M939 stuck motor

[Stevepala]

So I picked up an M939 real cheap and was aware it was not running. First sign of problem was a battery with a melted post, then pulled the engine oil dip stick and found oil contaminated with water, it was creamy brown. Then got under the bell housing with a pry bar to rotate the fly wheel. No movement, crap!

So has it been sitting so long the rings are stuck to the cylinders? Blown head gasket or cracked head letting coolant in cylinder?

Everything looks great and well maintained, hard to believe its stuck.

What should I do?

 

[Beerslayer]

Short answer is replace the motor with one that already runs or part out the truck. If there is water in the oil it is effectively done for considering the cost of the mechanical work to make it run.

Good thing is you can find a good running turboed 350 or 400 complete for around $2500 if you look around.

 

[sandcobra164]

If the truck was an M800 series, I'd say hook a towbar to it toss it in 5 high and gently rock it back and forth with another truck. I've seen many NHC 250's stick after sitting for quite some time. You could take it a step further and remove injectors and soak PB Blaster on the tops of the pistons and liners and try the above mentioned method with a prybar on the crank of a M939 series truck.

 

[doghead]

Isn't there a "compression relief" plug on the heads for each cylinder? If there is, you could put something in the cylinders through them and avoid the injector adjustment/removal.

I too, have an NHC-250 that is stuck(hopefully just from sitting).

I'll be looking at mine next week.

 

[Stevepala]

I'm going to put some panther piss in each cylinder and let it soak. Drain the nasty oil and then start turning the crank until it loosens up. If I can loosen it up I'll run it. If not a 400 will replace it.

They have compression plugs in the heads, any photos out there?

 

[WillWagner]

Cut the oil filter open to see if there is metal in it. If not, it's probably just hydraulic locked. You can pull the valve covers, screw an intake or exhaust valve down to open the valves on each cylinder...only do it about 1 to 1 1/2 turns...and try to bar it over.

 

[doghead]

Will, is there a compression plug? I'd like to put some penetrant or lube into my cylinders, before forcing it to crank over.

 

[WillWagner]

Early ones had a compression release, the blocks on the NHC have the hole for it but they are not installed. Only way to get anything into the cylinders is to remove the injectors and put the penetrant through the injector hole.I can honestly say, I have never seen a stuck piston from rust. Seen LOTS of failures that result in water/coolant in the cylinders, but never a rusted stuck piston, even if the engine sat for weeks before repair. Post pics when you guys find your issues.

 

[wreckerman893]

The 250 in my ill fated wrecker adventure was locked up due to the air cleaner being removed and moisture getting into the engine that way.....nothing I tried would unlock it and the engine was replaced (another ill fated endeavor) with a take out.

With the number of these trucks being parted out you can prob replace it cheaper than repairing it.

Csm Davis in Mississippi may have a couple of engines laying around.

 

[Stevepala]

Drained the oil and found nice chunks of piston skirt, so much for the great deal I paid. What are these Army guys doing that causes this type of damage to so many motors?

So I need another motor, big cam 350 or 400 seems the way to go. Is it a direct fit other then the exhaust and intake? Who has done this swap? How about a good source on a motor?

Thanks

 

[BigWill1985]

It could have been caused by many things. I have read of the governors being set higher and the operators over-revving them. I have also heard stories of operators adjusting the injector pumps and over-fueling the motors (EGT's get too high, and things start to melt). These engines don't take well to over-revving especially. That's a lot of unbalanced iron flinging around!

~Will Courtier~

 

[dougski]

So I picked up an M939 real cheap and was aware it was not running. First sign of problem was a battery with a melted post, then pulled the engine oil dip stick and found oil contaminated with water, it was creamy brown. Then got under the bell housing with a pry bar to rotate the fly wheel. No movement, crap!

So has it been sitting so long the rings are stuck to the cylinders? Blown head gasket or cracked head letting coolant in cylinder?

Everything looks great and well maintained, hard to believe its stuck.

What should I do?

Not sure what you should do BUT, a big cam engine 350-400 will rip that Allis trans up. Too much power. Just a thought to save ya a head ache in the future.

 

[rangereter]

For about a thousand dollars, (you) can inframe your existing motor (crank and heads having a 75% chance of being ok and not requiring any work), it would be an adventure for you. My experience has been that you can always inframe cheaper than replace (wet liner advantage).
regards, Bob

 

[timntrucks]

i have a 250 in the barn that im selling. i am along way off. had it in the classified sec for 750.00 but i think the ad ran out

 

[WillWagner]

For about a thousand dollars, (you) can inframe your existing motor (crank and heads having a 75% chance of being ok and not requiring any work), it would be an adventure for you. My experience has been that you can always inframe cheaper than replace (wet liner advantage).
regards, Bob

Only if the liner is in tact and the crap flying around didn't mess up the lower receiver, only way to tell is to take it apart. If it is OK, you'll need to find used parts and slap a set of new rings on the piston.

 

[Stevepala]

I think at this point the best course of action is to replace the motor. It's a shame because it has less then 10 hrs on it, if the Hobbs meter is correct. But because of the large chunks of piston skirt in the oil, an in frame rebuild is not the chance I'm willing to take.

Having never worked on these trucks before, it looks like the easiest way is to pull the engine and trannie together.


I have located a few big cam engines that would be good candidates. Hopefully the trannie will last long enough for me to find a solution for the excess torque. I don't plan on hauling any heavy loads or show-boating so maybe it will last.


If anyone has information on a good running big cam 350 or 400 for sale at a reasonable price that would be helpfull. If anyone has any information on the big cam install that would also be helpfull.

 

[dougski]

My 2 cents, just a thought to consider. I ran the same problem a few months ago on NHC – 250. I knew I had it frozen up cylinder and water was in the crankcase. Just on a whim I got the idea to remove the exhaust manifold and inspect exhaust ports. To my amazement I noticed that on cylinder number one exhaust port it was wet, the other five exhaust ports were all dry. When I removed the head for cylinders 1 and two the problem was noticeable. There was a broken valve and the piston was split down one side. I could tell this because I remove the oil pan and the crack was at the bottom of the piston also. This caused the piston to expand the Boer thus locking up the engine. I needed a new head piston sleeve kit and injector oh also the pushrod. With some hunting around I was able to get all parts needed for under 400 bucks. That's a lot cheaper and easier than buying a used engine and R&R it. A Cummins factory new in the box piston sleeves ring wrist can was only $150 and free shipping. Head gasket 40 bucks, could use injector and pushrod 60 bucks. The only problem you will have maybe is that the heads need to be torn down to 300 foot-pounds. I had to go onto eBay and buy a torque wrench that big. Good luck just more food for thought.

 

[rangereter]

Steve,
I would hate to see you throw good money after bad. Since you have already drained the oil, you could take another hour, drop the base pan and with a mag light look up at the bottom end and probably see where/what your problem is.
regards, Bob

 

[Vintage iron]

I have cummins 350 and 400's ready to install. We can do the swap too. It would be a easy in and out for us. Let me know if I can help.

 

[Stevepala]

Matt, I would like to speak with regarding the engine and swap. Email me at stevepala1@aol.com.

Steve