M923 Thermostat diagnostic

[DerangedMadman]

I was driving my M923 around the yard cutting and loading some branches in the back and noticed there was a little steam coming out from under the hood. I immediately checked the temp guage which seems to be working fine and it was only at 160 degrees. I shut it down and popped the hood to find out the reservoir was steadily spewing antifreeze out of the overflow tube. I was falling down on the exhaust and steaming up. From here I could hear air bubbles coming up from the engine into the reservoir. It seems to me like the thermostat must be stuck but I would think the engine would have gotten hotter than that. Any thoughts?

I am guessing the thermostat is located in picture 3 in the housing the hose is attached to on the top of the motor. I am on Verizon WiFi so I have not downloaded the TM's yet. I did download the P2P but it has nothing on it except for electrical and pnuematic.

 

[nf6x]

That sounds like symptoms of a blown head gasket to me.

 

[DerangedMadman]

I waited 2 hour, let it cool down. Checked the fluid level and it is still just under the fill tube. Started it back up. It got back up to about 165 degrees and the same thing happened again. I hope it is not a blown head gasket. Even while it was cool when I took off the cap to the reservoir it started bubbling again. I does not leak any antifreeze at all other than it overflowing right now. There is no antifreeze in the oil and no oil in the antifreeze. I thought maybe the reservoir cap is bad and it is releasing overflow at a lower psi because the spring was worn out but that still does not explain the bubbling I hear coming back into the reservoir when it is shut off.

 

[Bob H]

The bubbling is typically combustion pressures escaping through a bad head gasket into the cooling system.

 

[nf6x]

I had blown head gaskets in my HMMWV many years ago. As it turned out, the heads were also cracked. And I found the problem 250 miles from home. So, it could be worse!

One complication that confused the heck out of both the mechanic I hired and me was that my surge tank was foaming up so vigorously that he couldn't even get a good sniff with a block tester without it ingesting coolant. It looked like the truck was making water somehow. I eventually figured out that the breather hoses leading from the surge tank to other high points on the engine were clogged with rusty crud. That kept the coolant from being able to level out in different parts of the system, so the leaking exhaust gasses formed a pressurized pocket of exhaust at the top of the engine, pushing the coolant down out the bottom of the engine, and up into the surge tank. If the breather tubes were open then the exhaust gas probably would have passed through them without foaming up the coolant as much. I haven't studied my M939-series engines carefully enough yet to know if the same thing could happen to them, but since they also have remote surge tanks like my HMMWV, it's something I would look into.

My suggestion is to buy or borrow a block checker to see if those bubbles are exhaust, and to also check for any blocked coolant breather tubes which could complicate things further. I bought one from NAPA a while back to have on hand.

Edited to add: If you're not familiar with a block checker, here's a quick description: It's a glass cylinder with a rubber plug at one end that you cram into the radiator cap opening, and a check valve in that plug. You fill it with a chemical fluid that changes color in the presence of exhaust, then close the top of the cylinder with a cap that has a squeeze bulb. Then you suck air from the coolant reservoir through the checker with that squeeze bulb. If exhaust gas is getting into the coolant, then the liquid in the checker will change color.

 

[nf6x]

NAPA block tester kit:

NAPA AUTO PARTS

 

[DerangedMadman]

OK, new idea. I was just reading through the TM under the -1, 3-55 it states there is an Air compressor supply and return tubes.

As the truck sits there is a definate air leak in the system somewhere that I have not been able to find.

Is it possible that the air compressor is somehow leaking pressure into the cooling system causing the bubbles and overpressurizing the cap causing it to overflow?

I will buy one of the testers to see if there are exhaust gases in it. Thank you for that.

This engine seems to run too good to have a blown head gasket.

 

[juanprado]

That engine looks like a new rebuild. Head Gaskets might have not been torqued properly or heads/block may not have been machined properly.

Super heated exhaust boils the coolant at the leak regardless of the Thermostat and it needs to go somewhere.

Good Luck and I hope it is not that!

 

[DerangedMadman]

The coolant is not boiling when it comes out of the overflow, It is a steady liquid. There is no vapor coming from it until it hits the outside of the exhaust manifold that is underneath the overflow tube.

The Napa near me has the tester in stock so I am going to pick it up today, I just dont think (at least hope) that is it.

 

[DerangedMadman]

The question remains can the air compressor somehow leak into the air compressor coolant lines?

This would explain why I am still getting bubbles after the temperature has cooled and there is still pressure left in the air tanks.

 

[DerangedMadman]

Okay, Its been a little over an hour and the temp has cooled on the engine. About every 60 seconds or so there will be about a 5 second run of bubbles coming up into the reservoir. This is pretty steady.

I drained both the air tanks, completely depressurized them from the drains. The bubbles stopped.

I guess now I need to know why.

 

[nf6x]

If you have an air compressor, then maybe you can repressurize the system with the engine off to see if the bubbles come back.

 

[rrrr]

Check your oil to make sure you are not getting coolant in it as well.

 

[doghead]

Your air compressor is water cooled, right?.

 

[DerangedMadman]

Yes it is water cooled. I took nf6x's advise and repressurized the air tanks with my air compressor and it started doing it again. It is losing air from the secondary tank and is bleeding into the cooling system. I guess I am going to have to rebuild the top of the compressor where the water inlet and outlet come out.

Any ideas on the gasket set or rebuild kit?

 

[rrrr]

Great job trouble shooting !

TNJ Murray is probably the place to go. Maybe White Owl or Boyce also.