Solved my CRAPPY MPG's, Spoiler Alert it was the COIL PACKS

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Daniel644

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Original poster
Feb 27, 2015
533
OK, so I bought my Trailblazer the better part of a decade ago at this point (at least 7 years) it had over 200k miles when I bought it, it's now at 245k and had the transmission replaced 6-7 years ago, the whole time I have owned it it averaged 14.5 MPG, In my research when I was first looking at these trucks I had seen posts on the old site about guys saying they where getting 20-22 MPG on the highway and just could never believe it, I had changed my spark plugs, did the fuel filter (after running fuel system cleaner through it), since it's an 03 and has a fuel filter, replaced the VVT Solenoid since the connector was full of oil, cleaned the throttle body and even went as far as doing the K&N intake as some people claimed to see 1-2 MPG increases with it, all for not, it always has averaged 14.5 when driving it for work and 13.8 driving just to town and back (that chunk of my route is less fuel efficient then town to work), the best single tank average I had ever seen was one time I did a 16.5 MPG average for an all highway driving tank.

ALL these mileage numbers are calculated by hand using the method of filling till the pump clicks off and resetting the trip odometer at each fill up and doing my best to always use the same pump and taking actual miles driven divided by gallons to refill (I have found pumps can vary by as much as half a gallon for where they click off at which is why I always prefer to use the same pump when possible).

Well anyway last weekend (Sunday a week ago) I had to drive out to my tax ladies house to take her a computer I had repaired for one of here customers, truck drove out there fine then when I went to drive to town it felt like it had the occasional miss but wasn't throwing a code or anything, but knowing these trucks (again from my research) can have coil pack issues I said screw it and grabbed an $80 set of new coil packs off Amazon, took it for a test drive and it felt like it had a little more power, so I decided to put it through it's paces and drove it on vacation this weekend, we will ignore the first fill up for now until I fill up again at the same pump to get the overall average of 4 tanks and instead focus on the 2 tanks of fuel, so heading south to my sisters house in North Florida I filled up at the Buc-ee's in Warner Robins, GA from there I drove to her house, parked it for a few days and drove it back, I had 353.5 Miles on that tank and it took 16.876 Gallons to refill at a Shell station about 30 miles south of the Georgia line (I should have waited but never having driven this far on a tank in this truck I wasn't sure I would make the state line or not), I then continue north till I reach the Buc-ee's again and fill up at the same pump I filled up at Thursday night, I had 170.3 Miles on that trip and it took 8.083 Gallons to refill, that calculates out to 20.98 MPG average for an all highway of the flat lands of South Georgia/North Florida from the same Buc-ee's pump as the "control", a basically 4.5 MPG increase over the BEST highway mileage I had ever done before, an over 27% improvement in MPG's, so moral of the story, don't you let anyone tell you the only reason to replace a coil pack is because of a misfire, they do definitely age and will effect your fuel mileage signifigantly, Oh and remember that 21 MPG highway is with 22" SS replica's, so not a light wheel, far more rotational mass, I would be curious to see what it would do with stock wheels and tires but I did the EXT front brake upgrade to the bigger rotor so 16's won't fit anymore, I think with smaller rims and the narrower stock tire width it could do 22 maybe 23.

Can't wait to see what my around town mileage is like now once I refill to clear the highway trip completely.
 

Mooseman

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Dec 4, 2011
22,550
Ottawa, ON
Nice! One thing that people also don't realize when people were getting 20 MPG, we had access to non-ethanol fuel. I remember before ethanol was mandatory, I experimented between pure gas and 10% ethanol and I consistently lost a couple of MPG. And now they're talking about 15% ethanol. At the same price, you'll lose even more MPG. No thanks.
 
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Daniel644

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Original poster
Feb 27, 2015
533
Nice! One thing that people also don't realize when people were getting 20 MPG, we had access to non-ethanol fuel. I remember before ethanol was mandatory, I experimented between pure gas and 10% ethanol and I consistently lost a couple of MPG. And now they're talking about 15% ethanol. At the same price, you'll lose even more MPG. No thanks.
Yeah I used to specifically get the Ethanol Free on some of my cars because it made a difference in Mileage and they where from the 90's before seals where updated to handle ethanols corrosive effects, it's actually the reason I started getting gas at the local station I get gas from because they have a single Ethanol free pump, but i stopped because it didn't make sense from a math perspective, the cost per gallon of Ethanol Free was higher then the reduced cost of operation from the higher mileage. It's also worth noting this 21 MPG highway was on 10% Ethanol Gas. Buc-ee's does have Ethanol Free but I didn't wanna go searching for what set of pumps was the ethanol free because that place is HUGE with well over 100 Pumps.

Definitely not doing no 15% Ethanol garbage.
 
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Street

Registered Member
Oct 2, 2021
12
Wilmington, DE
Whiskey Tango Foxtrot! I mean holy crap - my BEST tank was 17MPG which is an outlier from my usual fill-ups. My overall average is 15MPG, with 15.6 for my last tank. I figure that the 17 was a "pump cutoff early last fill" error. But you mean to tell me that 20MPG with the 4.2L is even possible?!?! E10 or not? I run up and down the highway most days 30m one way - 70mph(2k rpm+/-) - and still barely average 15. I love my Voy, but any MPG increase would help! (especially since filling up yesterday for $5 / g - OUCH). If it wasn't for the Wawa mobile app 15c/g discount - $80 wouldn't fill it.

I should mention that I live in Delaware - state is nearly as flat as glass.
 

Lckent48

Registered Member
Dec 4, 2011
33
Street, don't know what year you have. My 2009 average from day 1 is 15.6 About 80% in town. Best tank, driving downwind across Nebraska, was 23.4 Worst tank, cold, snowy, in town and in 4wd was 10.4
 

Street

Registered Member
Oct 2, 2021
12
Wilmington, DE
@Lckent48 2002; in 10k miles of tracking fuel mileage; not once have I gotten better than the 17 - but as low as 10.4 in Jan (oxygenated fuel sucks).

Hmm... for a guy who works from home, I can't believe I've managed 10k since Oct 2021.
 

Daniel644

Hobbyist
Original poster
Feb 27, 2015
533
Whiskey Tango Foxtrot! I mean holy crap - my BEST tank was 17MPG which is an outlier from my usual fill-ups. My overall average is 15MPG, with 15.6 for my last tank. I figure that the 17 was a "pump cutoff early last fill" error. But you mean to tell me that 20MPG with the 4.2L is even possible?!?! E10 or not? I run up and down the highway most days 30m one way - 70mph(2k rpm+/-) - and still barely average 15. I love my Voy, but any MPG increase would help! (especially since filling up yesterday for $5 / g - OUCH). If it wasn't for the Wawa mobile app 15c/g discount - $80 wouldn't fill it.

I should mention that I live in Delaware - state is nearly as flat as glass.
more driving will tell, but I felt confident enough in those numbers based on my control of the same pump being used, the previous tank (filled at my local gas station) coming down to Buc-ee's averaged 19.45 MPG, thats close enough that the half gallon variance you can see from 1 pump to another in where the pump clicks off could explain the difference, if I see anywhere around 22.5 MPG on my next refill back at my normal pump this weekend then that will confirm the 21 MPG average over 4 tanks of pure highway driving, we'll know more this weekend.
 
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Daniel644

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Original poster
Feb 27, 2015
533
OK, we have final numbers, filling up at the local pump (and I mean PUMP, not just station) that I filled up at to start this trip, I averaged 19.76 MPG on the bucee's to home route, so that brings the 4 tank average 20.3 MPG and clearly shows an increase of 1+ MPG in South Georgia/North Florida flat lands compared to the mountains of North Georgia, I suspect the slightly better return mileage is due to the fact I went thru Atlanta at like Midnight-1AM so there was no traffic slowdowns like there was when heading south.
 

vipergg

Registered Member
Dec 7, 2011
189
Whiskey Tango Foxtrot! I mean holy crap - my BEST tank was 17MPG which is an outlier from my usual fill-ups. My overall average is 15MPG, with 15.6 for my last tank. I figure that the 17 was a "pump cutoff early last fill" error. But you mean to tell me that 20MPG with the 4.2L is even possible?!?! E10 i or not? I run up and down the highway most days 30m one way - 70mph(2k rpm+/-) - and still barely average 15. I love my Voy, but any MPG increase would help! (especially since filling up yesterday for $5 / g - OUCH). If it wasn't for the Wawa mobile app 15c/g discount - $80 wouldn't fill it.

I should mention that I live in Delaware - state is nearly as flat as glass.
I always got 22 mpg on my 2008 Isuzu Ascender , it is a GMC Envoy with a Isuzu name stuck on it .Same engine and transmission and just different front and rear fascia . Mine is 2WD but I always got 21-22 going down the highway at 65-70 mph. Above 70 mph the lowly 4 speed auto begins to take its toll . I so wanted them to keep making this SUV and stick one of the modern 8 speed auto's behind it and that thing probably could have got 26-27 mpg . The I6 is wonderful engine and it is still going at 165000 miles . Original engine, tranny , coil packs , alternator , fuel pump and water pump .
 

Mooseman

Master Blaster
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Dec 4, 2011
22,550
Ottawa, ON
Heck, even the 6 speed would have been good. But these SUVs died with the old GM.
 
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Daniel644

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Original poster
Feb 27, 2015
533
If we wanna get super technical I'm running a 265/40/22, so in theory (if my math is right, feel free to correct me if i'm wrong) we actually need to add 2.7% to the miles driven number to account for the speedometer/odometer error from running the larger tire diameter so that would put the corrected MPG for the South Georgia drive up to 21.5 MPG and thats on the 22" SS Replicas and Nitto NT420S tires with a combined weight of nearly 70 pounds per wheel and tire.

Ended up getting a flat on one of the back tires, I pulled the tire off the truck to make it easier to find the hole, plugged it but while doing so found a small chunk of tread missing and saw how dry cracked the edges of the tires are, checked the dates, they are 6 years old (18th Week of 2016) so got some new tires ordered, the NT420S is discontinued, decided to go with the Continental EXTREMECONTACT DWS 06 PLUS XL, they are 7 pounds lighter a tire, so should be a significant reduction in rotational mass.
 

Mektek

Hobbyist
May 2, 2017
612
FL
six years and the tires are failing???? maybe if they were chinese specials I could understand.....
I had a friend with a vintage cadillac that had the same tires for twenty years!
 

Daniel644

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Original poster
Feb 27, 2015
533
six years and the tires are failing???? maybe if they were chinese specials I could understand.....
I had a friend with a vintage cadillac that had the same tires for twenty years!
this is what every tire looks like on the outside, there is a piece of tread about half-three quarters the size of a dime just flat out missing on the inside of this specific tire. Remember this is a street driven car parked outside in the elements, not a garage queen. Besides you are supposed to replace tires at 6 years anyway https://www.edmunds.com/car-maintenance/how-old-and-dangerous-are-your-tires.html
 

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Mektek

Hobbyist
May 2, 2017
612
FL
looks like an alignment problem.......
It hasn't worn to the base rubber so you should get a few thousand miles more out of them.
 

TollKeeper

Guru
Dec 3, 2011
6,886
Brighton, CO
Im at 10 years on my tires on my Aztek, but they only have like 30k miles on them too.

My Envoy, I got 7 years, and 72000 miles out of those Michelins.
 

Daniel644

Hobbyist
Original poster
Feb 27, 2015
533
looks like an alignment problem.......
It hasn't worn to the base rubber so you should get a few thousand miles more out of them.
look past the edge wear from the alignment and down into the space between the treads, see the cracks in the rubber down in the grooves? I'm not worried about the slightly balded edge of the tire, i'm looking at the cracks in the rubber and chunks of tread missing (which I can't get a good shot of unless I take the wheel off) either way new tires are here and will get installed this weekend. If I find the time to do the rear brakes before Saturday I'll get a picture of the chunk missing from the tread.
 

Mektek

Hobbyist
May 2, 2017
612
FL
Did you always have a wear pattern like that? I have a lumina that in the first fifteen years of life had even wear on the front tires. Something changed after that and then the front tires would develop a band of extra wear on the outer inch and half. The rest of the tread had normal wear. I haven't figured out why but I'm experimenting with changing the camber slightly to see what happens.

I noticed it after a few months even in new tires. The back tires wear evenly.
 

Daniel644

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Original poster
Feb 27, 2015
533
Did you always have a wear pattern like that? I have a lumina that in the first fifteen years of life had even wear on the front tires. Something changed after that and then the front tires would develop a band of extra wear on the outer inch and half. The rest of the tread had normal wear. I haven't figured out why but I'm experimenting with changing the camber slightly to see what happens.

I noticed it after a few months even in new tires. The back tires wear evenly.
Sometimes you can see weird tire wear due to Caster, which is something that usually doesn't have an adjustment, but Toe and Camber can do edge wear like this, too much toe in and positive camber or it could be not running enough air pressure causing the shoulders to bear more of the load, I run the stock air pressure of 32 PSI but I just found a website the other day that suggests (and even has a calculator) for when you upsize tires the new pressure you need to run to get the same load capacity and it said I should be running like 38 PSI based on it's calculator.

They are calling for thunderstorms this weekend so I stayed up all night doing the rear brakes (freaking HATE that damn rear caliper bracket mounting bolts, gonna shower and head off to bed here shortly, but attached are the pictures of what I saw on that rear tire that is the main reason i'm replacing these tires. At this point the tire only needs to last long enough to get the less then 10 miles to the tire shop and go through the rear pad bedding process for the carbon fiber ceramic pads (I got the Powerstop KC2062-36 kit) since I ate through the last set of front pads in under 40k miles (this is the EXT part that requires 17" or larger wheels to run the larger front rotor) and the are cross drilled and slotted rotors that are also coated so they aren't supposed to get all rusty like most rotors get, we'll see how that coating holds up.
 

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Mektek

Hobbyist
May 2, 2017
612
FL
yep, I junked my tires when I saw something similar.
I run closer to maximum pressures to reduce rolling resistance. 45psi on the front and 42 on the back. I did an alignment recently and set it for zero toe. I might try slightly toed out to check the effect.
 

Daniel644

Hobbyist
Original poster
Feb 27, 2015
533
yep, I junked my tires when I saw something similar.
I run closer to maximum pressures to reduce rolling resistance. 45psi on the front and 42 on the back. I did an alignment recently and set it for zero toe. I might try slightly toed out to check the effect.
My local tire shop (the only one open on a Saturday, which is the only day I can do truck things on due to my work schedule, around here) are some greedy bastards, they are up to 55 bucks a tire mount and balance and another 6 a tire for disposal, if those old tires weren't so unsafe to drive on I would have driven to a Discount Tire, they are 24 a tire for mount and balance and 3 dollar disposal fee. even having to drive the 100 mile round trip to the nearest Discount Tire even at these gas prices I still would have been a 100+ bucks cheaper. They are definitely screwing people who decided to save money by not overpaying for tires from them by hitting them on the mount and balance.

All that aside, LOVING the new Continentals, they ride SO SMOOTH and I can't hear them even with the windows down (unlike the Nitto's on my Civic), will probably bump up the tire pressure a bit to see how they feel at like 36-38, that should combat the shoulder wear.

Now onto the next issue that just popped up, gonna start another thread for that one.
 

Mooseman

Master Blaster
Moderator
Dec 4, 2011
22,550
Ottawa, ON
This thread has run its course and went off topic so

:lock_it:

Now onto the next issue that just popped up, gonna start another thread for that one.
Good idea!
 
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