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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.
 

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2009 chevy trailblazer_lt
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If not done already, it is due for 100k service. This includes spark plugs (AC Delco 41-103 only), transfer case and transmission fluid changes, transmission filter, air filter and engine antifreeze. In addition to 100k service, I would clean the throttle body thoroughly. There are plenty of write-ups on how to clean. Make sure you cut power to pcm while doing this so it will relearn settings when you are done. On my car,a 2009, this is done by pulling underhood fuses 10, 28 & 29. Some say just disconnect battery, but this sometimes causes problems with interior air flow controllers.

Another possibility for bad mileage is catalytic converter failure. This can be caused by a bad thermostat opening too soon causing engine to run too cold. Engine should be running at 210 deg.
 

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I just ordered AV delco plugs and an airfilter from advance auto parts, I put in new plugs and found the leaky gasket but frankly I dont recall what ones they were I just told them to give me plugs. So Iridium plugs it is then I didnt think it would matter before. I did a clean and seaform when i replaced the valve cover gaskets. I guess a transflush and filter are next up too much mess for me to do. I cant detect any shifting issues but it is time. I checked on a new Cat there about 200-250. I think I can switch that out myself if i have to.

So new plugs and new airfilter on the way, Fingers crossed,
I just did the termostat in my ram I hope it is that easy on the TB. That cant harm anything the temps on the live data looked like it was working but if its easy its only $20.
I guess the last resort is going to be the Cat replacement. Hate to do it and it not be the issue.
 

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2002 chevy trailblazer_lt
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I like noodling around with data in spreadsheets where I can color scale the data and look for things that stand out.

I'll have to admit I don't remember what my air intake temps are but can't help thinking yours look high at times. Will check against mine under similar conditions except I won't be doing any 0 to 45 mph WOT runs like we see in your log files!! I have too much mileage on mine to subject it to that sort of thing.

 

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I guess a transflush and filter are next up too much mess for me to do.
Do not power flush the transmission! Just do a pan drop and filter change. Many are the horror stories of people who have had their transmissions power flushed, and shortly thereafter needed a new transmission. A pan drop and filter change are all it takes. I would also check for dragging brakes, that can do bad things to gas mileage. A clogged catalytic converter can wreak havoc with gas mileage, as well.
 

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Discussion Starter #6
Do not power flush the transmission! Just do a pan drop and filter change. Many are the horror stories of people who have had their transmissions power flushed, and shortly thereafter needed a new transmission. A pan drop and filter change are all it takes. I would also check for dragging brakes, that can do bad things to gas mileage. A clogged catalytic converter can wreak havoc with gas mileage, as well.
I have a feeling im going to be buying a CAT. It just passed State emissions test last month so I wasnt thinking a CAT was the issue. I have heard the same thing about power flush. I go to the same mechanic for 20 years and dont go to jiffy lube or places like that. Dont trust them.
 

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If not done already, it is due for 100k service. This includes spark plugs (AC Delco 41-103 only), transfer case and transmission fluid changes, transmission filter, air filter and engine antifreeze. In addition to 100k service, I would clean the throttle body thoroughly. There are plenty of write-ups on how to clean. Make sure you cut power to pcm while doing this so it will relearn settings when you are done. On my car,a 2009, this is done by pulling underhood fuses 10, 28 & 29. Some say just disconnect battery, but this sometimes causes problems with interior air flow controllers.

Another possibility for bad mileage is catalytic converter failure. This can be caused by a bad thermostat opening too soon causing engine to run too cold. Engine should be running at 210 deg.
Quick question, why those spark plugs only? I used NGK spark plugs, have had no issues. Asking because I've never owned one of these before except for the one I just bought.
 

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2004 chevy trailblazer_ls
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Keep an eye on the long/short fuel trims. May point to something.
If I remember correctly, there was a member on here that had poor fuel milage and excessive long / short fuel trims and he went through a laundry list to fix it, but it did work.
Sorry, don't remember the posters name.
 

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41-103s because I've read way to many posts by people who have used different plugs who had problems. They last 100k miles, so a little extra cost is worth it to me.
 

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Your LTFT look high sometimes at idle. you can spot these in the spreadsheet posted above, rightmost column, look for the places where that column is dark red then zoom in there. There are times it is 12 or maybe greater. Ideally should stay in single digits.
 

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@MikeMc

I added another tab to the spreadsheet and put in a log file from my own 2002 Trailblazer.

As expected my intake air temps never approached what your logs are reading. Also your LTFT are way high in comparison to mine. My max LTFT was 6.25,,, hours was more than double this.

I have read that sometimes the PCM is seen dumping in extra fuel when it detects what it perceives as a lean condition and that can sometimes be traced to something like a cracked exhaust manifold letting in air before the O2 sensor #1.
 

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I figured if im logging data I should give it a varied idle highway and stomping on it to see how it acted and what data it presented.I dont hear any ticking or exhaust noise from it. I wish I had a lift to get under it but I have ramps to visual check it. My son just got back from the store picking up a new air filter and ACDELCO plugs. I just double checked the box and they are 41-103 plugs. The outdoor temps here are mid 80 to 90. Our plan is to change the plugs and airfilter tomorrow. Ill get under it and see what condition the exhaust is. I have a endoscope so ill see if i can take a look at the manifold also. I have been thru so many articles at this point nothing looks like a smoking gun. I was hoping for a Gotcha moment a simple o2 sensor.
 

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@MikeMc

Even at your outside temps that intake temp looks high. Mine runs no more than 15 degrees above ambient and that is when sitting still and idling after fully warmed up.

Your "smoking gun" is I think there in the data. Those Long Term Fuel Trims are high. The PCM is tossing in 10% to 15% extra fuel. Something is causing it and needs to be found out.
 

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@MikeMc

here is another discussion of the same thing.....
 

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@MikeMc

Even at your outside temps that intake temp looks high. Mine runs no more than 15 degrees above ambient and that is when sitting still and idling after fully warmed up.

Your "smoking gun" is I think there in the data. Those Long Term Fuel Trims are high. The PCM is tossing in 10% to 15% extra fuel. Something is causing it and needs to be found out.
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.
 

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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.
 

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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.
 

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something is amiss that's for sure, I sold my trail blazer and got a hd 2500 Duramax but I used to haul my Kubota l4400 with loader and mower and I weighed in at 14210lbs yeah way over weight!! But I drove 71 miles everyday with it to Aransas Pass, tx after hurricane Harvey. I got 11.0 to 11.8 mpg every day I had over 105k on it and only thing I added to it was some airbags into the springs in back ran 20psi in them. When not hauling load I would get 20 to 22mph
 

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We have a 2005 Envoy that had a thermostat stuck open issue a few years ago. No mileage decrease that we noticed, but there was a fault code that wouldn't go away. It came back after clearing. Something like engine temp error. I believe we had a check engine light also. The temperature guage was also reading low(190 to 200). Yes, it went away after I changed the thermostat(which was a pain, I don't care what anyone says!) Just my 2 cents worth since you're not getting an error code. By the way, there was an excellent post in here somewhere with pictures, showing the thermostat replacement. Good luck!!
 
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