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FMTV 3126 Engine options

MatthewWBailey

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I think it was @fuzzytoaster who wrote here the 3126b was the best long haul and overland engine? I could have purchased a C7 from him. Perhaps starting another thread: NOT for arguing, but sharing plusses and minuses?
Putting this thread up to ask a question and generally get more discussion on the "3126 boat Anchor concept". @Reworked LMTV @hike @GeneralDisorder were leading the charge on that topic so please continue.

my question before I spend >$3000 on oil pump and lube filter upgrades (oil pump $700, IFS kit $1100, oil cooler decking $1500, pre HEUI filter ?), is this: is"downgrading" to a rebuilt 3116 a better option? I assume it won't talk to the Allison so there's that.
 

coachgeo

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Putting this thread up to ask a question and generally get more discussion on the "3126 boat Anchor concept". @Reworked LMTV @hike @GeneralDisorder were leading the charge on that topic so please continue.

my question before I spend >$3000 on oil pump and lube filter upgrades (oil pump $700, IFS kit $1100, oil cooler decking $1500, pre HEUI filter ?), is this: is"downgrading" to a rebuilt 3116 a better option? I assume it won't talk to the Allison so there's that.
imho upgrade to a Cummins. There is some threads in here where that is discussed
 

GeneralDisorder

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I would get a M-ATV 370 HP C7 crate engine when one shows up in the surplus market again.

The issues I've seen with the 3126:

1. Engine oil pump's have problems with the idler design and at some point they changed the pump from a gear pump to a trochoid pump. Some pumps have a ball bearing for the idler and some pumps have a bushing that is fed oil pressure directly from the pump (thus unfiltered - another reason to keep your oil clean and install a bypass filter). When the idler bushing gets worn it leaks oil volume from the excessively large bushing clearance and drops oil pressure to the oil galleries and gauge. The number of design revisions to the oil pump speaks to the difficulties that CAT had with these pumps.

This video shows the issues. Note that this RV in the video below has 90k miles on it but the FMTV's with their stock gearing cruise at redline while this RV likely cruises at peak torque. So the extra 1000 RPM of constant operation seems to do in these oil pumps a lot quicker in our world.


2. Oil coolers warp and allow unfiltered oil into the clean side of the filter housing. This is compounded if the oil pump is making metal from the pump or the idler bushing. Decking the oil cooler isn't that difficult - any cylinder head shop can do this for under $100. Worth checking IMO.

3. The HEUI system relies especially on clean oil - see #1 and #2. @ckouba recently experienced all these pieces come together into a very large repair bill (in terms of parts - he did all the labor himself) - HEUI pump, engine oil pump, oil cooler resurfacing, many hours of labor, etc. All of this at only ~15,000 miles on the engine.

4. 3126 engine blocks of a certain serial number range have problems with bell-housing cracks.

5. The ECM is not that common and of the ECM's I have worked with I have only experienced complete failure on 3126 units. I have not had a C7 ECM that was actually "bad". In addition the 3126 units are much less common and tend to be 3x to 5x the cost of a used C7 ECM. There is some talk of the earlier units having internal batteries that can fail and render them inoperable - I have seen dead units but I'm not sure if that was related to this. All the ECM's I've flashed have no memory of the date and time so it seems that later units either eliminated the battery or do not rely on it's function. I've been meaning to open one up and see if the battery is replaceable or at least remove it if it's dead so it can't leak all over the board.

6. The 3126 is the lowest production volume of the various FMTV engines. It's the first generation of the HEUI system and so the C7 benefits from the mistakes made on the 3126.

7. CAT came out with a pre-filter kit for the C7 HEUI pump - and it's only like $400 for the parts! Sadly due to the design differences between the 3126b HEUI pump and the C7 HEUI pump there's no good way to adapt it to the older design. The oil supply inlet to the HEUI pump is much lower on the 3126 pump so there's no space between the block and the pump to install it. For whatever reason CAT has not developed this pre-filter for the 3126.

Theres's also a lot of other changes and upgrades that the A1R trucks got that A1's didn't have. And in terms of the market the A1 and A1R seem to fetch similar prices mostly because new buyers aren't aware of the differences between the two so any truck that is 2000+ model year gets the same higher pricing because those are the trucks that Acela and other refurbishers and integrators focus on (Acela won't even buy A0 trucks). There's a big jump in terms of pricing from the A0 to the A1, but not a correspondingly high jump from the A1 to the A1R. So the 3126's are over-valued IMO and it makes the most sense to start with the A1R unless you specifically want the A0 for the "mechanical" engine - that's a whole discussion on it's own though and typically people making that decisions fail to realize that even the A0 has electronics for the transmission and is less reliable in several other ways that make this argument questionable.
 
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hike

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When we owned a small fleet of CAT equipment we did oil testing. I have been wondering if we should restart with this truck and the comment about the likelihood the previous owner did not maintain the engine well makes me think again, that is worth exploring–
 

hike

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Found @fuzzytoaster post:

I covered this in a few past posts and believe the A1 trucks between 2003-2004 would be the clear winners in my opinion. They have the 3126 and fall in a narrow serial range where they received many of the newer truck upgrades that became standard on the 2006+ A1R models. Battery disconnect, engine remote start, and etc are nice to have and you've got ABS, exhaust brake, and more HP without physical modifications which are stock on all A1 trucks. These 2003-2004 A1s are still modern platforms which parts and support are available without being price gouged for having a C7 engine. It pays (or saves) to be one step behind the most cutting edge of tech and the slightly older stuff (early 2000's) will be more relevant out of country if you plan to expedition there. You'll have all the perks without any of the down sides. There's also a higher chance of catching an A1 truck that has gone though a rebuild/depot which is always nice.

When it comes to the C7 trucks, I've had 7 A1Rs and still have 3. They're a royal PITA when they want to be and any issues related to the truck, and not a specific component like the engine or transmission, you will be on your own. TM's are electronic and not for distribution legally. The HP flash sounds exciting on paper but the torque output is near the same between a 3126 @ 330 hp vs a C7 @ 370 hp. It's about 40 ft/lb difference and the C7 will be within 3 ft/lb of torque of the limit of the torque converter so unless you're all out in the most extreme conditions you won't see the difference. I do enjoy my C7 trucks but they aren't common enough yet in civilian hands to have any true support beyond major off the shelf components. You will need to be a master of your vehicle.

I have to disagree with the end of GeneralDisorder's last sentence about the A1Rs being newer trucks so everything is newer. The A1R trucks are nearing the end of their useful service life. Not many A1Rs have been rebuilt and remained as soft cabs. Those who are selected go to Oshkosh and become A1P2 and are retained for service. Most A1R trucks will be original but some A1 trucks (2001-2004) and A0 trucks (94-98 ) went through late rebuilds being full resets and are newer in every way. A prime example of this is the 2006 M1078A1R (non-reset) and the 1997 BAE Systems M1078 (2010 rebuilt) trucks I currently have. The A1R shows its age and the M1078 is absolutely mint! I have a M1083A1 w/w that was a late rebuild too.

At the end of the day you will have to evaluate each truck individually and make a judgement call. You can't take the internets word as a definitive so you need to evaluate what skills you have and challenges you're willing to take on should the situation arise. If you want a C7 truck then go for it, they're a blast! But again, evaluate each truck on its own and what you're willing to take on.
The whole thread is interesting and somewhat related to this one–
 
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MatthewWBailey

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I'd look for a crate C7, and have a plan for that installation. But continue to run the 3126 until it quits without doing any major updates beyond possibly the oil pump and basic maintenance...
That seems sound. I could do the oil pump easily enough and I already have a bypass filter.

Mine is already consuming oil at 14kmiles at a rate of 1gallon synthetic per 2k miles . Does that seem high?
 

ckouba

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3. The HEUI system relies especially on clean oil - see #1 and #2. @ckouba recently experienced all these pieces come together into a very large repair bill (in terms of parts - he did all the labor himself) - HEUI pump, engine oil pump, oil cooler resurfacing, many hours of labor, etc. All of this at only ~15,000 miles on the engine.
This is true, and I went a little overboard due to not having the resources of the US military supporting me (at least not directly!). I intend to use my rig to go places and after already experiencing an engine code 1000+ miles from home, I went with the pound of prevention is worth a ton of cure philosophy. I had decent oil pressure, but didn't want that to fail so got the updated pump. I had no evidence of oil bypass at the filter housing, but pulled it anyway and had it decked (the gasket printout looked like it was OK but the machine shop needed to take ~0.025" off to flatten it). Just guessing what the bill would be for an "out there" recovery, I chose to put the IFS kit in. It won't prevent a HEUI failure, but it will contain the damage and make it easier (less $'s) to get moving again. At the risk of jinxing things, I went through the effort to try and bulletproof my motor and based on the recent results, it was worth the effort.

Which brings me to my real point. None of this hobby is cheap or inexpensive if you want to enjoy it. I put some $'s into mine to try to make sure that I not only can get out to places I want to visit, but that I can come back too!

One motor may be better on paper but if it's that important to you, I imagine you'd be far better off picking your powerplant and then finding a truck equipped with it than finding *A* truck and repowering it. My real point- if someone gave me a completely free A0 chassis and Cummins motor and said to have at it, I'd flat out pass... If I wanted a C7 (or whatever) that much, I would find a chassis with one in it. If this motor ever pops, a 3126 is going back in the same hole. I can't imagine the hassle of swapping motor types would entail. Too much for this amateur!
 

MatthewWBailey

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This is true, and I went a little overboard due to not having the resources of the US military supporting me (at least not directly!). I intend to use my rig to go places and after already experiencing an engine code 1000+ miles from home, I went with the pound of prevention is worth a ton of cure philosophy. I had decent oil pressure, but didn't want that to fail so got the updated pump. I had no evidence of oil bypass at the filter housing, but pulled it anyway and had it decked (the gasket printout looked like it was OK but the machine shop needed to take ~0.025" off to flatten it). Just guessing what the bill would be for an "out there" recovery, I chose to put the IFS kit in. It won't prevent a HEUI failure, but it will contain the damage and make it easier (less $'s) to get moving again. At the risk of jinxing things, I went through the effort to try and bulletproof my motor and based on the recent results, it was worth the effort.

Which brings me to my real point. None of this hobby is cheap or inexpensive if you want to enjoy it. I put some $'s into mine to try to make sure that I not only can get out to places I want to visit, but that I can come back too!

One motor may be better on paper but if it's that important to you, I imagine you'd be far better off picking your powerplant and then finding a truck equipped with it than finding *A* truck and repowering it. My real point- if someone gave me a completely free A0 chassis and Cummins motor and said to have at it, I'd flat out pass... If I wanted a C7 (or whatever) that much, I would find a chassis with one in it. If this motor ever pops, a 3126 is going back in the same hole. I can't imagine the hassle of swapping motor types would entail. Too much for this amateur!
Also sound.
 

GeneralDisorder

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The 3126 and C7 are almost the same engine. And in the ways it matters for an engine swap they are almost identical. Such that it would take very little effort to swap one for the other. The wiring is setup so that the engine and it's ECM are a self-contained unit and the chassis side connection is the same plug for both engines.

Putting either into a 3116 truck would be an interesting adventure. Mostly from the transmission control module perspective because the 3126 and C7 will both run with just power and ground to their computer. You also need the e-throttle if you want to drive it, and of course if you want gauges you will have to plumb that in with whatever the 3116 had for sending units, etc.

I agree with @Ronmar - look out for a C7 but run the 3126 till it gives up something expensive.
 

Ronmar

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Yea as far as engine swaps go, I think a c7 in place of a 3126 would be way toward the easier side of the difficulty meter.

now a C7 in place of a 3116 sounds a little more for the advanced class, but I think it would be doable, and may be something I may contemplate at some point if the need arises. So far 3116 parts are still available. They sold that engine all over the world:)
 

MatthewWBailey

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I would get a M-ATV 370 HP C7 crate engine when one shows up in the surplus market again.
I assume there's a big difference in the C7 crate engine from military surplus vs the civilian over-the-road market? I see lots of New & Used 2004-2008 C7 engines on the web from/for buses and RVs, industrial. But They often don't look the same as the C7s from an A1R at least from pictures.
 

GeneralDisorder

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I assume there's a big difference in the C7 crate engine from military surplus vs the civilian over-the-road market? I see lots of New & Used 2004-2008 C7 engines on the web from/for buses and RVs, industrial. But They often don't look the same as the C7s from an A1R at least from pictures.
Military crate engines come with many of the accessories, etc for the vehicle they are intended to go into and they are sometimes differently configured than civilian engines for sure. The M-ATV engine would come with M-ATV accessories and bracketry and much of that has to be swapped out for the FMTV furniture. But it's straightforward enough to figure that out - it's been done already and I was involved in the process. It was pretty simple and all bolted together.
 

MatthewWBailey

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@GeneralDisorder @Ronmar (and others of course) do you know what this tag means specifically?

This engine is advertised as a rebuilt 3126b in 2009 "complete engine". But I can't find out what was "rebuilt", the whole engine or just the crankshaft and bores?
74A77A82-DBA9-4AFE-92E8-4BC41A48A537.jpeg15B68480-F36F-4A24-A7B6-9D2674D56151.jpeg
397E7EF4-E1BA-46B6-B92B-09747A6B3D68.jpeg
 

GeneralDisorder

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It would have been the complete engine rebuild. CSMS is a Combined Support Maintenance Shop - these are used by multiple units and are staffed with AGR and civilian contractors.

It means it got "overhauled" by the lowest common denominator. Underpaid government employees that would have to commit murders (probably plural) to be fired and even then it might take several years. You would not like the things that soldiers have to say about Depot rebuilt components with respect to getting good ones the first time around.

It's not one of the "bad" serial number 3126's that have block cracking problems - that's the "6PZ" serial prefix engines..... I would carefully inspect it including cutting open the oil filter and if possible pull the oil cooler and check it for flatness, etc. It would be a huge gamble IMHO and I'm guessing whoever is selling it considers these markings to indicate additional positive value while personally I would consider this a possible red flag and a detracting quality for resale. I would much rather have a low mileage take-out that was built by CAT than some rebuild done by a depot - hell it might be one of "those" engines that got installed and immediately pulled back out because it was garbage. It happens more than you think.
 

Skyhawk13205

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@GeneralDisorder @Ronmar (and others of course) do you know what this tag means specifically?

This engine is advertised as a rebuilt 3126b in 2009 "complete engine". But I can't find out what was "rebuilt", the whole engine or just the crankshaft and bores?
View attachment 925058View attachment 925059
View attachment 925061
The data plate is good info to determine if the crank was machined down and the bearings are oversized. The problem with oversized is that it removes a possible repair in the future if the crank has to be machined again. Most likely when it was overhauled, the crank was found to have a pit or damage that needed to be machined out and the bearings are replaced with a larger set to take up the clearance. Usually any oversized holes are marked with yellow around the bore.
 

MatthewWBailey

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It would have been the complete engine rebuild. CSMS is a Combined Support Maintenance Shop - these are used by multiple units and are staffed with AGR and civilian contractors.

It means it got "overhauled" by the lowest common denominator. Underpaid government employees that would have to commit murders (probably plural) to be fired and even then it might take several years. You would not like the things that soldiers have to say about Depot rebuilt components with respect to getting good ones the first time around.

It's not one of the "bad" serial number 3126's that have block cracking problems - that's the "6PZ" serial prefix engines..... I would carefully inspect it including cutting open the oil filter and if possible pull the oil cooler and check it for flatness, etc. It would be a huge gamble IMHO and I'm guessing whoever is selling it considers these markings to indicate additional positive value while personally I would consider this a possible red flag and a detracting quality for resale. I would much rather have a low mileage take-out that was built by CAT than some rebuild done by a depot - hell it might be one of "those" engines that got installed and immediately pulled back out because it was garbage. It happens more than you think.
The engine I have now is a 6PZ. I just got the new oil pump. Cat 496-4800. I gotta get that going. Oil psi is only 40 at 1500rpm and I'm getting check engine at every restart at temperature.
08B94FE5-560E-4E4C-AEBF-8F82613C53A1.jpeg
 

GeneralDisorder

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They like to crack on the drivers side at the rear - behind the starter. Plenty of non-military applications that experienced that failure and in our application the operating conditions can be significantly more harsh. Slowing down the drivelines with the ECO hubs helps for sure.

If I was replacing a 3126 though I would just go straight to the C7.
 
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