Towing and sucking gas like crazy.

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Edmduncan

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ideas from the group.
Truck= 1991 C2500 Silverado. Trailer package, 5.7 350 tbi with tbi riser plate and 4l80e trans.
120 Ltr tank.
Pulling 5200lbs travel trailer and about 1000# in the box of gear.
I only pull in drive, I dont tow in od.
Engine is strong, transmission has cooler and shift kit and is prestine when serviced by transmission shop.

My problem is this, decent gas millage when not pulling, about 6-700km highway.
But pulling at a steady 100-105km/h my gas millage is worse than bad.
I can get about 180-200km on the top 1/4 of my tank from full to 3/4 mark when not pulling.
But I used 3/4 of my tank when pulling 176km to our campsite. Ouch!!!!
Now my other issue is my trany isn't slipping but its boiling trany fluid out the breather, everything I tow. Advise my trany guy said to try a bigger cooler for the first attempt to address the fluid spew.
I just put a tack on to make absolute sure my trany isn't slipping.

What are you thoughts from the group?
 

SUBURBAN5

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Here's my input. 1 make sure you do a tune up if it needs it. Plugs,wires, cap,rotor, fuel filter,air filter, oil, ect. Also maybe rebuild your tbi. 2 put a bigger trans cooler if your always towing, you having a shift kit I believe holds the gears longer which means your burning gas more. Also you having tbi wont be as efficient as the vortecs fyi. 3 depending on your rear end that 350 gonna be working hard to pull. She's gonna be thirsty all the time. However I understand you dont wanna swap it for a different motor. What kind of milage are you trying to get?
 

SUBURBAN5

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It's not "apples to apples" comparison but my suburban does 480-640km on half a tank empty unloaded. That's about 83ltr. I average 24-28 k/pg total depending on city or highway. That's a vortec 5.7 with a 342 rear end. Put a trailer on it and tow it in 3rd all that goes down the drain maybe 16 -20k/pg. Never really did the math but she gets thirsty trust me. And filling her up is 166ltr. Its just for piece of mind like I said not a good example mine is a c1500.
Maybe somebody with the same/similar set up will chime in.
 
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RichLo

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With that much weight are you towing in OD or keeping it in 3rd? Keeping it in 3rd keeps the RPMs up but is less stress on both the engine and trans.
 

thegawd

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Are you absolutely sure the tranny cooler is hooked up? Just like the 7 way plug, the hd tow package comes with the proper parts BUT. they are not hooked up. They need to be installed. The oem tranny cooler on my burb was finally hooked up say 5 years ago when I had my tranny rebuilt.

Al
 

stutaeng

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Ok, just a quick calc for us 'Muricans:

Unloaded:
(180km x 1.0 mile/1.6 km) divided by (0.25tank x 34 gallon) = 13.2 MPG

Loaded:

(176km x 1.0 mile/1.6 km) divided by (0.75tank x 34 gallon) = 4.31 MPG!

You said 120 Liters, that's about 32 gallons. I used 34 gallon tank, which is what literature says, but relatively speaking the numbers are off the charts. That sounds REALLY bad, LOL.

OP mentioned '91, that's the first year of the 4L80. He additionally mentioned he tows in "drive," not "overdrive." I'm not sure what that means. I think that's on "3?"

Are there a lot of mountains there? Altitude? Have you had this truck for a while? Or just bought it? Has it always had poor fuel range while towing? Unfortunately, I don't have much experience with TBIs, much less towing. Perhaps someone has a similar set-up towing similar weight in the similar road conditions. I've read upper 8MPG towing on Vortec, just have no idea on TBIs. We really need to know road conditions.
 
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stutaeng

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Later gens PRND321. early gens PRNDd21. D is overdrive smaller d is 3rd gear for us I believe

Ok, sorry. It seems you had already mentioned it.

Now then, if it is flat terrain, then it's perfectly fine to tow in "Overdrive" (4th gear), right? I think that's what my 2000 C/K 3500 Owner's manual states. Not sure if that's always been the case on earlier 4L80s? If there's mountain passes, I guess one would tow in "Drive" (3rd gear) just to keep transmission from hunting for gear constantly? It seems that's what the ECU does for you on the newer vehicles with the Tow/Haul mode? It holds the gear in much higher RPM before shifting.

Maybe that's the problem? OP is always in 3rd without the need?
 

SUBURBAN5

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I always thought 3rd was the power gear and 4th or overdrive was cruising mpg. Never had a 4l80 but for the 4l60e thats what it says in the manuel. I think the fact hes sporting a tbi it's not as efficient on burning the fuel. I could be wrong but on top of that he has a shift kit. Doesn't it stay in gear longer??
 
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