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Cat Man
Cat Man, Caterpillar Engine Expert
Category: Heavy Equipment
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Experience:  23 Years Caterpillar Engine experience.
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Peterbilt C 15 I have excessive pressure in my overflow tank

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I have excessive pressure in my overflow tank displacing water and causing a overheating problem on a C15 cat engine s/n mxs97350

Hi. Thanks for using JA! I will try and help you out.


Does the engine push the water out under load only?


Or can you raise the engine to High Idle and it push the coolant out?

Customer: replied 5 years ago.

This vehicle pulls rail road cars at slow speeds but under full load and have noticed it then, but also when driving the truck down the highway it will loose water also. This problem started after the fan exploded and tore through the radiator, I have no clue as to how hot the engine may have got when that happened .The radiator, fan, tensioner,and thermostats where replaced at that time.Since then we have had this problem.

Thanks for the information.


I have seen many MXS engines push water out over the years. Most of the time it is a blown head gasket or cracked liner flange.


The reason I asked about the problem happening under load is because if you can get it to push out coolant at no load high idle, you could possibly have an air compressor with a blown head gasket. (I know you have bypassed the AC)


Assuming the Air Compressor is OK, the next place is the JWPC or Jacket Water Pre Cooler. I think you referred to this earlier. It is possible that they can fail and boost pressure pump up the system. However, if this is the case, you should be able to pressurize the cooling system and see coolant in the inlet piping.


If the JWPC is OK, I would think you most likely have a blown head gasket or cracked liner flange. I have seen these happen after an engine has been run hot before. I am not sure why this did not show up in your combustion leak test but I have seen it not show up before and the engine end up having a blown head gasket.





Please let me know if you need more help or have more questions.

Customer: replied 5 years ago.

If I have a blown head gasket should'nt I have found combustion in coolant? The cracked liner flange that you refer to, would it be visible upon head removal or is it where the liner meets the lower step in the block? I'm going to try to pressurize the cooling system to check the JWPC an will let you know. Thanks!

From my experience, if the head gasket is severely blown you would have found combustion in the coolant. But when the head gasket has just a small leak, it can go undetected.


If the liner flange is cracked, it is most visible under the top flange. The liner would need to be removed to see it.


I normally remove the head and inspect the head gasket. If it is not blown, I then send out the cylinder head and have it checked for cracks. If it is not cracked, I then remove all the liners and look for cracks.


I agree that you should inspect the JWPC first. You may get lucky and see coolant or at least a white residue in the pipe area that lets you know there has been coolant in there.



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