Towing with a 305 V8?

[Bextreme04]

I have a 2015 K2500 HD 6.0 gas/6L90 with with 4.10 gears. I get around 12-13 mpg empty. If a modern engine and transmission does this bad, I'd hate to see what a small block connected to a 4 speed would do. With that said, I can tow some heavy loads with ease but the 6-8 mpg makes me sad. I got this truck used, but if I had ordered it new, it would have 3.73s.

TLDR: Your catalytic converters or O2 sensors are bad.

My 2011 Suburban used to get 15-16 around town and 20 on the freeway. 10mpg loaded pulling a 6k lb trailer at 70mph. The mileage just started to slowly drop out and now it is down to about 10mpg around town, 13-15 on the highway, and 8mpg towing. I pulled my hair out trying to figure out what was going on... and then I saw some posts about catalytic converters on the Gen IV LS platforms causing loss of fuel mileage and how to see if your cats are bad using HP Tuners and datalogging the O2 sensors. Sure enough.. did a run and looked at the sensor feeds and there was no #2 O2 sensor switching after the initial warmup. They basically loaded to rich after warmup and stayed there. This stayed the same even after replacing all 4 O2 sensors with GM OE sensors. When logging all four sensors, you SHOULD see the rear o2 sensors switching about 20% of the rate that the front sensors are switching.

I have a new exhaust from headers all the way to exhaust tip going on this next weekend with new flowmaster high flow cats going on. I'm hoping I should get all of that mileage back. I also ended up disabling the flex fuel because the bad cats were causing the virtual flex fuel algorithm to decide that I had anywhere from 15-45% ethanol content in my ~10% content max standard gasoline. It was causing it to put WAY too much timing in and also run pig rich, which was also a part of the problem.

 

[Velder]

@Bextreme04 That sounds very interesting. I just got home from a trip pulling my 4k lb trailer with my dad's 2008 Yukon.
It is the 2500 3/4 ton with the 6.0L, and when pulling we were only getting 8mpg. I am not sure I follow how you did the testing, but I would love to know if that is the issue with the Yukon. Could you elaborate on the testing?

 

[Bextreme04]

@Bextreme04 That sounds very interesting. I just got home from a trip pulling my 4k lb trailer with my dad's 2008 Yukon.
It is the 2500 3/4 ton with the 6.0L, and when pulling we were only getting 8mpg. I am not sure I follow how you did the testing, but I would love to know if that is the issue with the Yukon. Could you elaborate on the testing?

Depending on how hard you are pulling, speed, grades, etc... you might see 8-10mpg pulling, but it should really be more on the 10 side. The unloaded MPG is really where I see the biggest issue. There is no reason that a double overdriven 6.0 should be getting 12-13mpg. Anyways, if you have a way to graph or log live data, you just log all 4 O2 sensors. B1S1, B1S2, B2S1, B2S2. You should see both Bank 1 and Bank 2 Sensor 1 regularly switching between high and low voltage. You should see Both Bank 1 and Bank 2 Sensor 2 staying high for a while, then switching low for a while, then back again. The rate of switching should be ~20% of the rate that Sensor 1 on both banks are switching. If it isn't, then you have bad cats. You can try cleaning them out with various stuff, but really OEM cats are only designed and required to last for 6 years.

Here's a video that shows what the switching graph looks like and explains what to look for. Old O2 sensors will get lazy in their reading and take longer to switch, which can also cause poor fuel economy.

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[Velder]

@Bextreme04 thank you so much! I hope I can measure those with my obd scanner, we'll see.

 

[Rusty Nail]

4wd welding truck > 2wd welding truck.

These dudes know too but not everyone will tell you. Grit Dog is way more friendly than I am and he's no dummy.

I'm readin this thinkin aww man, if this thread turns into more+other LS swap bullstuff i'm out like a lightswitch. Couldn't care less - might as well break it down for the new guy since he's still here.
I work enough off road country gigs / oil and gas type stuff to have experience to speak up but old boy is right.

not everyone who asks actually wants advice.

Let's stop here and take a break. Ok? Real talk.
Is the truck capable of it? Yes, no question. Would a different gear set be beneficial ? Yes, no question.
Is a 305 capable? Yes, without question.

Is that what you really want though? Only because it's grandpas truck? Maybe. I can't answer that question. Exactly how much welding to you intend to do in the old square? Where and what type? On the concrete in the city? Only in nice weather? Well, we're at a fork here see?
One way is man style that fixes anything he wants and the other way? Well that's the road to home for dinner in time. That guy is cool, don't get me wrong but he don't got no money. If you intend to say, hit the road or fix ANYTHING in the country, the 2wd is not up to it. No matter the engine, SBC or LS, you've already spent too much time and money thinkin about it.
If youve got kids and dont plan to follow oil rigs around, the 2wd will probably be ok but make sure you get a posi.

That's all.
Reckon I was reading some other thread or something. Dude was choosing between a lowered c10 and a k5. He's gonna let the Blazer go and thus  guarantees  he's gonna get stuck and be useless. He's not a workin man like us though, he drives a concrete queen.
It's shiny. I can make fun of him if I want to - welcome to the internet.

 

[Grit dog]

^Well said @Rusty Nail. My point to him as well, albeit in a slightly different light. But yours is a very accurate light.
We don’t know this dude, nor what, where, how far, how long or like you said if he wants to be home in time for dinner every night with his family or is content/predisposed to running marginal setups and the headaches that inevitably are greater with that.
And to your point, even if a “city” welder, I’ve employed rig welders all over the place and again, inevitably if a guy is actually hustling work, hard, it ain’t all gonna be on pavement. Unless maybe he carry’s 1000’ of welding lead.
But even if so, at the end of the day, he’s pressing a now antique truck or marginal capability for the use, as designed, into daily duty, expecting that old truck to outkick its coverage every day.
Possible? Sure. Doable, yup with some considerations. But time is money and stuck or broke down somewhere in Lost my Cattle, Texas don’t pay the bills or get you home for supper.

 

[Grit dog]

And again, I’m sure SRO will probably chime in again. And using him and his truck as a comparable example is not really that accurate.
1. He’s a master mechanic and can repair/maintain anything on that truck in his sleep. And he appears to take VERY good care of it.
2. It’s 90% a dd and infrequently hauls and tows. Yes it’s for his business but it’s a side gig. He’s not rolling 4 barrels of feed in the truckbed and 6 on the trailer, over the Pass daily. Or even weekly or even monthly. Big difference there.
3. He is very well aware of what to do and what not to do as to not break his truck. Not everyone is that way. (Don’t read into that @Velder, just providing context)

 

[Grit dog]

And lastly, if a guy is a “professional”, as trite as it sounds, “looking the part” is part of the equation.
Let’s face it most everyone judges a book by its cover at some point or other. And some more than others. And as someone who has employed maybe 1000 different subcontractors it’s generally easy to spot the guy who appears slobby and unmotivated and shows up running “junk”. Just as easy as it is to spot the other type of “Joe” who has his bro-dozered out $100k rig when he really only needs a half ton work truck.
Both have their issues more than the guy who either appears to be very well put together and has clean newer equipment or the guy who appears to be frugal and has clean not so new (but not antique) gear.
There’s a balance there that one should be trying to hit as a businessman.

 

[Velder]

@Rusty Nail Thanks for the elaboration, that's a really good take on 4wd vs 2wd. I am a rural welder, would far rather be way off pavement in the middle of nowhere, than in downtown Houston any day. So yes your right, I really need to get 4wd.
I think you can rest easy about the engine swap, when the guys on here started going through the ins and outs of it, I realized I don't have the knowledge or the time to tackle that. So for now I am looking for a affordable diesel for my welding rig, and I will just doll up the C10 for a run around truck.
@Grit dog Thanks for all your input, I really do appreciate it. I know what you mean that some guys ask for help, but doesn't really want it. But be assured when I ask, I really do value the advice of guys older than myself with a whole lot more experience. And I really like your take on the professional appearance aspect. That is something that I would like to get right.

 

[JBswth]

I run 4.56 in my '82 Scrambler and '93 F700. Both have 35 inch tires though.

That changes

Hey @Rusty Nail, what do mean Dude has practically guaranteed himself to be stuck in the snow or mud?
would really appreciate your take on doing an engine swap, though I think I have already been talked out of it.
@JBswth thanks for the input, that is probably just what I am going to do, what gear ratio would you recommend?

I would use 4:10 with a 700R4, or even 4:56. If you change to a Turbo 350, then, you probably don't want to go any deeper than 4:10, or else it will be screaming at 60MPH. If you have a stick, you probably want a 4:56, especially if you have Overdrive.

Mike

 

[SirRobyn0]

And again, I’m sure SRO will probably chime in again. And using him and his truck as a comparable example is not really that accurate.
1. He’s a master mechanic and can repair/maintain anything on that truck in his sleep. And he appears to take VERY good care of it.
2. It’s 90% a dd and infrequently hauls and tows. Yes it’s for his business but it’s a side gig. He’s not rolling 4 barrels of feed in the truckbed and 6 on the trailer, over the Pass daily. Or even weekly or even monthly. Big difference there.
3. He is very well aware of what to do and what not to do as to not break his truck. Not everyone is that way. (Don’t read into that @Velder, just providing context)

And lastly, if a guy is a “professional”, as trite as it sounds, “looking the part” is part of the equation.
Let’s face it most everyone judges a book by its cover at some point or other. And some more than others. And as someone who has employed maybe 1000 different subcontractors it’s generally easy to spot the guy who appears slobby and unmotivated and shows up running “junk”. Just as easy as it is to spot the other type of “Joe” who has his bro-dozered out $100k rig when he really only needs a half ton work truck.
Both have their issues more than the guy who either appears to be very well put together and has clean newer equipment or the guy who appears to be frugal and has clean not so new (but not antique) gear.
There’s a balance there that one should be trying to hit as a businessman.

@Velder Yea, all of this is true about me. I will point out that my truck is a C20 not a 10 and there are differences and I would not feel or be as comfortable doing what I do with the 20.

Yes, I DD it to the shop. I think Grit is understating the amount I haul with it, but I'd probably guess 75% of the time I'm running empty or lightly loaded. 25% of the time I've got some sort of load of some substantial weight. I probably haul feed barrels about every 4 weeks to 6 weeks and it's generally around 100 miles loaded but it depends on who I'm getting from the exact mileage or if the pass is involved. Now sometimes that's 4 barrels in the bed sometimes it's 6 and once in a while it involves the trailer. I suppose I probably pull the trailer on average of once every 2 months. The trailer is weight rated for 7K I don't go over 6K but loads vary....

Yes I take very good care of the truck, I maintain it to be reliable on these trips and to handle loads without a hiccup. I'm very well versed in towing and hauling with older vehicles and quite frankly I really enjoy the heck out of it. I would use a square for daily hauling, and towing, but then I'd have to have a C/K30 with 4.10 or 4.56's not just want them like I do now. I'd also want the SM465 BC those trannys can take whatever you throw at them maybe the TH400. And preferable a 454, and oh, ya a full floating rear end. It would probably work in say a firewood selling business, or a tree trimming business, or lawn maintenance, but would probably be not work out as well on anything remotely over the road where time is money and were rolling along at 55 with a thirsty gas engine. Yes I would love the rummble of the engine going 55 turning 3K, but would it be cost effective?

That's all me and my take. Is that a good program for just anyone? Probably not.

 

[SirRobyn0]

@Velder Also keep in mind whatever the trucks legal GWVR is will not change after you make changes to it. If it's only got (for example) 2K left for towing and your running around with a 5K trailer you could get slapped with some serious issues if you were ever involved in a accident your fault or not. So another something to think about.

 

[Velder]

@SirRobyn0 Thank you so much for all the input, I really appreciate it! I wish I lived close enough to just bring my truck to your shop to have any work done.
That last point about the GVWR is something I really haven't thought about, I would have been quite shocked.
Like I said before, think I'll just leave the the truck for what it is designed for, I can't remember if you already gave your opinion on this, but would you advise trying to rebuild the Quadrajet carb. or switching to a Holly?

 

[SirRobyn0]

@SirRobyn0 Thank you so much for all the input, I really appreciate it! I wish I lived close enough to just bring my truck to your shop to have any work done.
That last point about the GVWR is something I really haven't thought about, I would have been quite shocked.
Like I said before, think I'll just leave the the truck for what it is designed for, I can't remember if you already gave your opinion on this, but would you advise trying to rebuild the Quadrajet carb. or switching to a Holly?

I would never advise switching to a Holley (or only very rarely) certainly not on a stock engine. You'll get a lot of opinions if you did a thread dedicated to that question, but the bottom line is that any flavor (within reason) of carburetor can be step and made to run your engine. However GM picked the quadrajet, they were designed and tuned to run the engines they were installed on. You'll be hard pressed to get any other carburetor to perform as well. With that said the Q-jet is one of the most complicated 4 barrels to rebuild. Guys will sometimes switch to Edelbrocks because they cost less to purchase new and are one of the easiest to adjust and tune, which is good because they need much more tuning and routine work than the Q-jet. Edlebrocks are sensitive to fuel pressure, and prone to fuel percolation. Holley's biggest downfalls in my opinion are the side hung float bowls are prone leaking sometimes the metering blocks will warp and leak, but they are very custom tunable. Q-jets in old age are prone to leaking from the welch plugs in the bottom of the carb, which will cause dripping fuel into the manifold. Most guys when we rebuild them cover those plugs over with epoxy to ensure they seal, that is not the best solution but removing the plugs and staking in new ones is not an option for the vast majority of us.

In my opinion on your truck the best option would be to rebuild your existing Q-jet.

Please do not purchase a remanufactured unit at an auto parts store. While that may seem like the easy button, they are notoriously not setup correctly or are remanufactured to be much more universal than they were intended.

Hopefully that's helpful.

 

[Rusty Nail]

Wooo! Nine pages. That was a great thread, congratulations!