M939a2 overheating/excessive coolant pressure

[Ryanpowell167]

I've got a 1991 m923a2 with the 8.3 cummins, if overheated on me a while back and I flushed the coolant, because it was full of rust...ever since then it has been pushing coolant out the cap right after it is started, and it overheats if you try to drive it. I am a diesel mechanic but every thing I work on is light duty... I refilled it with all the air ports on the heater and after cooler open like it say and there is no air pockets, remove belt and try again, still build excessive pressure at idle, I can tell by pinching the overflow line and holding my hand on degas bottle and it builds pressure. So I know it's not the water pump, tore it down yesterday and replaced the head gasket, retry and it still is building pressure as soon as it's started. I'm not getting coolant into my oil and I'm not burning any. So today I had this epiphany that my shutterstat could be leaking air into my coolant, causing the over pressure and ultimately the overheated. I took it off and hooked 140 psi shop air to it and put it in a bucket, no leaks. So at this point I am stumped....I used a combustion gas tester to see if there was any combustion gas in the cooling system and I could never get it to show any so I'm thinking I may have something leaking somewhere that I don't know of. Like maybe the compressor is water cooled or something stupid....

 

[Suprman]

If it is overheating something is definitely wrong. The cooling system is much larger than that engine actually needs. Is coolant flowing? There are 2 thermostats it's possible if the system was gunked up they could both be stuck. How much overheating are we talking here? If the engine is damaged from overheating then running it again and again will just cause more damage. Maybe time to pull the head and see what's going on in there. It's not that bad of a job.

 

[Ryanpowell167]

Replaced the head gasket yesterday, old gasket appeared ok. The root cause of the over heating is the over pressure I'm getting because i def should not be able to feel pressure built on cap at idle right after I start it.

 

[Suprman]

No I don't have pressure build up in my cooling system even after running for some time. If I remember correctly the air compressor only has oil cooling lines not coolant. Have you taken hoses off and tried to peek into the radiator? I don't know what else would pressurize the coolant other than combustion pressure. Is it blowing coolant out the overflow vent?

 

[goldneagle]

I remember reading a similar issue where a gasket leak in the air compressor was pressurizing the engine cooling system. The member complained about air bubbles coming out the overflow tank. Check the air compressor for leaking head gasket. If I am wrong please correct me.

 

[Ryanpowell167]

That sounds feasible, how might I rule that out, is there a way I can disable it and run the engine?

 

[goldneagle]

That sounds feasible, how might I rule that out, is there a way I can disable it and run the engine?

Is your fan clutch kicking in when engine temp is higher than 190? Do you have a infrared thermometer to test engine and radiator temperatures when "overheated"? Your temp sensor could be defective. It's a common issue.

Basically is the water boiling or bubbling. One is temp related and the other is air in coolant.

 

[73m819]

You could have a pitted liner. A pitted liner will put air in the coolant causing it to run hot and will pressurize the cooling system even right at startup.

 

[goldneagle]

You could have a pitted liner.

He said there was no coolant in the oil. Would a pitted liner allow contamination both ways?

 

[Suprman]

I think he checked for combustion contamination in the coolant. There is a tool mechanics can use for it. I think it's time for a visit to the TM.

 

[73m819]

A liner has COOLANT on the other side for most of the liner length, With as much RUST the OP said he found, there is a very good bet that he will find that he has a bad liner, if one is bad REPLACE all 6

A pitted liner WILL cause the events that the OP stated, if he has gas showing or not, there is not many causes that will cause these issues

 

[Suprman]

Wouldn't the combustion test fail then and show contamination in the coolant? Maybe that test needs to be redone.

 

[Coffey1]

It was butch's 944 and coolant does go to air compressor and will pressurize the cooling system.

 

[Ryanpowell167]

Tested for combustion gasses in coolant several times. Pitted liner would cause this however with no combustion gas detected in coolant, sort of rules that out, especially with no water in oil and no oil level growth. When the engine is shut off the higher pressure coolant would fill the crankcase and or hydro lock the engine. I will try to mess with the air compressor tomorrow

 

[73m819]

Tested for combustion gasses in coolant several times. Pitted liner would cause this however with no combustion gas detected in coolant, sort of rules that out, especially with no water in oil and no oil level growth. When the engine is shut off the higher pressure coolant would fill the crankcase and or hydro lock the engine. I will try to mess with the air compressor tomorrow

WRONG, if the pit is below the combustion point then the rings will be above the pit, if the pressure is going into the coolant, there would be NO co0lant in the oil. Could be a small pit, allowing pressure out but not large enough to allow the low pressure/coolant back. If it is a cracked liner with a small crack, the internal pressure will expand the liner over ridding the outer pressure, causing the issues, but once the engine is shut down, the external pressure will push against the liner, closing the crack, hence no leak.

 

[Ryanpowell167]

If the rings were above the pit then I wouldn't be having gasses enter the coolant because compression rings would be keeping it out. At least it would only be blow by entering coolant which would still set off the combustion gas tester

 

[Ryanpowell167]

I ordered the compressor gaskets, the head gasket and plate gasket was like 3 dollars so I guess we will find out if that is iy

 

[Ryanpowell167]

Well, here's an update. Took head off compressor, there is not gaskets separating a coolant and air passage, so a leak isn't possible unless the head was cracked which I ruled out by running it with the coolant lines off and no pressure built in it. I'm going to dump a bunch of coolant sealer in it and see what happens, worst case scenario is I need liners anyways so no big whoop I guess....

 

[Coffey1]

Was the compressor all gunked up butchs was and its showing on mileage and hour meter low
7K

 

[Ryanpowell167]

Yea it was but it's intake comes from the engine intake so it had some carbon and stuff. The head has a coolant inlet and outlet though and there is no place a gasket leak will cause air into coolant. Like there is not place a gasket touches coolant