LMTV or?...

[Aernan]

the block on a 5.9 is the same as a 3116 no sleeves I own one of those too they don't have the torque out of the box the 3116 has and juicing them up will also shorten the life of them I only got 500 k out of my 12 valve. the hydronic heater is a good idea and also something I'm looking at. The air in the fuel system can be cured with a good cat primary filter water separator and a cat primer pump. I haven't done mine yet because its still working good. A change from 15w40 to a 5w40 synthetic will improve your oiling performance when cold and is an approved lubricant for cooler temps I believe up to 80 degrees and down to -30. Taken care of it should be a million mile motor. The problems you hear of are usually the result of abuse or contaminated fuel these problems will also plague the cummins cornbinder and Detroit engines. It is a great cold weather engine when outfitted right just that the low pressure injection means it starts like an 855 cummins when cold and there are several cold weather starting aids used on them the military chose to use either injection its simple and works well maybe I will remember to take a video tomorrow when I start it up

I like the thinner oil idea.

Can you tell me more about this Cat primer pump and fuel water separator?
What are the symptoms of air in the fuel system?

 

[justin22885]

I am going to have to look up more about the fuel system and injectors of the 3116 as that seems like the root of 99% of its problems. If I understand this more or even find a solution around it then that is good, especially if I would have to remove a Cummins just to rebuild it as well. After I have my machine shop constructed and if faced with a rebuild scenario I could in all likeliness sleeve it myself, and it would appear accessing the engine to remove it is going to be a lot easier on the LMTV.

 

[big block 88]

When I look at the 5-ton what I see is a significant increase in weight, engine size, fuel consumption, etc over the other two options. Is it going to perform better off-road at 22,000lbs? In terms of LMTVs breaking down, what is generally the culprit behind this? Is it related to the reputation of poor reliability surrounding the 3116 engine that I would consider replacing anyway or is it something else?

Achiles heal for the lmtv is S&S had no business getting the contract in the first place. They were overwhelmed and built trucks with inferior quality and even worse electronics and driveline designs. Granted the trucks i was around were all 07-10 trucks, they were so bad, examples, no start, no power with good batteries and slave, no air building, driveshafts missing retainers, wouldnt go in gear any gear, cabs wouldnt tilt and i could most likely go on. These were on almost all new trucks. Even the mraps never had this kind of issues and they were rushed to theater.

The lmtv's were better at one thing than any other vehicle in the pool though!!! They held the parking lot down like it was made for it.

Yep 939's are heavy. But you will be VERY hard pressed to find anthing that a lmtv can go through that 939 wont go through. Head to head both will do better than the other at some for of off roading.

But im biased. I never experienced a good fmtv and never had a 939 let me down. They both have their quirks.

3116 is a solid motor in thousands of trucks and equipment. Pretty sure it cant be sleeved though....

 

[Aernan]

I figure any vehicle can be reliable as long as you find and fix issues before they become catastrophic. Given this is a commercial engine it has long service intervals and can be operated at limited capacity to limp home. For it to be reliable for me it's about learning how it fails and how to troubleshoot the symptoms. Given there is a great community around this truck and engine I feel confident in being able to keep it on the road.

 

[justin22885]

Wow, alright. When you guys said the 3116 can't be sleeved I was thinking really? you just bore out the cylinder, press a sleeve in and machine it down. I didn't know many diesel engines actually have sleeves that can pretty much be removed and reinstalled by hand as easily as changing a piston. I definitely want that feature in an engine for long term ease of maintenance. I wish my gasoline pickup truck could do that.

 

[Coffey1]

Lmtv or MTV way better cab dry warm way way better turning radius seems like my Ctis works way better. Spare tire way way easier to get down and change. I don't think I could go back to 939 series truck. Cab more comfortable and roomy.

 

[Aernan]

It's going to take a whole lot of driving to wear out this engine. If you get the 500k at 6.5 mpg average that's ~77k gallons of diesel. That's $269,500 in diesel at $3.50 a gallon. Replacement engines are some $3-$16k. It's a drop in the bucket for operating such a large vehicle. As far as total cost it would be much cheaper to buy a spare motor and stockpile it in the event you do wear yours out.

That said there are things on newer diesels that can make the motor longer lived and more reliable. Mostly involves replacing the old plumbing and adding additional filtration. Also methanol injection can help control EGT which will help your engine and turbo life a great deal. Factory tuned engines have no water injection so they are set conservative to not overheat. Despite that you can build a lot of heat climbing big hills even unloaded in hot weather.

 

[scottmandu]

the c-7's and 3126 have a hui pump that is more prone to failure.

Heui pump failure is mostly related to dirty oil. Caterpillar has a retrofit kit that installs a secondary filter between the engine oil supply and the heui pump

 

[justin22885]

It's going to take a whole lot of driving to wear out this engine. If you get the 500k at 6.5 mpg average that's ~77k gallons of diesel. That's $269,500 in diesel at $3.50 a gallon. Replacement engines are some $3-$16k. It's a drop in the bucket for operating such a large vehicle. As far as total cost it would be much cheaper to buy a spare motor and stockpile it in the event you do wear yours out.

That said there are things on newer diesels that can make the motor longer lived and more reliable. Mostly involves replacing the old plumbing and adding additional filtration. Also methanol injection can help control EGT which will help your engine and turbo life a great deal. Factory tuned engines have no water injection so they are set conservative to not overheat. Despite that you can build a lot of heat climbing big hills even unloaded in hot weather.

Well I am just over 30 years old and plan to use it for the rest of my life, so. I imagine I will be doing a whole lot of driving over the next 40-50 years. Also when I no longer need it for construction I may put a camper shell on the back and use it as an offroad RV to do some traveling.

 

[Awesomeness]

Justin,
I think that you're overthinking or over scrutinizing this. The FMTV is fine, and according to the military the most reliable vehicle they have ever had. The 3116 engine is fine, and used in hundreds of thousands (millions?) of dump trucks and other medium duty stuff (not to mention marine engines, generators, construction equipment, etc.). The 1st Sergeant at the depot down the street told me, "I'm glad you got an M1078A0 [with 3116]. I would keep it as mechanical as I could get.". The parts for the 3116, the transmission, and the axles are commercial off the shelf, and readily available. A whole replacement engine is like $3500... less than a rebuild for any engine, so who cares if it is resleevable or won't last more than 500k miles, or whatever?

I'm not sure I understand the argument that the older trucks are better. The military replaced those for a reason, and they are very happy with and proud of the FMTV. Even the "tried and true" argument seems to get deflated when we realize the FMTV is now 25 years old, a large portion of that at war. I'm not saying the old trucks are bad, they served their purpose for a long time, and then they got replaced. This just reminds me of arguments like "carburetors are better than fuel injection" or "revolvers are better than semi-autos" or "Windows XP is better than Windows 10" or whatever... everyone always dreaming of the past, resisting change, and sticking with what they are most comfortable with. You take a hit either way, either with older stuff that has been improved upon in newer things, or with newer things that have issues without improvements yet. For me, I see that as a wash, and will stick with the newer stuff.

 

[big block 88]

Yeah newer always better. Ask ford how dropping the 7.3 to go to the 6.0 and 6.4 worked out... perhaps the trucks are better now and we saw growing pains and the issues have been worked out now. A quick search of this site shows the gremlins in the wiring and drive line in those trucks.

But if your savy with electronics maybe you will be good to go. I will say the ride was very nice when i was being tow barred by a 925 to depot...

 

[justin22885]

Then maybe if I keep the 3116 I should keep an eye out for a low cost, low mileage replacement when I can find a good deal and then grease it up and pack it up for a backup in case I ever find myself needing to do a swap. If they are that cheap and I will be somewhat remote, a backup can't hurt.

 

[Awesomeness]

Yeah newer always better. Ask ford how dropping the 7.3 to go to the 6.0 and 6.4 worked out... perhaps the trucks are better now and we saw growing pains and the issues have been worked out now. A quick search of this site shows the gremlins in the wiring and drive line in those trucks.
But if your savy with electronics maybe you will be good to go. I will say the ride was very nice when i was being tow barred by a 925 to depot...

Yes, this is the heart of the ridiculousness of the argument every time it comes up. Essentially yes, newer is better. Those opposed to change always point out some edge case where it isn't. I'm also a mechanical engineer, so I know better than anyone that not every new idea works out great the first time. But as I said, these trucks are decades old now, so we're beyond the growing pains.

I get it, you don't like them and had a bad experience. So I think to myself, "Do I trust this one anecdotal story (or even a hundred of them), or numbers like the 100,000 FMTVs the military owns, their reliability studies that show them to be the most reliable truck they've ever had, etc.?" There's really no comparison, and it just tells me you aren't interested in actually understanding. I have nothing against the old trucks. You're going to spend similar amounts of time/money/resources on old or new, so why not have the new?

If you wanted to make an actual logic-based argument why the newer trucks are worse, I think it would go something like this... The older trucks are being retired because they have been replaced, but are otherwise in well-kept condition. The newer trucks are largely getting auctioned because they are broken, or else they would keep them around longer. Both trucks are going to succumb to the age of rubber seals/o-rings/gaskets/etc. before they realistically reach the mechanical life of almost anything involved (e.g. transmission, engine, axles, etc.), and the older trucks have probably passed that life and had most everything replaced already, while the newer trucks are being got rid of exactly because those things are starting to fail (so they are going to need to be serviced by the new owner in the near future).

 

[Aernan]

So comparisons to the 3116 and the C7. This is all from reading so if I get it wrong someone please do step in.

The governor on the 3116 is completely mechanical and takes fairly limited number of inputs to determine the max RPM of the engine. It governs down the fuel delivery. I know for certain speed and turbo boost are two of the inputs.

When Cat went to an ECU they gained the ability to meter many more values which they can use to Govern the engine. Here are some things I know modern ECU can monitor but I don't know which sensors are on the 3126 or C7:

• Oil Temp
• Coolant Temp
• Intake Air Temp
• Exhaust Air Temp
• Ambient outside Temp
• Boost Pressure
• Turbo RPM
• Exhaust gas Temp
• Mass Air Flow
• Oxygen Sensor in exhaust
• Engine Speed
• Crank Position
• Altitude
• Catalytic temp/DPF Temp
• DPF high side low side pressure

With all the sensors you can correct for:

• Moisture in Air (hot dry vs cold wet)
• Oxygen Content (altitude)
• Fuel Value (cetane and total burn)
• Cold weather warm up
• Ambient (cold hot)
• Light Load vs towing
• Fuel economy vs performance
• SMOG regulation (completeness of burn)
• Exhaust gas excessive temp

Certainly the 3126 and C7 the designers tackled a whole lot of those issues. The 3116 is a one size fits all with local adjustments for conditions. So that means less peak power and not as fuel efficient. But the flip side it's such a well known engine it's easy to diagnose and has been made reliable by years of experience.

 

[Aernan]

Well I am just over 30 years old and plan to use it for the rest of my life, so. I imagine I will be doing a whole lot of driving over the next 40-50 years. Also when I no longer need it for construction I may put a camper shell on the back and use it as an offroad RV to do some traveling.

For passenger cars/trucks most people do under 30k miles a year. My dad is a truck driver and crane inspector. He spent his life on the road. He managed 55k a year. If it's not your primary vehicle you might get 10k on it.
10k * 50 years = 500k = one engine
30k * 50 years = 1.5 million miles = 3 engines

More realistically
5k * 30 years = 150k

Cause:
50k miles = ~8k gallons of diesel at $3.50 a gallon = $28k a year in fuel.
Cat dealer service $6k
Other broken stuff. $2k
28 + 6 + 2 = $36k a year.

In other words. Since it's a commercial diesel the engine will last and outlast any passenger car you have. That is if you start the engine and driver for 4 hours a day and keep the revs down. If you hot rod and do 15 minute trips you will shorten the engine life a bunch.

 

[justin22885]

What is the difference in fuel economy between an electric C7 and a mechanical 3116? If there isn't a noticeable difference then I don't see much benefit to using EFI. I do not know the fuel economy of the truck in question, my guess is somewhere between 7mpg and 10mpg.

 

[Awesomeness]

What is the difference in fuel economy between an electric C7 and a mechanical 3116? If there isn't a noticeable difference then I don't see much benefit to using EFI. I do not know the fuel economy of the truck in question, my guess is somewhere between 7mpg and 10mpg.

My guess is that the difference is trivial in the small scale (e.g. mileage from a tank). The truck is designed to get about 6MPG, 300 miles on a tank full. Any changes due to EFI are going to be small percentages - I would guess maybe 5%, and probably not more than 10% improvement.

The military cares about these kinds of things because even seemingly insignificant changes add up to huge amounts in large scale (they own 100,000 of these trucks, so a few percent means millions of gallons / dollars per year).

 

[justin22885]

I found data from Cat saying 4% improvement in fuel economy. This is very, very low. I could save more fuel than that just by tweaking my driving habits a tad.

Hmm, it gets the same fuel economy as a 5-ton cargo truck weighing a good 6,000lbs more, why is that? And people are claiming 10-15mpg with the M35A3 using the same engine and the same weight as the LMTV.

 

[simp5782]

I found data from Cat saying 4% improvement in fuel economy. This is very, very low. I could save more fuel than that just by tweaking my driving habits a tad.

Hmm, it gets the same fuel economy as a 5-ton cargo truck weighing a good 6,000lbs more, why is that?

Different engines. Power bands. Drivetrains. All kinda factors come into play vs a 5 ton truck. Most 923A1s get 6mpg empty driving. I get 5.5 grossing 60,000lbs in a 923A1 running 65mph. You dont buy em for mileage. No truck, especially older generations is going to get good fuel mileage. Unless you are going down hill with a tailwind.

In my 923 i have all the power she needs to maintain 65mph loaded with 12 percent throttle. My foot just resting on it. Every LMTV i have driven. Especially 3116s your foot hurts from holding it to the floor to maintain 58mph. Basically wasting fuel you dont have to.
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[Awesomeness]

I found data from Cat saying 4% improvement in fuel economy. This is very, very low. I could save more fuel than that just by tweaking my driving habits a tad.

Hmm, it gets the same fuel economy as a 5-ton cargo truck weighing a good 6,000lbs more, why is that? And people are claiming 10-15mpg with the M35A3 using the same engine and the same weight as the LMTV.

Because the key things that determine fuel economy aren't very different between any trucks like this. The weight doesn't matter much, except during acceleration... so a fully loaded semi isn't far off these numbers either (probably 4 MPG). The air resistance is the biggest factor, followed by rolling resistance (e.g. tires, bearings, gears turning through oil, etc.) and those are mostly the same for all vehicles this size and shape.

The other factor is that changes to fuel economy are percentage based. So going from manual fuel injection to electronic fuel injection adds 4%... in a car that was getting 30 MPG you see a couple MPG and that seems interesting, in something like this getting 6 MPG the difference is smaller than the error difference you get based on where the tank happens to stop filling with gas. Essentially unnoticeable in small quantity view.

I don't own an M35, but 10-15 MPG sounds overly optimistic. That's too high a change, even to accommodate that these are full-time 4WD and an M35 isn't. They are smaller and slower (less air resistance), but they also have 6 wheels (more rolling resistance). Even very modern and aerodynamic medium duty trucks don't do that well. I would expect to see 8-10 MPG max from one. (Again, the changes are always percentages. There's pretty much no way to explain getting 100% extra to go from an FMTV to M35. You would have to find 20 of those little 5% advantages.)