M923A1 coolant system overpressurizing when COLD and not running ?

[rivercreek]

I have an unusual problem with my M923A1 that i'm hoping someone can offer some insight on. The coolant system is creating pressure when COLD. While running, I am not losing a drop of coolant. When I park the truck and let it sit overnight, there is a small puddle of coolant underneath the truck. There is so much pressure building up in the system when cold (not running) that coolant is actually being forced out of several coolant hoses and even seeps out of the ends of the radiator slightly. I've replaced the cap, but it baffles me how I could be getting pressure while cold and not cause any issues while hot ?? In the meantime, I've been leaving the cap loose while parked - but that, of course, is just a band aid. Any ideas would be greatly appreciated !

 

[R Racing]

Most likely a long shot . but maybe your leaking air back through your air compressor . it has a coolant passage and lines going to it? also worth a look at your shutter stat to.

 

[rivercreek]

Thanks for the reply R Racing. My shutter stat is not leaking externally - what are you suggesting I do? Do I need to tear it apart and check o-rings, etc. ? Secondly, do you have any ideas how I should go about troubleshooting the coolant tube to compressor? I'm not sure how to proceed..

 

[Alexsha]

Have you tightened all you hose clamps on the coolant lines? They get loose in the cold.

 

[R Racing]

I'd rebuild the shuttle Stat first. If I had the same problem after that them I would either open up all the tank valves or remove the air line from the compressor. Then start the truck and let it run til it comes up to temp not letting it build up any air pressure. Then shut it off and see if your coolant issue is the same.

 

[Carlo]

I agree with R Racing. I think this would be a fast way to find out. Don't leave out head gasket leaking as a maybe!

 

[Coffey1]

On Butch's 944 it was the compressor he rebuilt problem solved.
When running compressor heats up and closes any gaps
When cool leaks into coolent.

 

[rivercreek]

Thanks for the responses, apologize for the delay on my end. Very busy with work. Will hopefully have the truck back at my place in the next day or two to try and figure this mystery out once and for all. Alexsha, yes - all my hose clamps, radiator bolts, etc., are tight. Checked, crosschecked, triple checked. I believe that the pressure that is being created is greater than what the coolant system is meant to contain. R Racing - I will swap the shutter-stat with a known good one from another truck and give it a try first before I go deeper. As far as starting the truck and running it up to temp - that's a slightly different story. I've never had any luck heating these trucks up in summer, much less winter without driving them. Even with the entire radiator face covered with a contractor trash bag, I've never been able to get the daggone things hot enough to open a thermostat. Course I can't use the throttle levers on any of the three 923's i've got either. The throttle levers are so unbelievably and unpredictably sensitive, I'm absolutely convinced that all 855 Cummins throttle assemblies are female. At least all mine are. It's either idle or WOT. Ain't no in between possible unless it's directly controlled by my foot. Anyways, shutter-stat first. Then i'll go and ride her hard, bring her home and make her sit in the corner to cool off and think about all the bad things that she's done. Will post update on results. Thanks again for everyone's insight.

 

[R Racing]

Thanks for the update. Hopefully it will be something simple !!

 

[Jon Giovani]

When you say it builds pressure when cold, do you mean ambient temperature cold, or within a few minutes or hours after running it?
The reason I ask is because most laymen don't realize that an engine reaches it's hottest temperature and cooling system pressure after the engine is shut down. This is do to the coolant not circulating through the cooling system and sitting stagnant thus creating heat and pressure until it reaches is apex and begins cool and pressure resides, much like expansion and contraction of a canning jar.

 

[RobertoGatos]

I put one of these on my shutter stat incase it ever fails. Not sure if thats your problem, but figured its worth mentioning.

 

[todds112]

I put one of these on my shutter stat incase it ever fails. Not sure if thats your problem, but figured its worth mentioning.

View attachment 536357

That looks like a good idea. Mine failed and was venting all the secondary air out through it (luckily not into coolant system). I turned the secondary valve off at the tank, but that would be much better.

 

[rivercreek]

When you say it builds pressure when cold, do you mean ambient temperature cold, or within a few minutes or hours after running it?
The reason I ask is because most laymen don't realize that an engine reaches it's hottest temperature and cooling system pressure after the engine is shut down. This is do to the coolant not circulating through the cooling system and sitting stagnant thus creating heat and pressure until it reaches is apex and begins cool and pressure resides, much like expansion and contraction of a canning jar.

Jon, it is leaking coolant when cold as in cold cold, as in sitting for a couple days cold. And it's winter, to boot. I can't imagine any substantial daytime heat expansion this time of year, and the system stays pressurized for days and days after its last run. Thank you for the response though. Not afraid to admit I need all the help I can get. I'm reasonably convinced that the problem is connected to the compressed air system - although it still remains to be seen exactly where they are interacting. I still have not had the opportunity to get ahold of the truck long enough to do the troubleshooting I have been previously advised to do. This time of year it can be very difficult for me timewise to address a "non-critical" problem with the trucks. I've got 3 - 923A1's, an A3 Deuce, and a couple of smaller trucks on the road for snow removal with the state this year for snow removal and it's been an extraordinarily frustrating year so far with "weird" truck problems. As I mentioned previously, my plan is to replace the shutter-stat with a known good one to eliminate that as a possibility before I dig into the compressor. One related and perhaps premature question that I'd really appreciate some advice on is this: Compressor head gasket OR reman compressor? It is a low mile/hours motor - so it's unlikely to me that the pump is worn out, it is working just fine. If the problem turns out to be a compressor head gasket, is it as simple as it appears to replace one? It seems to me like a reasonably more substantial job to replace the compressor (injector pump removal, etc.). Anyone with experience with both have an opinion on this? Now, I'm making the assumption that the compressor head gasket can be replaced while on the truck. Am I wrong? Just putting my eyeballs on it, the head gasket seems a very simple procedure, although not one covered in the TM that I've been able to locate. The U.S. Government seems more inclined to simply replace the part in question and just bill it back to us, the little people, as opposed to repairing said part. The only potential problem with replacing the head gasket on the truck that I see is that the front inside head bolt is pretty close to the bottom of the intake manifold - will this bolt clear the intake allowing me to remove the compressor head with the compressor still installed on the truck? I'm looking at a different truck now - or i'd have pulled it out just to check. Not trying to remove parts from a perfectly good compressor on a different truck to check if I don't have to... Also, Roberto, that shutter-stat valve is a great idea - a good, cheap failsafe that I think I may do to mine. Thanks again all for the input.

 

[rivercreek]

Post update: I finally found the problem. Turned out to be a bad shutter-stat. I replaced it with a known good one and that took care of my problem. No more cold pressurization. One interesting thing that I found while trying to troubleshoot the compressor - it has an air check valve disallowing tank air to flow back though the compressor. Had I known that previously, I think that it would have excluded the compressor as being the culprit. Am I wrong on this? Thanks again all for the input and advice - couldn't have done it without you.

 

[DIESEL GUY]

I'm glad after reading this entire post you found your issue. I'm new to this site and thought I might offer some info for future reference. Yes the air compressor could cause the very same issue. The check valve you are referring to is downstream of the compressed air. When the head gasket fails on the compressor it will force air into the cooling system. Usually causing a much more dramatic pressure issue though. Actual brake system pressure (120 - 130 psi) in your cooling system can create some serious leaks. In addition this issue doesn't go away once the engine is warm. Unless the compressor stops at governed pressure which wont happen because half you system air is leaking into the engine. Compressors fail like this all the time so I felt this might help someone experiencing similar issues.

 

[rivercreek]

Thanks for the response diesel guy. So I am clear on what you are saying, this head gasket leak forcing compressed air into cooling system could only occur while engine (compressor) is running, correct? The check valve that I mentioned should prevent any air leakage through head gasket while the engine is off with the exception of whatever pressure is in the line between the check valve and the compressor is my thinking. Although I have corrected my current problem, I'd still like to know for any future issues. Thanks again and welcome to Steel Soldiers !