6.5 liter turbo diesel

When buying a one-ton truck the thing that saps your mileage is that they generally have a very high numerical rear end, usually 4.10:1 - great for moving heavy loads but bad for highway mileage. If you are looking for a larger capacity truck that will only occasionally haul heavy loads and will get its greatest use commuting, then I suggest you look into a 3/4-ton truck with a 3.73:1 rear axle.

As for the 6.5L turbo diesel compared to the 350 gasser, the cost of maintenance is about the same, but the 6.5 has two issues that you should investigate before you buy a vehicle with that motor. The first is in '97 or '98 GM moved to a higher capacity water pump with dual thermostats and a crossover. The earlier versions had a problem with burn-through of the head gasket in the rear cylinders when the motor was used for heavy hauling or towing and the new pump and thermostat set-up all but cured this issue. If you are getting an earlier ('94-'97) truck with this motor, see if the water pump and thermostats were upgraded to the later design.

The second issue is the Fuel Solenoid Driver (FSD) mounted on the front of the fuel injection pump. This little black box generates a lot of heat which (in theory) is supposed to transfer the heat to the fuel pump body and be cooled by the fuel flow. Unfortunately, this may have worked well on the test bench, but when in a truck's engine bay with little or no airflow around the FI pump, the FSD has a nasty habit of overheating and frying. If you don't catch it quickly enough, the heat will also fry the optical sensor inside the FI pump and then the entire pump will need to be replaced at a cost of $1800.00 or more. Plus the added heat from the FSD also thins out the diesel fuel and that's not good for pump lubrication.

The cure for this is an FSD Cooler (you can search for this on the net and get a better explanation that I can provide) which is nothing more than a finned heat sink that the FSD is mounted on away from the FI pump. Any heat the FSD generates is bled off into the atmosphere instead of the pump body and everything works fine. I suggest getting a new FSD at the same time as the cooler instead of trying to take the old one off the pump - which is a tremendous PITA because you have to remove the intake manifold and all the plumbing to even get at it. You can even buy an extended harness to move the FSD even further away from the motor for better cooling, but I used the recommended mounting position and no extension with no problems and my '95

6.5L gave me great service.

Cheers - Jonathan

Why not just buy a quarter ton chevy or somthing, still get the 350 and have better highway milage

I would, however I have dump bed that i will be putting on it and the frame width on the one ton is the correct size. I think the frame on the 3/4 ton is wider by several inches.

thanks for the feedback

From personal experiences (after owning gassers for over three decades), I would tend to disagree with diesels being too high maintenance, that is, compared to gassers: no spark plugs, no plug wires, no distributor, no coil/ignition module, no O2 sensor, no lean/rich adjustments, no vapor lock, etc.

Just like most gassers, change the oil/oil filter (religiously), air and fuel filters as needed, and follow manufacturer's recommended maintenance schedule.

Adding to Jonathan's list: the oil lines to/from the oil cooler, and the heater hose from water pump/thermostat housing.

Regards, Franko

Hi maint. ????? !!!!

I have 252,000 on my cummins & it's never been looked at...just filters & oil. My friend had a 6.5..it was no power house for sure, but it ran great & got the best mileage I've ever heard of(32+).

Diesels are great,but the hi price of the engine and hi fuel prices force a person to keep the vehicle long enough to re coop the $$....as I have-15 yrs.

I disagree, especially since we are talking about diesels here. Keeping the RPM's in the "sweet zone" for a diesel means keeping them low where the engine is most efficient, which means a 3.73 will be just fine for the application the original poster mentioned (mostly commuting, occasional hauling). The most efficient RPM for a gasser is much higher so it will not suffer as much from a higher ratio rear end. My 3/4-ton Chevy is rated at

15K+ lbs on a fifth wheel hitch with a 3.73 rear end, but still gets 21+mpg empty on the highway. If I had 4.10's in the rear I might gain 1000+ lbs towing a fiver (which I don't need), but running empty on the highway at the same speed would keep my RPM's higher than my motor likes to cruise at and would sap a few MPG (which I don't need either, especially at $2.30/gal for diesel). Of course I could run slower, but the speed limit is still 70mph regardless. When commuting with a larger truck, you're not looking to launch like a Corvette from every streetlight, so a 3.73 rear would be just fine and help bump up the mileage around town with the diesel - I get just over 18 when I commute with mine. My old 6.5L diesel with 3.73's got 17 commuting and 22+ highway empty religiously.

I don't know how heavy a load the OP will be hauling or how often, but since the 6.5L turbo diesel generally put out around 200HP and 400 ft-lbs of torque in it's final version, then it'll be up to him to decide if hauling is more important than commuting mileage.

Cheers - Jonathan

I tend to agree somewhat with this on a diesel engine but it depends on load too, If you plan to haul heavy with a 6.5 I would still go with a 4.10 because it would give you more towing power and you would still have about a 2000 RPM cruise at 60 mph or so in OD (better pulling power in OD) and the 6.5 with its much higher compression and prechambers has a wider useful RPM range and rev a bit freer than a Dmax or a Cummins. With those engine I would go with a 3.73 unless regularly towing 14k or more. If you plan to run oversize rubber too, go for deeper gear to offset power loss from bigger tires too.

Original poster stated he won't be pulling heavy loads, just carrying heavy loads on occasion. With that, the 3.73 should be fine. 63 mph/1800 rpm (max torque for 6.5TD) should give him the optimum mileage performance (22 mpg on our 4x4 2-Dr Tahoe).

Regards, Franko

I just bought a 94 GMC 1 ton crew 4x4 with the 6.5. Just sold the same exact truck but with a 350 gas motor. Mileage is much improved from the gas truck. Getting 19-21 mpg empty. The gasser got 12-14 mpg empty. The warranty on the inj. pumps is 11 years or 193,000 km. They had numerous troubles with inj. pumps and FSDs. Also some lift pump troubles from what I've heard. All that said, I much prefer the diesel over the gas for moving a heavy truck like a 1 ton crew.

Merc(converted Ford man)

94 GMC 1 ton crew 4wd

here is a link to the FSD cooler.

Here is the cooling system upgrade for older 6.5 diesels. Have not had any overheating issues with my 94 with the old cooling system though, even when towing 8000 lbs.

Merc

"gas mileage is better that the 454. I am under the impression that the

6.5 will get mileage in the low 20's and have a lot more muscle than the 350."

I wouldn't necessarily jump the gun on that.

My stealership owner/mechanic friend had (and quickly sold--in a good way) a K3500 dually XCab 6.5L/5-speed/manual t-case, 98 model.

He got 14mpg's no matter what with that truck.

He also had (end result as above, too) a 4x4 94 model 6.5L "heavy half-ton" truck (K1500) that got 18 tops cruising to and from Florida, doing 80, but not awesome as one thinks a diesel would do.

He currently drives for himself a 2000 F350 CC w/PSD/auto/2wd.

13mpg's. He does use jet fuel mix, but deinitely NOT 20+mpg's.

Don't fall for the semi-myth of VASTLY better mpg's from a diesel in such a big truck. My Stepdad's 3/4-ton 2000 Xcab/long bed Chevy with big block gets 14+ on the highway.

To boot, a 350 in that big of a truck will struggle a bit more compared to the 454....therefore any wished for mileage efficiency has gone "adios".

Actually, the mileage you quote on the vehicles you list isn't totally unexpected. The '98 K3500 extended cab dually is an extremely heavy truck and I'd wager that 14mpg with the 6.5L would be about 5mpg better than you get with an identical truck with the 454 gasser.

The same for the '94 K1500 (which has the earliest, least powerful version of the 6.5). Speaking from personal experience, if he had slowed down to

72mph on his trip to Florida he most probably would have gotten 21+mpg. At 80mph the 6.5L in that 4x4 was pretty far outside the sweet spot in the RPM range and anything will suck gas at 80mph with the aerodynamics that a pick-up has. He still did better than what the same truck would have done with either the 350 or 454 of that year regardless.

As for the Ford F350, this is a motor that is not known for it's fuel efficiency at all, and 13mpg is exactly what I would expect for that truck.

On the other hand, my current Silverado 2500HD 2wd crew cab with the 6.6L Duramax and Allison tranny with 3.73 rear end will get me over 21+mpg on the highway at 72mph empty, and 18.5mpg religiously in combined commuting-type driving. Not bad for a truck that weighs like a tank.

Cheers - Jonathan