5.7 vortec mpg

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CRF450R

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A tune will help IMMENSELY. I have a blackbear tune on my '98 K3500 5.7 CCLB SRW on 33s and got my truck calibrated for premium octane fuel, the larger tires, aftermarket intake and exhaust, cat delete, and rear 02 delete. I can see this 7000 pound truck get 1-2 mpg better now than it did before all the mods. Even with stock tires I only got maybe 12 mpg on the highway. Now, i can get 13.4ish and I've seen as high as 16.8 mpg on flat roads in western Colorado (mile high elevation helped).
I will not have a tune done to my truck! Premium fuel is hard to get in my area within driving 45 minutes to the nearest gas station that has premium.Im doing the mpfi swap over the holidays hopefully that'll do something this thing feels like a 6.2 gutless diesel with 454 mpg. Once the trucks painted it'll basically be parked as I'm looking into a new / new used daily driver a 14+ Tundra 5.7. with the Iforce I can expect the same mpg but least have some balls under the hood. But I'll always keep my 98 Z71 it'll be a nice looking truck once it's painted I love my 98 but not happy with the 5.7 vortec under the hood my 94 Silverado 5.7 TBI would walk all over my 5.7 vortec in every possible way then some. This seems odd to me so I'm trying to get the 5.7 vortec to were it should be something must be causing this vortec to be so gutless and hard on fuel. My buddy Kevin has the exact same truck just different colour and it just eats my truck alive.
 

RawbDidIt

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My truck has the MPFI upgrade, it's pretty gutless under 2000 rpms or so, gets 11-12 mpg (75% city, 25% highway). I wouldn't put my eggs in that basket. The fuel pressure regulator is super easy to replace, but my understanding is that the test to make sure it's operating correctly is to take the pressure off the fuel rail, if it's within spec and holds pressure over 60 psig for 2 seconds (IIRC) with the pump off, that shouldn't be your problem either. That same fuel system has plenty of headroom to run a 383ci engine, so while it can certainly be improved, I don't think improving it will make a marked difference for you. You mentioned making some pretty good modifications to your exhaust, but have you made any improvements to your intake? My next modification is the 454 throttle body mod posted above (thanks for the suggestion, didn't know it was an option until reading through this thread). I'd look into that first. If you're getting poor fuel economy your fuel system is definitely putting out enough fuel, I'd spend my time trying to improve the ability to pull in air, rather than trying to push more fuel into it. May be worth pulling the intake completely and giving it a thorough cleaning, I may do the same, but I'll likely wait until my 383 is built and do it at the same time. You can spend your money how you want, but if you have exhaust and fuel abs you're not getting power, you need air.

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CRF450R

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My truck has the MPFI upgrade, it's pretty gutless under 2000 rpms or so, gets 11-12 mpg (75% city, 25% highway). I wouldn't put my eggs in that basket. The fuel pressure regulator is super easy to replace, but my understanding is that the test to make sure it's operating correctly is to take the pressure off the fuel rail, if it's within spec and holds pressure over 60 psig for 2 seconds (IIRC) with the pump off, that shouldn't be your problem either. That same fuel system has plenty of headroom to run a 383ci engine, so while it can certainly be improved, I don't think improving it will make a marked difference for you. You mentioned making some pretty good modifications to your exhaust, but have you made any improvements to your intake? My next modification is the 454 throttle body mod posted above (thanks for the suggestion, didn't know it was an option until reading through this thread). I'd look into that first. If you're getting poor fuel economy your fuel system is definitely putting out enough fuel, I'd spend my time trying to improve the ability to pull in air, rather than trying to push more fuel into it. May be worth pulling the intake completely and giving it a thorough cleaning, I may do the same, but I'll likely wait until my 383 is built and do it at the same time. You can spend your money how you want, but if you have exhaust and fuel abs you're not getting power, you need air.

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Dam well I'm doing the mpfi swap anyway as it's hard to no start when it's cool/cold outside and yes did the temperature sensor already. I don't believe in these stupid so called cold air intakes when they should really be called hot air intake. If the mpfi and new fuel regulator doesn't fix the problem that'll be were I stop. I'll paint the truck and it park it (nice weather only truck) and buy my Tundra as my new daily driver.
 

evilunclegrimace

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My '98 K 1500 Suburban 5.7 just pulled 15.8 mpg on a trip from Knoxville to Erie fully loaded w 2 adults,4 mounted wheels and tires, a dog, my spare tools and all of our luggage running 75 MPH through the hills on I-75. It currently has 254 thousand plus miles it.
 

RawbDidIt

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Dam well I'm doing the mpfi swap anyway as it's hard to no start when it's cool/cold outside and yes did the temperature sensor already. I don't believe in these stupid so called cold air intakes when they should really be called hot air intake. If the mpfi and new fuel regulator doesn't fix the problem that'll be were I stop. I'll paint the truck and it park it (nice weather only truck) and buy my Tundra as my new daily driver.
Cold air intake would make trivial difference at most. It pulls air from the same place so the only thing it would do is reduce restriction due to rough edges on the intake tube. The throttle body is a bit of a bottleneck. It may also be worth looking into a new intake manifold. I'll be putting a new intake manifold and headers on mine with the 383. Current engine has a bit of piston slap during warm up, so I'm not adding a ton of power to this block.

MPFI may help with the cold start issue you have, it doesn't get much colder than 20* here in Dallas and even then it's rare, so not much experience with real cold weather here, but it always starts up within a second of engaging the starter.

I do have a slight issue with idle that just started, wonder if anybody could weigh in on it. Occasionally after startup the idle will cycle between 500 and 800rpm. It's intermittent, but I've tested vacuum and fuel pressure all good. I did recently replace my exhaust, manifold back (new flowmaster cats, magnaflow side exit cat-back). New O2 sensors as well. Reset all bolts to spec after having it installed for a month. Does a new set of cats and/or O2 sensors require the ECM to be reset?

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RawbDidIt

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My '98 K 1500 Suburban 5.7 just pulled 15.8 mpg on a trip from Knoxville to Erie fully loaded w 2 adults,4 mounted wheels and tires, a dog, my spare tools and all of our luggage running 75 MPH through the hills on I-75. It currently has 254 thousand plus miles it.
What's your gearing? Mine runs 3.73 gears if I remember correctly.

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