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  1. #1
    Join Date
    Jun 2008
    Up North, in Canada !

    Question 6.5L diesel engine

    Hi there, I'm looking to buy a Suburban in the near future... I found a nice one... 1999 with the 6.5L diesel engine. I was just wondering what king of fuel consumption can I expect from this truck when I drive it on the highway, without towing anything ?

  2. #2

    Join Date
    Feb 2008
    Red Wing, Minnesota, United States


    Not sure about mileage but not a great engine.
    99 K1500 Suburban LT "THE BEAST"
    Hypertech III, K&N, true dual

  3. #3
    Sr. Mechanic John W's Avatar
    Join Date
    Sep 2007
    St Peters, Nova Scotia, Canada


    I would leave the 6.5 right where it is.
    2000 4x4 Ext. cab 5.3 Z71
    2009 Corolla

  4. #4


    the 6.5 diesel were not a good engine as they said and the fuel mileage is still to gonna be very good.
    95 GMC Sierra 2500
    3" Body Lift, 33" Goodyear MTR's
    5.7L 350 TBI Edelbrock SDT Exhaust, Flowtech Headers, K&N Intake
    Sold :(
    1999 Jeep Cherokee, 3" Rustys lift, 33" BFG MT's, locked, armored: Rolled :(
    2000 Jeep Grand Cherokee Limited. K&N Intake, Custom Kolak Exhaust

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  5. #5


    The 6.5 isnt as bad as everyone makes it out to be.
    When properly tuned with good maintenance it can go over 250K before needing a rebuild. Mileage in the neighborhood of low 20's is possible unloaded, loaded mileage runs in the high teens. The 6.5 isnt a torque beast like the newer Diesel engines but it can hold its own against similar sized gasoline engines while towing and still return better MPG and longer time between rebuilds.

    With a few tweaks it can do much better.
    The Fuel control can be adjusted to improve HP, but at a loss to MPG, and I wouldnt attempt this unless I installed a Pyrometer first, tail pipe temps can get high enough to melt the manifolds if you dump to much fuel in.
    Add a Gale Banks turbo kit and it'll run with a stock newer Diesel engine.
    A Banks exhaust will improve mpg and performance even more.
    And the addition of a Propane injector makes it run like a whole new truck giving it more power and better mpg.
    I've had a couple trucks running the 6.5 and I wouldnt hesitate to buy another, I would beef it up as time and money permitted, first I'd put in a Pyrometer, then a Propane kit, Exhaust, and finally do the turbo.

  6. #6
    Join Date
    Jun 2008
    Up North, in Canada !


    thanks everyone for your comments

  7. #7


    I have a 98 6.5 4door long bed dually. I toped my tank of at 280miles and spent $80.00 so not to bad
    2000 Ls s10 V8 extended cab drag truck
    1999 s10 Ls Droped 2.2L
    1998 s10 blazer red stock

    1985 s10 blazer 5 speed v8
    1982 4X4 355ci 450hp k1500 trail truck
    1978 4x4 short bed F150 460ci 540hp mud truck on 38.5 super swampers boggers

  8. #8


    With the right set up you can look at decent fuel mileage. I had one built by a local diesel shop and it dynos at 450hp i still get 24mpg on the highway. This motor isn't great stock but with a little work they can be a great motor and very reliable. I have nothing bad to say about them.
    2000 Chevy Silverado 4WD Classic
    2'' lift Torsion Keys and air shocks
    33/12.50 Hankook Dynapro M/T
    3 1/2'' down pipe back 6" Tip
    Dodge Towing Mirrors
    Smoked Tails, Cab Lights, and Side Marker Lights
    1% Rear and Side Window, 20% Front Tint
    American Racing Wheels/ With a custom black paint
    Cold Air Intake
    Edge Performance Chip
    Custom Audio System
    LED Tails
    Black background headlights and parking lights

  9. #9


    There's nothing wrong with the 6.5 other than most people either didn't maintain the ones they had, or the people they took them to for repairs butchered the snot out of them, therefore they thought they were bad engines. There's a few things GM could have improved upon before production, but those were most addressed throughout the years, like the cooling systems.

    There are plenty of 6.5's running out there with over 500k which have not had any issues or needed to be taken apart. Mostly this is because the owners maintained them!

    Most 6.5's I've come across get mid-teens for mileage thought some have been low 20's and some as low 11-12.

    If they have a complete history of the vehicles maintenance and ALL the items GM requires as maintenance has been near flawlessly done --oil changes, injectors, harmonic balancers, and cooling system service, then it may well be worth looking into.
    Have You Smoked A Hemi Lately? Why, Yes I Have!
    2008 Sierra RC/SB, Iron Block 5.3L(LMG)
    , 3.42, Locking Diff, HD Cooling, HD Suspension, Towing Package.
    Raptor Stainless Steel Step Bars, Folding Tonneau Cover, DiabloSport Predator

  10. #10


    The 99 truck should be turbo equiped from factory. There are common issues that crop up from time to time, but the same can be said for any engine. A few simple checks will tell you if the truck you are looking at is worth buying, or running away from. As stated above, maintenance is key with these trucks. Also a basic understanding of what they are, and are not, will help you get the most out of your purchase. The original 6.2/6.5 diesel engines in GM vehicles are made by Detroit Diesel, and are an all new design for a light duty diesel engine, with fuel economy, emmissions reduction, and low maintenance as the design priority. The Ford, and Chrysler diesel engines by comparison, are medium duty truck engines modified to fit in light duty trucks. Therefore comparison is unfair. Also GM miffed the production by implementing cost reduction moves that further reduced the engines potential. However, most of the GM diesels still running today have had the early issues fixed, unless they are extremely low mile units. Now then, an easy way to spot a keeper, watch it start cold, pull the oil fill cap and the radiator cap or the overflow cap, and look for lots of blue smoke out the tailpipe or the oil full tube, and air bubbles from the radiator or overflow. Blue smoke out the back or out the fill tube means worn rings. Air bubbles in the cooling system mean combustion gasses in the coolant, IE a head gasket or other internal cooling system leak. If you see a little smoke out the back, that's normal, but if it clouds your view, or likewise at the engine, run away. Drive the truck, and listen for any issues, and try a hot restart to see if it fails. Look under the truck for oil leaks near the front on the drivers side. Let us know what you find, and good luck.
    Brian L.

    Brian Larson CST. KC9DAK.
    Janesville WI.
    87 GMC G20 6.2 diesel.

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