Better fuel economy

Mooseman

Moderator
Dec 4, 2011
25,260
Ottawa, ON
What are you getting now? What has been done for maintenance?
 
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Trekie47

Original poster
Member
Aug 5, 2019
16
Brimley Michigan
What are you getting now? What has been done for maintenance?
according to log that i keep i average 15.72 Mpg, and since i've come into ownership of the vehicle 15,000 miles ago i've changed the oil around every 3k, new plugs, and coils, suspension bits. I plan to service both Differentials in a day or 2.
 

Mektek

Member
May 2, 2017
656
FL
Inflate your tires. If you follow the factory recommendation, you will have best ride quality and worst fuel economy. Look at the sidewalls and see what the maximum is and keep it a few psi below that.
 

Trekie47

Original poster
Member
Aug 5, 2019
16
Brimley Michigan
Inflate your tires. If you follow the factory recommendation, you will have best ride quality and worst fuel economy. Look at the sidewalls and see what the maximum is and keep it a few psi below that.
tires say 44. Depending on the temp they sit at either 42 or 45. Thanks Michigan weather.
 

coolride

Member
Aug 23, 2019
596
Adirondacks
Lighten up the load. Look around to see what sort of stuff you're hauling around all the time. Maybe there's some heavy stuff you can remove.
 

Mooseman

Moderator
Dec 4, 2011
25,260
Ottawa, ON
That isn't an awful MPG but about average for these trucks. Mine averages between 15 and 17 MPG. Follow the above recommendations to try and improve it.
 

TollKeeper

Supporting Donor
Member
Dec 3, 2011
8,047
Brighton, CO
If you have the 4.10 GT5 gear set, fuel mileage is going to be very subjective..

The GT4 3.73 gear set seem to get the best all around fuel mileage
The GU6 3.42 seem to give the best highway fuel mileage

I have the GU6 with a V8, and my mix of 65% highway, and 35% city, nets me about 15mpg winter, and 16.5 summer. The GU6 does not like city driving for fuel mileage.

The few trips I have done with my truck, I can easily net 18-27 mpg, depending on speed limits, and road conditions. My sweet spot for my truck seems to be right at 68mph turning 1900 rpm. 69 is where the fuel mileage starts to decline noticeably. But again, remember I have the V8, so the power curves of my engine are completely different than your I-6. Just gotta find that sweet spot.

My truck just turned 200,000 miles on Thursday, so you could say I have had many years and miles to find that sweet spot. I also run my tires at maximum pressure in the summer, and recommended pressure in the winter.
 

2x1968muscle

Member
Dec 4, 2011
17
Ensure that your thermostat is allowing the engine to fully warm up. If the computer thinks the engine is below normal operating temp then extra fuel gets used trying to heat up the engine.
 
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Trekie47

Original poster
Member
Aug 5, 2019
16
Brimley Michigan
Lighten up the load. Look around to see what sort of stuff you're hauling around all the time. Maybe there's some heavy stuff you can remove.
I don't carry much if anything other than myself. The heaviest thing that i carry is an extra jug of oil.

Ensure that your thermostat is allowing the engine to fully warm up. If the computer thinks the engine is below normal operating temp then extra fuel gets used trying to heat up the engine.
I have replaced the thermostat so i sure hope it's working properly.

If you have the 4.10 GT5 gear set, fuel mileage is going to be very subjective..

The GT4 3.73 gear set seem to get the best all around fuel mileage
The GU6 3.42 seem to give the best highway fuel mileage

I have the GU6 with a V8, and my mix of 65% highway, and 35% city, nets me about 15mpg winter, and 16.5 summer. The GU6 does not like city driving for fuel mileage.

The few trips I have done with my truck, I can easily net 18-27 mpg, depending on speed limits, and road conditions. My sweet spot for my truck seems to be right at 68mph turning 1900 rpm. 69 is where the fuel mileage starts to decline noticeably. But again, remember I have the V8, so the power curves of my engine are completely different than your I-6. Just gotta find that sweet spot.

My truck just turned 200,000 miles on Thursday, so you could say I have had many years and miles to find that sweet spot. I also run my tires at maximum pressure in the summer, and recommended pressure in the winter.
i'm not exactly sure which rear end i have, but i will find out when i service the Differentials.

That isn't an awful MPG but about average for these trucks. Mine averages between 15 and 17 MPG. Follow the above recommendations to try and improve it.
is there anything else i could do IE: a preset engine tune, intake or exhaust modifications ?
 

TollKeeper

Supporting Donor
Member
Dec 3, 2011
8,047
Brighton, CO
i'm not exactly sure which rear end i have, but i will find out when i service the Differentials.
Nothing that hard.. Open you glove box, on the inside of the lid will be a sticker with a bunch of 3 digit codes. Look for..

GT5 - 4.10
GT4 - 3.73
GU6 - 3.42

This sticker is the build sheet for your truck. Lots of info on it.

You can also go to http://www.free-vin-decoder.com/ and put your VIN in the box at the top right, and click on 'Decode VIN' and it will give it to you as well.
 

Mooseman

Moderator
Dec 4, 2011
25,260
Ottawa, ON
is there anything else i could do IE: a preset engine tune, intake or exhaust modifications ?
Not much available for accessories. Exhaust is already pretty free flowing (check for any back pressure), nothing available for intakes except HAIs (Hot Air Intakes) which would worsen it. Tunes are available, including from our own @limequat however, results are mixed regarding MPG improvements. Jeremy might be able to fill you in more on that. It would improve drivability.
 

Reprise

Lifetime VIP Donor
Supporting Donor
Member
Jul 22, 2015
2,724
Hi, i have a 2007 trailblazer, and i was wondering things that i could do to improve fuel economy other than "drive better".

Welcome to the site. Since you're looking for info about the 4.2L that came as the base engine in these, you're definitely in the right place. I have the 5.3L in mine, so I'm not an expert on the I6 -- but most of the GMT360s got the I6, not the V8.

To start...
- Your post above says your TB is a 2007, but your profile says it's a 2005.
I'll assume that it's an '07, but if it's an '05, let us know. Reason: GM modified the cylinder head ever so slightly, and I think '06 was when they did it. So if yours is an '07, you've already got the better head. IIRC, GM changed the head a second time, for '08 -- but that head only works with '08-'09s (and '08-'09 trucks can only use that head). Some of this is immaterial for you, but since I'm talking about improvements in the cylinder head...

In a larger context, fuel economy (FE) is the bane of this engine. It has great power, thanks to it's DOHC and variable valve timing (VVT) -- but those same things limit what could be done as far as cam / timing changes (compared, to say, an engine with fixed timing, like the early LS engines). Being an inline-six, it also has the inherent advantages of that form factor (superior balancing in the rotating assembly, as one example).

The other bad thing is that since this was the only vehicle the 'Atlas' LL8 (4.2L) was installed in (for 2002-2009), there's not a lot of aftermarket support for the engine -- and what little there is / was, was focused more on power, than FE. There were other versions installed in GMs 'midsize' pickups (the Colorado had a 5-cylinder version of the Atlas, for example, and there's also a 4-cylinder version, IIRC)

That leaves us with two other potential avenues -- maximizing FE via keeping the engine perfectly in tune (OEM specs), and things you can change (like tire pressure, the way you drive, etc.)

I'll leave the 'tune up' part for you; those things are on this site. One thing I would tell you -- use ONLY the listed AC Delco spark plug for this engine, as just about everyone here states that the engine runs best with it. The others will give you the correct #, if you need it / can't find it.

Make sure your catalytic converter is in good shape (not clogged / plugged), as well as your primary O2 sensor. If you don't have a 'pro' scan tool, you can get a cheap OBD-II reader and a smartphone app to show you what's going on with the O2 (again, there's posts & even whole forum sections dedicated to this; a couple of our members know how to create custom parameters to read specific engine functions, too (see: TJBaker57's posts)).

Tires -- you can inflate the tires to max sidewall pressure, which *will* decrease / lower rolling resistance (RR, or LRR). However, the ride will become harsh -- perhaps harsher than you would like.
A good 'middle ground' is to inflate a good 5-6 lb over what's on the door placard. People who work at tire shops tend to run their own tires like this. Besides decreasing RR, your tires will perform more / most optimally at this pressure.

As far as driving habits -- Google 'hypermiling techniques'; there are sources that will tell you how to change your driving style / habits, to improve your FE (further than what you've already done). I'll give you a couple of pointers, but there'll be a lot more that I don't mention here.

First -- As a rule, you want to be in the highest gear, at the lowest RPM to get there. Generally, this means keeping the tach at 2000 RPM or lower while you accelerate to speed. That means you're going to be accelerating 'leisurely', to put it politely. The right lane is your new best friend, here.

Second -- and this is really important -- you need to train yourself to 'lock the throttle' by keeping your right foot at a constant position, once you're up to speed, and more or less pretend there's an egg between your foot and the gas pedal (if you move it too much, you 'break the egg'; -- but don't try putting an actual egg there!) 🥚🐔

This includes uphills / downhills -- most people will accelerate when on a hill. You will need to learn *never* to do this. Instead, keep the throttle at the same position; the vehicle will bleed off speed, but you'll gain it back on the corresponding downhill. Another name for this is "driving with load" (popular with truckers; many use this), or, from a physics standpoint, "trading kinetic for potential".

This will take a toll on your ankle as you first start out, until your muscles in that area adapt to not moving for long periods. This is especially noticeable if you drive a lot of highway miles, which will bring me to my final point.

City driving is a killer (with any vehicle), getting up to speed, having to stop, idling in traffic, etc. The longer you can keep a constant speed (or, as stated above, throttle position), the better your FE will become. So if you have a mostly highway commute, the potential for improvement is greater.

As an example... I recently took a long highway trip with my Envoy XL (LWB, or 'Long Wheelbase'), which also has the 5.3L V8, as I mentioned. It also has the optional larger gas tank (3 gal; 26 total).

Normally, with mixed driving around where I live, my range indicator in the dash will display about 325 miles or so, when I fill up. But... when I drove to Ohio... by using some of those things I spoke of above, when I filled back up in OH, my range indicator was up to... 525 miles, IIRC (there's a discussion on this, if you want to search my posts). I didn't have my tires inflated to max sidewall, and I didn't empty the truck of everything including the jack & spare tire. In fact, I tend to drive with a *lot* of crap in my truck. Not *really* heavy, but there's stuff in the cargo area, and I keep both the 2nd and 3rd rows folded 100% of the time (no kids, etc.)

So... I was basically able to drive from way up north in IL (near the IL / WI border) all the way to Ohio -- on one tank of gas. That's why I say that it's easier to improve highway mileage vs. city. And because my engine is an '03, all eight cylinders run all the time. So it's not shutting down cylinders to improve fuel economy, like the later LS engines do.

You're probably wondering... "where did he get all of this?" :yikes:
I used to drive hybrid cars... and I spent time learning from a hypermiler. I don't follow all of their techniques (like drafting trucks close behind), but a couple of things stuck with me, and I can generally beat the EPA rating on anything I drive, if I choose to. Sometimes by a little... sometimes by a lot, depending on variables (and which vehicle I'm driving -- I also have a pickup that I use to tow my camping trailer with -- and with that, I'm generally in the 8-9 mpg range on the highway; nothing to be done, pulling a 30ft long, 7000lb, 11ft. high 'sail'). It hurts to have to fill up every 4hrs / 200 miles or so (26gal tank, again), but "it is what it is".

To wrap up...
In city driving, and especially as you're first starting out, you may find a difference of only 1-2 additional MPG per tank (so, maybe 40 miles or so of extra range). And you may decide that it's more trouble than it's worth. But given the limitations of what can be done to modify the 4.2L for better economy... most of the potential -- such that it is -- lies with you.
 

Trekie47

Original poster
Member
Aug 5, 2019
16
Brimley Michigan
Welcome to the site. Since you're looking for info about the 4.2L that came as the base engine in these, you're definitely in the right place. I have the 5.3L in mine, so I'm not an expert on the I6 -- but most of the GMT360s got the I6, not the V8.

To start...
- Your post above says your TB is a 2007, but your profile says it's a 2005.
I'll assume that it's an '07, but if it's an '05, let us know. Reason: GM modified the cylinder head ever so slightly, and I think '06 was when they did it. So if yours is an '07, you've already got the better head. IIRC, GM changed the head a second time, for '08 -- but that head only works with '08-'09s (and '08-'09 trucks can only use that head). Some of this is immaterial for you, but since I'm talking about improvements in the cylinder head...

In a larger context, fuel economy (FE) is the bane of this engine. It has great power, thanks to it's DOHC and variable valve timing (VVT) -- but those same things limit what could be done as far as cam / timing changes (compared, to say, an engine with fixed timing, like the early LS engines). Being an inline-six, it also has the inherent advantages of that form factor (superior balancing in the rotating assembly, as one example).

The other bad thing is that since this was the only vehicle the 'Atlas' LL8 (4.2L) was installed in (for 2002-2009), there's not a lot of aftermarket support for the engine -- and what little there is / was, was focused more on power, than FE. There were other versions installed in GMs 'midsize' pickups (the Colorado had a 5-cylinder version of the Atlas, for example, and there's also a 4-cylinder version, IIRC)

That leaves us with two other potential avenues -- maximizing FE via keeping the engine perfectly in tune (OEM specs), and things you can change (like tire pressure, the way you drive, etc.)

I'll leave the 'tune up' part for you; those things are on this site. One thing I would tell you -- use ONLY the listed AC Delco spark plug for this engine, as just about everyone here states that the engine runs best with it. The others will give you the correct #, if you need it / can't find it.

Make sure your catalytic converter is in good shape (not clogged / plugged), as well as your primary O2 sensor. If you don't have a 'pro' scan tool, you can get a cheap OBD-II reader and a smartphone app to show you what's going on with the O2 (again, there's posts & even whole forum sections dedicated to this; a couple of our members know how to create custom parameters to read specific engine functions, too (see: TJBaker57's posts)).

Tires -- you can inflate the tires to max sidewall pressure, which *will* decrease / lower rolling resistance (RR, or LRR). However, the ride will become harsh -- perhaps harsher than you would like.
A good 'middle ground' is to inflate a good 5-6 lb over what's on the door placard. People who work at tire shops tend to run their own tires like this. Besides decreasing RR, your tires will perform more / most optimally at this pressure.

As far as driving habits -- Google 'hypermiling techniques'; there are sources that will tell you how to change your driving style / habits, to improve your FE (further than what you've already done). I'll give you a couple of pointers, but there'll be a lot more that I don't mention here.

First -- As a rule, you want to be in the highest gear, at the lowest RPM to get there. Generally, this means keeping the tach at 2000 RPM or lower while you accelerate to speed. That means you're going to be accelerating 'leisurely', to put it politely. The right lane is your new best friend, here.

Second -- and this is really important -- you need to train yourself to 'lock the throttle' by keeping your right foot at a constant position, once you're up to speed, and more or less pretend there's an egg between your foot and the gas pedal (if you move it too much, you 'break the egg'; -- but don't try putting an actual egg there!) 🥚🐔

This includes uphills / downhills -- most people will accelerate when on a hill. You will need to learn *never* to do this. Instead, keep the throttle at the same position; the vehicle will bleed off speed, but you'll gain it back on the corresponding downhill. Another name for this is "driving with load" (popular with truckers; many use this), or, from a physics standpoint, "trading kinetic for potential".

This will take a toll on your ankle as you first start out, until your muscles in that area adapt to not moving for long periods. This is especially noticeable if you drive a lot of highway miles, which will bring me to my final point.

City driving is a killer (with any vehicle), getting up to speed, having to stop, idling in traffic, etc. The longer you can keep a constant speed (or, as stated above, throttle position), the better your FE will become. So if you have a mostly highway commute, the potential for improvement is greater.

As an example... I recently took a long highway trip with my Envoy XL (LWB, or 'Long Wheelbase'), which also has the 5.3L V8, as I mentioned. It also has the optional larger gas tank (3 gal; 26 total).

Normally, with mixed driving around where I live, my range indicator in the dash will display about 325 miles or so, when I fill up. But... when I drove to Ohio... by using some of those things I spoke of above, when I filled back up in OH, my range indicator was up to... 525 miles, IIRC (there's a discussion on this, if you want to search my posts). I didn't have my tires inflated to max sidewall, and I didn't empty the truck of everything including the jack & spare tire. In fact, I tend to drive with a *lot* of crap in my truck. Not *really* heavy, but there's stuff in the cargo area, and I keep both the 2nd and 3rd rows folded 100% of the time (no kids, etc.)

So... I was basically able to drive from way up north in IL (near the IL / WI border) all the way to Ohio -- on one tank of gas. That's why I say that it's easier to improve highway mileage vs. city. And because my engine is an '03, all eight cylinders run all the time. So it's not shutting down cylinders to improve fuel economy, like the later LS engines do.

You're probably wondering... "where did he get all of this?" :yikes:
I used to drive hybrid cars... and I spent time learning from a hypermiler. I don't follow all of their techniques (like drafting trucks close behind), but a couple of things stuck with me, and I can generally beat the EPA rating on anything I drive, if I choose to. Sometimes by a little... sometimes by a lot, depending on variables (and which vehicle I'm driving -- I also have a pickup that I use to tow my camping trailer with -- and with that, I'm generally in the 8-9 mpg range on the highway; nothing to be done, pulling a 30ft long, 7000lb, 11ft. high 'sail'). It hurts to have to fill up every 4hrs / 200 miles or so (26gal tank, again), but "it is what it is".

To wrap up...
In city driving, and especially as you're first starting out, you may find a difference of only 1-2 additional MPG per tank (so, maybe 40 miles or so of extra range). And you may decide that it's more trouble than it's worth. But given the limitations of what can be done to modify the 4.2L for better economy... most of the potential -- such that it is -- lies with you.
Oh yea i forgot my profile said an 05 blazer, as i was pursuing this form a while back to help me in my journey to fix my mothers blazer which is the 05, but i went and bought myself an 07. Thank you for sharing some of your wealth of knowledge, my current employment is delivering pizzas so mostly city driving with a relaxing 40 miles of commute to and from work. With how gas prices have been rising i was hoping y'all would have an ace up your sleeve that would make a large improvement at once, but i guess life (and driving) really is about the little things. Once again thank you for the info guys.
 

Dadwagon

Member
May 21, 2020
141
West coast
15-16mpg is very easy to achieve if you drive the vehicle to keep up with traffic, or do anything but drive at a constant speed on the highway. Even going 70mph is going to net you 1mpg less for the tank. Seems pretty normal.
I have an I6 and the 3.73 diff with "eco" truck/SUV tires. The best thing I find is to do as much coasting as you can. I don't even think slowly accelerating helps fuel economy, just don't constantly hammer on 3/4 throttle. Get up to speed in final gear as fast as you can and do your best to brake as little as possible, using the tires to slow you down. The fuel economy isn't in getting up to speed, it's getting down in speed without having to brake since you didn't waste any of the throttle you gave it.
The onboard computer is mostly accurate unless you do a lot of idling with A/C on.
 

TollKeeper

Supporting Donor
Member
Dec 3, 2011
8,047
Brighton, CO
So you all might think I am crazy, but I am currently netting 16.3 mpg, driving at 95mph (speed limiter on my truck), on my way to work, and on my way home.
 

Reprise

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Member
Jul 22, 2015
2,724
I do think you're crazy, but not due to your reported MPG.
 

TollKeeper

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Member
Dec 3, 2011
8,047
Brighton, CO
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Reprise

Lifetime VIP Donor
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Jul 22, 2015
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Oh - that's different; +10, relative to the other traffic, isn't *so* bad. Hell, you're probably driving with the flow, at that speed. :laugh:
I thought you were consistently 30-40mph over, relative to everyone else.
And I didn't realize CO had an 85mph limit, TBH.
 

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