1991 BMY M923A2 loses heat when temp gauge hits 200 degrees

[Western PA M923A2]

Hey there fellow SS'ers! New 'private' here with a heating/coolant issue with my 5 ton. She's a 91 BMY, 8.3 Cummins 6CTA, Allison 5 speed auto. Suddenly over the past few days, once the temp gauge hits 200, I lose my heat. Maybe bad thermostats? I'm guessing they are opening because the truck does have heat after she hits around 160 or so on the gauge, but once it hits 200, heat fades out and then the temp goes over 200, even up to 220-230, and that worried me so I shut her down once it got that hot. Could the thermostats be flaky and closing back up? Weak water pump? Air in the system? I'm kinda stumped, so I thought I'd throw it out to the community. I topped the system of with coolant and it's full, but still have the same issue. Any thoughts? Thanks in advance!

 

[simp5782]

Have you verified your actual temperature reading with a thermo gun rather than just using the gauge? They are known to be junk and read high when they fail. Your heater issue is more than likely a waterpump since it is not circulating especially once it gets hot. If it was 250 engine I would say forsure you had broken the plastic impeller but the 8.3s don't have those.

1. Verify your actual temp. Check oil for water mixture.
2. Run the truck up to 200 degrees and shut down. Check your temp nearest to the pump. Drop your belt off your engine and spin the waterpump by hand. Do this for about 3 minutes straight. Check your temp nearest the pump again and see If it has changed. If it hasn't then you need a waterpump or there is a near by restriction. If it does drop significantly re install your belt.
3. Pull heater hoses off of heater core, run engine to see if there is circulation. While you have them off attempt to flush heater core. If there is no water circulation then you know it is a water pump.
4. Pull the thermostat and check to make sure if it not gunked up with crap and working freely..
5. Replace waterpump, thermostat, flush radiator. Pull any coolant access covers on the engine and clean them out free of gunk and rust. Add 7gallons of Concentrate antifreeze 3 gallons of distilled water and 2 bottles of prestone SCA additive

Please check the 5 ton sticky section for all part numbers listed in the Quick Reference Parts Spreadsheet.

 

[Cape Coastie]

Welcome to the site. My M923 has the Cummins NHC 250 but mine very rarely goes over 180-190. The fan kicks on and keeps it around 185. Sounds like you have a circulation issue. The tank may look like it is clean but may be mudded up in the block and heater core. Sounds like a flushing is in order.

 

[Shirehorse]

Also, something to keep in mind in the future when overheating. Stop your vehicle and allow the engine to idle instead of shutting the engine off. Do not open hood or remove engine side vents to maintain proper designed air flow. Ensure engine fan is working. More info can be found in your TMs.

Damage to engine and sanity will result otherwise.

 

[simp5782]

Also, something to keep in mind in the future when overheating. Stop your vehicle and allow the engine to idle instead of shutting the engine off. Do not open hood or remove engine side vents to maintain proper designed air flow. Ensure engine fan is working. More info can be found in your TMs.

Damage to engine and sanity will result otherwise.

View attachment 659391

I hate when people break their waterpump belt and they do nothing. Get out and spin that puppy by hand. She will still cool down even with the truck turned off. Especially on the NHC250. Stupid 939s only have 1 crap belt.

 

[Western PA M923A2]

Thanks for the replies so far. I've only had this truck since June, and she was my first MV.... popped my cherry with a 5 ton! Anyway, it didn't have heat at first and after reading posts on here I checked the petcocks that send the flow to the heater core, and as I read they are often closed, and so it was with mine. I opened the valves, and VOILA! HEAT!! I also opened the valve on the core to bleed out any air and had coolant flowing out. Now, to the present issue - I did notice today that the coolant in there is REALLY MURKY and rusty brown, so Shirehorse you may be right... a good flushing is probably in my future... a 5 ton cooling system enema! LMAO

 

[simp5782]

Thanks for the replies so far. I've only had this truck since June, and she was my first MV.... popped my cherry with a 5 ton! Anyway, it didn't have heat at first and after reading posts on here I checked the petcocks that send the flow to the heater core, and as I read they are often closed, and so it was with mine. I opened the valves, and VOILA! HEAT!! I also opened the valve on the core to bleed out any air and had coolant flowing out. Now, to the present issue - I did notice today that the coolant in there is REALLY MURKY and rusty brown, so Shirehorse you may be right... a good flushing is probably in my future... a 5 ton cooling system enema! LMAO

Both of those valves being closed would not cause the engine to get anywhere near 230 degrees unless you have a faulty gauge or something else going on. Even with it being murky it wouldn't be my first guess to cause overheating unless all the radiator tubes are stopped up.

 

[lindsey97]

Radiator is probably stopped up. It will be plugged on the left side, which is the hot side, with the entrance up high. Pull the radiator and have it rodded out and cleaned. Before pulling the radiator, get several 5 lb. boxes of cascade, drain the coolant, fill the truck with water and cascade, then drive it for one hour, drain it, then repeat a couple times. Replace the thermostat no matter what. They can and do go bad.

Lots of good advice in the above posts. Of all the issues I have experienced with these trucks, being 8-10 of them, the cooling system on every one of them had blockage of some sort.

 

[Western PA M923A2]

Well as the truck warms up and hits around 190 or so, I'm guessing that the thermostat is opening since the tube that connects from the thermostat housing to the upper left of the rad gets hot, and the heat does seem to work well right about then, but as the temp gets to around 200 on the gauge (which I just replaced a few weeks ago) there goes the heat. BTW, there are 3 thermostats on these, so which ones would cause heating problems - the two internal ones or the external one?

 

[snowtrac nome]

I agree on t- stat I would also flush the system, likely cause is air in the system mixed with flakey thermostat. I would also top off the coolant tank get some one to help you stall test the truck. While the truck is under load and boost look for bubbles in the cooling system indicating a possible head gasket failure. I have seen c series with this problem from almost new and no body bothered to check head gaskets. good luck

 

[Western PA M923A2]

Head gaskets??? YIKES!! I cringe at the thought. As to the thermostat, there are 3 thermostats on these, so which ones would cause heating problems - the two internal ones or the external one?

 

[simp5782]

Head gaskets??? YIKES!! I cringe at the thought. As to the thermostat, there are 3 thermostats on these, so which ones would cause heating problems - the two internal ones or the external one?

I bet you only have 1 thermostat and a plug in the other hole. They can have 2 side by side. Replace them both. It is available from cummins with both of them and the appropriate gaskets. Take your engine serial number with you.

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[Western PA M923A2]

I already picked up the 2 thermostats - NAPA cross referenced for the Cummins 3928639. What about the 3rd one - the external bypass thermostat?

 

[simp5782]

I already picked up the 2 thermostats - NAPA cross referenced for the Cummins 3928639. What about the 3rd one - the external bypass thermostat?

Some A2s dont have it. There are different types of the CTA engine in those trucks. I don't know how your truck is setup.



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[simp5782]

Parts can be found here. https://docs.google.com/spreadsheets/d/1-6vGwwEsD-flXsMFBn7VomUUJDuk_aG_to5pzHqzkBc/htmlview#gid=0

Weird. The nsn for the A2 thermostat crosses to a 358-180 or a #185 thermostat from napa. I will have to investigate that

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[WillWagner]

Which issue are you trying to troubleshoot? No/low heat to the cab or the high temp? You need to troubleshoot your issue BEFORE you start throwing parts at it.

After reading your posts, the stats don't seem to be the issue...upper radiator hose get hot, that tells me the stat is working, opening. It is quite normal for cab heat temp to drop when the thermostat opens, remember, the opening temp is the starting point, if the stat is a 185, it starts opening at 185 and is usually fully open by 195. In that time, stat has blocked off the block loop and started to pull cold coolant from the radiator into the system. The stat modulates until the complete system is just above the opening temp, 185,188 and will stay there until the engine is loaded. Idle cabin heat will be low. If the fan runs all the time, the heat will go away fast if the radiator is in good condition, it's doing it's job. Fix the fan if it is always running.

Also, what you describe can be caused by the supply and return heater hoses plumbed to the same area on the engine, check the plumbing. It is very common for "mechanics" to route hoses wrong. The heater doesn't care which port on it is supply and return, it is where the hoses start and end at the engine that makes the heater work. Supply needs to be pressure, somewhere on the block, preferably below the head gasket. That is the highest system pressure due to the orifices in the gasket. It is OK to use the ports in the head but the heater won't work well, the flow/pressure is low above the gasket. Return needs to be at a suction. Only one place to get that, at the w/p inlet. The C series has a couple of pipe plugs at the volute area that are used for returns, depending on the inlet option used, there are can be a few on the inlet...but IIRC, these had only one option and it was just a straight out casting. The other option was OEM supplied in the lower radiator pipe.

I don't argue that the system needs a service. Low cabin heat cannot be caused by a w/p or a belt. Spinning the pump by hand will not tell you the pump is circulating coolant, these move upwards of 60 gpm at rated rpm and in the time it takes you to remove the belt and start turning the pump, the system will have heat soaked enough so you won't be able to tell anything about coolant temp drop, cast iron heats fast and retains heat.

Understand your stuff and troubleshoot issues before just throwing parts at it, your wallet, (and wife), will thank you!

 

[Western PA M923A2]

Hi Will, and thanks much for the reply and info! As you may have seen in my post, I'm basically a 'newbie' and this baby is my first MV. I failed to include one detail on here - there was a coolant leak from the inlet line from the block to the air compressor (see TM 9-2320-272-24-1, page 3-169) right where the line connects to the 90 degree elbow from the block, righth at the nut that holds the line to the elbow. I'm guessing the seal there is the culprit. Well I patched it from the outside just to stop the leak and that seems to be holding, but could this be related?

 

[Western PA M923A2]

Also forgot to say that I've never seen the fan kick in. From what I've read the shutterstat should engage at around 190-200, and the temp gauge (which is new since the one that was in the truck when I got it never worked) in the cab shows I've reached that or above,

 

[WillWagner]

Overheat is different from cab not getting hot. The leak at the air comp line won't have an effect on the cab heat or over heat issue, it is just an annoyance. The seal there is only a 1/2 in id, 1/2 inch wide rubber sleeve, the nut crushes it against the fitting. Only difficult part is getting the old sleeve out. Cummins calls it a 1/2 water grommet. Still in use today.

If the temp has been over the fan on temp, you need to check that out too, most likely a junk gauge. Best to verify with a known good gauge, IR thermo will get close, but not as accurate as a good Fluke digital capillary thermometer.

BTW, a failed head gasket, low liner can cause an overheat, but the related symptom you will see with either one is coolant pushing out of the radiator filler. The early Cs did have issues with head gaskets leaking coolant externally, usually on the fuel pump side, center of the block area, between 3 and 4 cyls. No overheat there, just a loss of coolant...guess it would lead to an overheat if coolant got low enough

Judging from your location, i'd say a heater is muy important about now. If it were me, i'd verify coolant temp gauge and focus on the heater issue.

 

[Jbulach]

Get a no contact temp gun and start verifying temperatures. If you understand the system and flow path you can do a lot of quick trouble shooting without hurting yourself. They are also great to keep in your truck for checking drivetrain temps on the road, anything from engine, u-joints, breaks, to tires, and everything in between...