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6.5L Diesel Engine Discuss the 6.5 GM diesel engine & associated components. Automatic transmission questions & problems belong in the 4L80/85 - 4L60E - 6L90 Transmission Forum


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Old 07-26-2019, 03:13 PM   #1 (permalink)
JWJ2017
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6.5 fuel economy?

Hey all,


I've searched this forum and seen some discussion, but I'm looking for help w my fuel economy.


Right now its 13+MPG in town, and 15+MPG hwy. Its dipped as low as 12 in town.


I use odometer reading and track fill-ups to same level in fill tube every tank.


The truck used to have a Heath PCM. I removed it and put in a stock PCM. (I thought it might be behind low MPG)



I run 235 85R16 tires, instead of stock 245 75R16. I think this accounts for a 1.04 correction to odometer miles, which doesnt change MPG much.


I bought it w straight thru exhaust, but had a local shop put on muffler, not brand name or anything.



steps I've taken (w no change):
* new PMD
* reinstall vac pump and vac turbo control (solenoid and wastegate)
* swap out 4.10 gears with 3.10


The only things left I can think of are new injectors? Could it be a transmission issue?



Can I expect to get more than 13-14 in town w K2500 Suburban? Folks list some pretty high MPG in their sigs, but I'm not sure how realistic that is...


Any ideas or feedback would help. Thx!

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1997 K2500 Suburban, 6.5L VIN F
4L80E, 208K miles, pretty much stock
(prev owner did a lot, I removed aftermarket intercooler, reinstalled vacuum pump w/wastegate control etc after I bought it)
relocated PMD w/resistor, mounted behind grille
Webasto diesel-fired coolant pre-heater
EGT, boost and trans temp gauges
3.73 gears (was 4.10)
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Old 07-26-2019, 04:35 PM   #2 (permalink)
K2500 Burb
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I have a 96 K2500 Suburban and I average right around 10-12 mpg city/hwy with 4.10 gears, 35x12.5 tires and a DE exhaust. Also, have a S&B cold air intake and dual e-fans from an LS.

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Old 07-26-2019, 05:00 PM   #3 (permalink)
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In your signature it states you remover the intercooler... why...


never seen 3:10 gears... is your converter locking?
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Old 07-26-2019, 05:02 PM   #4 (permalink)
sctrailrider
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Not a Burb but my K3500 dually gets 17 hwy with 4:10's & 245/75/16's...
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Old 07-26-2019, 07:29 PM   #5 (permalink)
57diesel
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3.10 ? your sig says 3.73. Madison is big enough to have some traffic which is never good for MPG. What sort of highway driving do you do? 70 down to Janesville? Or 80? Do you use the cruise? Are you correcting for the ratio gear ratio change too?


I had a stock 96 with 3.42 and it would hit 20 at times. My 3.73 and 4.10 don't do that.
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98 Burb K25 4.10's 250K miles, new to me, pretty clean and stock.
95 Burb K25 3.73's CRASHED 250K miles, looks to have been rebuilt at least once, stock, bad rust, DE exhaust w/walker muffler, 8' western plow.
81 Buick Riviera GAVE AWAY with the infamous Olds 5.7 diesel possibly still going strong.
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96 Burb K25 3.42's SOLD at 340K miles, told it was all original, close race between engine wearing out or body completely rust out.
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Old 07-27-2019, 09:07 AM   #6 (permalink)
56pan
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Quote:
Originally Posted by 57diesel View Post
(snipped) "Are you correcting for the ratio gear ratio change too?" (snipped)

My '93 has the VSSB, (vehicle speed sensor buffer), and behind the glove compartment. I had to reprogram it with the jumper connectors when I changed tire sizes to get the speedometer/odometer correct. Don't know about a '97, but probably has something similar? And why did you remove the intercooler?
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Old 07-27-2019, 12:14 PM   #7 (permalink)
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Needs injectors.
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Old 07-27-2019, 04:05 PM   #8 (permalink)
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I got 20 in mine frequently a few years ago.

Roof Rack - Removed;
Air Dam Modification Added (larger than stock
Shell T6 5w40 Oil Was Used;
Tires Were Stock & set to 80 Cold;
Cut a hole in the K&N Air Box for a cold air intake;
Used a Scangauge II for Current Fuel Economy Readouts (the calibration was about -70%.)
Never filled the tank all the way (judged by the needle. 15 Gallons seemed to last 300 miles easily.)
Differential Gearing: Unknown but I highly doubt it's a set of 4.10s. I was able to get to 70 MPH easily enough.

I'm sure with the right gearing and mods and conditions, a 6.5 could get 30 MPG HWY. I seemed to be able to get at least 25 HWY (never tested that from a tank to tank read out, the gauge just seemed to go that high) and forget that it's a 3 ton brick; it somehow just worked.

I will note that the few times I did fill the tank up all the way, the car felt like it was lugging and using more fuel (makes sense; holding an extra 20 gallons = about 139 extra pounds and more weight on the rear tires.) Then again, maybe it was just the kind of fuel I bought. No idea.
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Old 07-27-2019, 05:06 PM   #9 (permalink)
Tommygunner
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Agree with Diesel pro about injectors. If they are stock OEM with over 200K miles. They should have been replaced miles ago. New injectors should help.

On truck in sig I get an honest 22 highway at 73mph 2200RPM. 16 mpg town . 15-16 highway mpg with Lance 8.5 foot SC Camper in back. All Checked with GPS. My tune is a combo of performance /economy. It is not a towing tune. Computer control Vacuum wastegate.
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Old 07-27-2019, 05:12 PM   #10 (permalink)
JWJ2017
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Hey thx all!



Yeah my bad, my sig is correct just a typo on 3.10 gears. I put 3.73's in. Just put in used ring and pinion in rear assembly and swapped in used front diff axle carrier. I did verify and count teeth on rear gears.



Honestly not sure if I have to do anything else to correct for new gear ratios. I'll investigate reprogramming the VSSB. Also for different tire sizes. I bought it with the current tire size (non-stock).


The converter is locking up, yes...but tbh it shifts a little softer since I re-geared it. I was wondering if the converter was going. I did a rough test on a hill in drive and the truck still creeps slowly uphill at idle, so I'm hoping it's ok. I should get a scanner on it and see how lockup is working.



the intercooler was on it when I bought it and was a cheap aftermarket job that was kind of cobbled together. it was prone to popping clamps at joints and was in the way of a lot of stuff. I guess I wanted to bring it close to stock and see if I could get MPG up starting from there. I mean w the 6.5 it seemed like the intercooler wasn't gaining me a lot. Thoughts?



I'm tempted to agree w you DieselPro that injectors should be next step.


I'm a little worried that even w 3.73's it wont be enough to get up to the legendary 20MPG.


I also have a big 42 gal tank and fill all the way.



As far as driving habits, I commute about 20 mi to near Sun Prairie and mostly hwy. But yeah it is a little trafficky, so it's not great. On long trips to MN, it tops out at about 16MPG, and I'd just expect better cause I use cruise etc.


I apreciate the comments!

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1997 K2500 Suburban, 6.5L VIN F
4L80E, 208K miles, pretty much stock
(prev owner did a lot, I removed aftermarket intercooler, reinstalled vacuum pump w/wastegate control etc after I bought it)
relocated PMD w/resistor, mounted behind grille
Webasto diesel-fired coolant pre-heater
EGT, boost and trans temp gauges
3.73 gears (was 4.10)
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