Overflow tank filled with coolant, pouring out of overflow hose

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~100° right now, just ran some errands around town, got home and left it parked for 20 minutes and walked out to see a cosmic river flowing from underneath my car. Overflow tank is absolutely filled (I haven’t ever needed to add coolant, I do oil changes myself so no idiot tech to top off the coolant reservoir) what could cause this? Should I get new radiator cap? No signs of oil mixed in with the water
 

bryson

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Radiator cap could be to blame - you mention the overflow is full, but not exactly where the coolant is coming from so I assume it's coming out the vent hose on the overflow?

*edit* Realized you mentioned in the title that it was coming out of the overflow hose.
 
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What does the coolant look like?

Red?
Brown?
Green?
Chocolate milk?

Yes, radiator cap could be to blame if you're feeling hopeful.

Is the cap on the overflow tank oriented correctly? Some have been known to be installed backwards so they will blow out, but cannot suck back in.

What year truck and any maintenance history on a head gasket?

Any recent work to the cooling system? Anywhere else?
 
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Coolant is green. In January I replaced the water pump, no issues since then. I have a aftermarket water temp gauge but yet to install it, need to get my hands on one of those things u put mid coolant hose.
 
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What does the coolant look like?
Red?
Brown?
Green?
Chocolate milk?

Yes, radiator cap could be to blame if you're feeling hopeful.

Is the cap on the overflow tank oriented correctly? Some have been known to be installed backwards so they will blow out, but cannot suck back in.

What year truck and any maintenance history on a head gasket?

Any recent work to the cooling system? Anywhere else?

Truck is a 94. No idea on maintenance from previous owners. I did do the water pump in January just because it was leaking. Aside from that no other maintenance besides oil changes every few k
 

mudgudgeon

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What does the coolant look like?


Truck is a 94. No idea on maintenance from previous owners. I did do the water pump in January just because it was leaking. Aside from that no other maintenance besides oil changes every few k

Based on unknown history, best to
- install engine monitor, watchdog etc
- replace thermostat, and hoses.
-Flush radiator and block. And refresh coolant (also check radiator codes are open, not blocked)
-Check operation of the fan clutch and verify that it is OEM,
-Replace and upgrade the silicone oil in fan clutch, or replace with modded OEM BLUE fan clutch (witsend or DIY)
-Replace radiator cap
 
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What does the coolant look like?


Truck is a 94. No idea on maintenance from previous owners. I did do the water pump in January just because it was leaking. Aside from that no other maintenance besides oil changes every few k
Basically it's either some kind of cap issue (best-case scenario), a cooling system issue such as the thermostat being stuck closed (medium-case scenario), or a HG issue (worst-case scenario).

I would try to use techniques to rule out the first two possibilities as they are pretty easy to deal with. If it's door #3, well I'm sorry my dude.
 
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You should follow the advice above, but I hate to say it... my money is on the head gasket. It sounds like you're definitely getting air into your cooling system, but the system is still under pressure. Most likely cause is gases getting forced into the cooling system from a leaking head gasket IMO.
 

mudgudgeon

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If it is a head gasket, bite the bullet and desk with it ASAP.
Not necessarily the worst news ever.

Good time to freshen up in the engine bay, and do some PM on things like
Heater hoses
Vac hoses
Injector and engine harness
Injectors
PCV valve + hoses
Rocket cover gasket
EGR delete
etc etc

If you're a halfway competent mechanic, it's all achievable without a huge spend
 
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My truck does this occasionally, when it idles for extended periods and it's hot outside. Sometimes it does it when I shut it off- come back 10 minutes later to a green puddle. It may be a bad cap, or head gasket. It could also just be thermal inertia causing the coolant to expand and do what it's supposed to do- go into the overflow- and if there's not quite enough room it becomes a puddle and a panic attack.
 
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Is the cap on the overflow tank oriented correctly? Some have been known to be installed backwards so they will blow out, but cannot suck back in.
From my understanding, this set-up is not capable of drawing radiator fluid back into the system once it is in the overflow, correct?
 

leonard_nemoy

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Coolant is green. In January I replaced the water pump, no issues since then. I have a aftermarket water temp gauge but yet to install it, need to get my hands on one of those things u put mid coolant hose.

Your rig could be completley fine and the tank could just be doing its job.

If you did a new water pump in January and replaced your coolant at the same time than it is very possible that this is the first time your rig has gotten hot enough to actually "overflow" and find its sweet spot in regards to coolant capacity.

In other words you might have overfilled the cooling system in January and it finally got hot enough to push the extra out.

If I was in your position I would let it puke as much coolant as it wanted than wait overnight. If the radiator was full the next morning and overflow tank wasn't below the low line than I would leave it and see if it happens again.


This very thing always seems to happen to me Just once after I have replaced my coolant. But I also don't measure my coolant, I just add as much as I can and I most likely overfill my system. Hence my rig always overflows the first time I really get it hot after a coolant change.
 
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From my understanding, this set-up is not capable of drawing radiator fluid back into the system once it is in the overflow, correct?
They are absolutely designed to draw coolant back in as it cools.

As the coolant heats and expands, it pushes out past the radiator cap, into the overflow bottle. The overflow hose from the radiator neck to the overflow bottle should be attached to the side of the OF bottle with the hose that reaches down into the bottle. As the engine cools, the radiator cap opens and sucks the coolant back into the radiator.

If the OF bottle cap is plumbed backwards it will push out but cannot suck back in and will continue to fill until it overflows.

This sounds like a sudden event, but if you recently did a water pump you may not have purged all the air in the system and hence the sudden issue.

Your thermostat must have the jiggle valve at the top to allow air to purge while closed. If installed wrong, you will get more air pockets.
 
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Not sure if already mentioned but try starting with the cap off and checking for bubbles. If bubbles present a fairly bad sign that the head gasket has failed.
While you have the cap off wash it under the tap then check the spring still works well, also try blow through it from the seal side. if you can blow past the little metal bit in the middle of the rubber cap has failed.
Otherwise when at operating temp check that both radiator hoses feel the same temp ruffly. If one is cold then themostate stuck closed.
Hope this helps
 
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My truck does this occasionally, when it idles for extended periods and it's hot outside. Sometimes it does it when I shut it off- come back 10 minutes later to a green puddle. It may be a bad cap, or head gasket. It could also just be thermal inertia causing the coolant to expand and do what it's supposed to do- go into the overflow- and if there's not quite enough room it becomes a puddle and a panic attack.
Not everyone lives on the surface of the sun like us. Im sure it's a short list of people on here that can say they don't have to warm up a car in the morning.
 

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