DD 8-92 Coolant Tank Overpressure when Cold

[Passages]

While performing a routine engine check, my starboard engine spewed about a half gallon of coolant all over when removing the cap. The engine had not been run in several weeks and was stone cold.

Replaced the cap after things settled down and cleaned up the ER. Did not start engine.

5 days later it had been a month since the engine had run and I figured it was time for an in-slip start up. Repeated pre-start engine check and again pressurized coolant spewed out whencap was removed.
Again, I cleaned the ER and started the engine with cap off. No exhaust smoke on start up and engine ran perfectly. No bubbles or increase in coolant tank level. Increase RPM to 1000, 1250, 1600. Coolant Temp went to about 130 which is about as high as it goes in a no-load state. Slowly reduced RPM to idle and contents of coolant remained stable.

Replaced cap and let sit for 3 days. No repeat of spewing coolant.

I am at a loss for what is happening. How can a cold engine develop pressure? Could a bad thermostat have retained pressure that released over time?

I don't know if this was just a hickup or something much worse.

 

[Captain Ralph]

Do you have a recovery bottle?

 

[Passages]

Do you have a recovery bottle?

No. Just a 7 psi cap and a dump tube

 

[Captain Ralph]

No. Just a 7 psi cap and a dump tube

Is the dump tube clogged?

Sorry to ask, Can you go over this again?
If you very slowly open the cap, Before that last little catch, does it blow out then?
Or after that last little catch when you have to push down a bit, turn just a little more to remove the cap completely?

I don't have a clue,, yet.
But I'll ponder it thru the night.

Answering one of your questions, a thermostat can not hold any pressure to cause this.

 

[SKYCHENEY]

I agree with clogged dump tube. I had one do this. It acted exactly the same.

 

[Passages]

Do you have a recovery bottle?

You might be on to something. I removed the elbow and the dump tube and blew air thru it. That's not the problem. However, the cap has never been replaced. If it's degraded, its possible a piece of gasket is blocking the tube. Off to NAPA for 2 new caps.

 

[Fanfare]

Be sure to get the proper pressure for the caps. I can't check from here, but I think the proper pressure for 92 engines is 7 psi or so, not the 15 you might see for your car.