....GO PAY SOMEONE that does. The information you can get here is INCREDIBLE!!! Finally, THE BEST ADVICE I EVER GOT FOR AUTO REPAIRS....IS to go back to the LAST THING THAT WAS FIXED “FIRST”!!!!! This may involve tracking down the previous owner....and that they WILL tell you the truth! With regards to your transmission....I Completely agree on NO POWER flush....BUT I highly recommend you flush or find someone who WILL do it....it’s messy but it is pretty easy to remove cooling lines from radiator, some old fuel hose, and a big bucket....THIS IS NOT A POWER FLUSH....as you are using transmission pump to push out old fluid.....I have had my 2004 Bravada since 30K....I do the fluid every 60-80K....HAD ZERO ISSUES!
Finally, this is a GREAT sight and forum...BUT PLEASE SLEEP ON IT....INCLUDING what I have sent!!! BEST OF LUCK SIR!!! I love this platform so much.... I just bought a Envoy XUV with 95K on it for $2,900...
 

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I got a trailblazer for my Son paid 4K total it had low mileage and looked fairly clean inside. There were a few things I knew about like New shocks, rear trailing arms i had to fix then found it needed valve cover gaskets because there was oil on the spark plug boots, then new front hubs and steering sensors because of abs lights and codes, and a few odds and ends.I fixed all this myself and thought it was good to go I was wrong, he kept going to the Gas station alot. I accused him of idling and listening to the radio goofing with friends you know the normal teen stuff. So I filled it up drove it for a while and Poof I checked the MPG and its under 10 mpg over 3 tanks now no matter who is driving. I purchased a Blue driver tool and pulled 2 logs from a trip to home depot. There are no codes in the system and Im at a loss where to go from here.? I thought Fan but the fan looks like it is functioning properly, Then O2 sensors but im not sure they are bad. Thought termostat but the numbers look right there also. I have attached the logs as a text file but they are exports from bluedriver and csv files. Anyone have any insight. Its an 07 TB 4.2 i6 4wd LS 105K miles. It idles fine no stalling the power seams ok but its hard to compare it to my V8 truck. He loves the truck but sub 10 MPG is not acceptable.
Not sure where you live, but fuel with ethanol cause the "thermal efficiency", think that is what it is called, to be lower MPGs. The concept of ethanol was to reduce oil consumption, but ethanol added to fuel lowers MPG and still causes more trips to the gas station to fill up more. Ethanol vs straight gas gets different gas mileage numbers, many articles on it.

I have a 2004 Envoy, 252,000 miles and around town driving, 12-13 MPG, but Hwy 21-22 MPG. I have changed plugs, new coil packs all AC Delco. New transmission was installed at dealer at 231,000, original one literally quit, so I know trans is not slipping. Have new fan installed as well.
 

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I fill my 2004 Envoy up every 2 weeks. I do a lot of in town driving and today before zeroing out the trip miles, Avg MPG showed 13.7. That is from a mix of driving fast and slow, but for what it is, a suv, didn't think its bad. I have over 251k, only used AC Delco replacement parts.
 

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This is my experience with my truck. I have had it for 200k and it runs like a dream with 15+ mpg (lead foot)

Step 2 should be a sticky on this site.

Step 1: Clean MAP and MAF sensor (2007 up for maf)

Step 1.5: clean your throttle body.

Step 2: check your PCV hose (driver side coming out of the intake manifold). (Disconnect from manifold leave block side connected) The car has an orfice not a valve. you should be able to blow into this hose with Little to moderate Resistance. If it is clogged or partly blocked pressure will build up and engine oil will shoot out of the breather hose on the side of the resonator and onto the sparkplugs. It also blows onto the throttle body making it gunky and messing up fuel trims as well as burning oil. Unclogging this also fixes rough idling because the air is technically unmetered. Use an air compressor to blow out the hose.


Step 3: use a cheap elm 327 or similar like blue driver to read your fuel trims. They should be below 10 percent. If they are higher it is excessively rich. (Mine are under 7 percent in any driving condition)


Step 4: change your thermostat if your temp needle is any lower than 210, if this is the case I would recommend adding a transmission cooler because the "correct" engine temp is hard on the tranny since the radiators are very close to each other.

Please don't replace your cat. Your truck needs a modern day tune up. Almost everything in this post is free and will save you a headache.

Throwing parts and money at a problem before making sure everything that could cause the problem is clean and working is a big no no.
 

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Step 3: use a cheap elm 327 or similar like blue driver to read your fuel trims. They should be below 10 percent. If they are higher it is excessively rich. (Mine are under 7 percent in any driving condition)


Step 4: change your thermostat if your temp needle is any lower than 210,
Re:Step 3; that data is already done and posted above in this thread.

Re:Step 4
Also uploaded, charted, color coded and discussed!
 

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I appended your most recent data to the spreadsheet. In looking it over just briefly and comparing to my own data also in the spreadsheet for comparison I noted an interesting item regarding engine coolant temps. I go from 122 degrees up to 205ish in under 5 minutes. This in an ambient temperature of mid 70s F. Your truck starts 10 or so degrees warmer, in an ambient of mid 90s F and takes about 25 minutes to reach and maintain the same temperature. Normal operating temps for these engines seems to be 205 to 210 or thereabouts.

You might consider a thermostat replacement.
Well, New plugs and an air filter are installed. I took a longer trip up the highway. It was a little hotter today 94-96. I think it looks better. I filled the tank on the way home so ill have to see about the MPG on the next fillup. For your enjoyment I posted the data on this trip. The long term fuel trim is consistently lower I believe. From here the other suggestions are replace the exhaust manifold and or Catalytic converter.. Lastly deal with the low mpg or trade it in. Im not there yet but it was absolutely running rich the plugs i put in when i got it were black around the outside of the plug. no fouling on the tip but i could see they were blacked. here is hoping the filter and ac delco plugs at least help it.
suggestion remove the throttlebody clean it thoroughly replace your thermostat and water pump, My 2008 trailblazer 4.2 LT went from 300 miles to a tank of gas to 250! The dealer was absolutely clueless as to what the problem could be I noticed my thermostat sticking a couple of times and after going on the forum I read where some people had replaced those items and their fuel economy improved just as mine has I am now over 300 miles to a tank of gas!
I have 140,000 miles on my trailblazer and it runs like a bear!
 

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I just got my 04 TB and it hasn't been easy figuring out what is wrong with it. I finally got it started today but it's misfiring. I haven't bought anything that I didn't know for sure was bad. Test test test. I've invested in a few testing tools and it's made all the difference in the world. Your already on the right track.
 

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Discussion Starter #32
The new termostat came in today. But im leaving for vacation for 2 weeks so it will have to wait till i get back. Im off on a fishing trip.ill check back in when i get it replaced. See you in 2 weeks.
 

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Welllllllllllll. I replaced the thermostat, changed the coolant, Mass air flow sensor and the map sensor. The numbers are a huge change. I filled the tank just now and will take it on a long distance drive maybe 75 to 100 miles. What do you think?
 

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I added your latest log to the spreadsheet linked earlier in the thread. Quite a difference in the trims as you noted. Also noted the downstream O2 sensor seems to indicate better catalytic converter operation. Still seem like your engine is slow to come up to temperature, maybe. I would keep an eye on that fan speed. Does your Bluedriver give access to cooling fan speed?
 

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Discussion Starter #35
It doesnt have access to the Fan speed. I was checking it out this weekend and it is spinning as soon as you turn on the motor. With it off you can spin the fan with a bit of resistance. I have a fan clutch in my amazon saved for later. I was thinking it was harder to turn than I expected and never really turns off. Looks like am narrowing down to lack of maintenance from the former owner. Or he just didnt want to pay the dealer to figure out whats wrong.Depending on work I may take a drive tomorrow night. The Fan clutch as about $65 so not a huge deal to replace from a cost standpoint. How hard is your fan to spin when it is cool? I can move it but it doesnt "spin".Is it possible to hate to have to work on a car and love doing it at the same time? Maybe its just me figuring out things is a passion. The thermostat didnt "look" too bad but you cant tell by looking at it.
 

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I wouldn't replace the fan clutch just yet. I would want some more time on it with all the recent work and see how it performs in a variety of circumstances.

They generally do not free spin hot or cold, there is always some amount of resistance. Mine runs about 700 rpm at idle and maybe 1200 at 65 mph. I can monitor the actual speed as well as the PCM requested speed with an ELM327 clone and a smartphone app.
 

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Discussion Starter #37
Hey well I took a drive finally. 75 miles mix of some city and most highway. 15.76 MPG. I think it was the MAF sensor or the MAP sensor changes. Gonna track it on a complete tank next.
 

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That's not bad mileage for one of these bricks! I'm happy when I get 14, but most all of my driving is in the city. 13.5 is about what I generally get.
 
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