CAT 3116 coolant under pressure in tank - overflow empty

k9medic

Active Member
Mar 27, 2013
597
Florida
Boat Info
1998 420 Aft Cabin
Engines
twin 3116 CATs
We ran the boat a great bit moving it over to the Bahamas at the beginning of the March and yesterday, while in the engine room, I noticed that the overflow/ expansion tank on one of the engines was empty.

I thought this was odd since during the pre trip it was at the "cold" mark. I started to take the cap off the tank to see what was us and realized it was still under pressure!

Thoughts on where to go from here? I'm thinking, take the cap off, try to keep the coolant from going everywhere and see if there is a blockage in the overflow tank line or perhaps an issue with the cap?
 
Interesting - it should never be empty; lower cold and higher hot. If none in the bilge then either a large bubble in the system or the HX is leaking between the raw and coolant sides. On a rarity the aftercooler could be leaking but you would smell antifreeze in the exhaust and the turbo pressure would probably blow out the coolant and overflow the bottle.
Do a pressure check of the system and see what happens. And, pull an oil sample from the engine.
 
I thought about the leaking but the coolant tank is still under pressure and the engine hasn’t been run in almost a month.

Wouldn’t it have bled the pressure down by now if there was a leak?

No engine work has been done.
 
Also Check the oil fill cap and oil dipstick for evidence of coolant contamination.
 
Under pressure. I started to take the cap off and it pushed coolant so I put the cap back on until I can come up with a plan.
 
Under pressure. I started to take the cap off and it pushed coolant so I put the cap back on until I can come up with a plan.
If the engine is cool and the system is still under pressure then it's a bit of a mystery. As the engine cools down the fluid contracts and should pull the liquid level in the overflow bottle down. Overflow bottle is empty and coolant system is pressurized.... I would get that engine oil sampled right away....
 
An oil sample is never a bad idea but the fact your cooling system holds pressure for such a long time means it is NOT leaking. If it was there would be no pressure. I would top up the overflow jug and keep an eye on it. I recently changed the coolant to cat ELC and have been dealing with a similar problem ever since. Toromont Cat tech calls it Extra Leaky Coolant and apparently seeps out of places other coolants don’t.
 
So I went in to check on the lines and now the tank is not under pressure and there is some fluid in the overflow tank.

The main tank is filled to the top though.

No water in the oil.
 
Under pressure. I started to take the cap off and it pushed coolant so I put the cap back on until I can come up with a plan.
So I went in to check on the lines and now the tank is not under pressure and there is some fluid in the overflow tank.

The main tank is filled to the top though.

No water in the oil.
I'd put a new cap on, refill tank and overflow bottle, bleeding off any air while filling, run it and monitor. I'd also send in an oil sample to make sure none is ending up there. Any disappearing coolant after that, then check heat exchanger.
 

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