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Coolant not getting taken in by Radiator

Noticed the heat stopped working in my vehicle, when I first purchased it was slow to heat. At the suggestion of a family member I checked the radiator for coolant, it was empty but the reservoir was at the full line.

I manually filled it, heat still didn’t kick in. Checked the reservoir and now it is past the full line. The engine does not seem to be running or cold according to the temp gauge on the dash. Looking for suggestions on where to start.

Quick update:
Replaced the radiator cap, checked the coolant hoses going to the heater core and both are equally warm after running. The 2004 Vitara doesn’t seem to have a heater control valve to check.

Did you purge the system of air?

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The problem is probably with a stuck open thermostat.

First, you shouldn’t be able to touch the heater hoses because if the thermostat were working, you would have gotten burnt.

Second, it the thermostat is stuck open, the coolant doesn’t heat up enough to create the pressure to force coolant into the reservoir when the engine is shut off. And if you don’t create pressure, you can’t create a vacuum to draw coolant back into the radiator when the engine cools down.


How long have u owned it? Heater was ok at start and now radiator is empty and heat is gone but gauge says motor is not overheating? With little coolant?

I’ve not purged the air. I’ll give that a shot

I’ve owned it since October so just a few months. The heat worked at first but only after driving it for a few miles (never at idle).
The radiator currently isn’t filled like it should be to the top but is not overheating per the gauge and I’ve not had any other kind of performance issues that I’ve noticed.

Are you sure you stated that the way you meant to?
The coolant only creates pressure when it’s being warmed up. It creates vacuum (not pressure) when the engine is shut off and the coolant is cooling and contracting. It doesn’t need to have had to create pressure in order to create vacuum. Fill a plastic water bottle with hot water and put it on the fridge (not the freezer), As it cools it’ll contract and pull the sides of the water bottle in without ever having had to create pressure.

Then we should get rid of what’s called the pressure cap?


The pressure cap allows the system to build pressure in order to elevate the coolant’s boiling point. It releases the pressurized and expanding fluid into the reservoir above (typically) 15 to 16 PSI, closing again when the pressure drops below the 15/16 psi, keeping the system at a predetermined pressure.
It allows free flow of the coolant from the reservoir back into the engine, enabling the engine to draw coolant back into itself as it cools and the pressure and volume drops. Heated matter expands, cooling matter contracts. It happens with all matter. One of the rare exceptions to this rule is water as it freezes.

So? If the thermostat is stuck open where the coolant doesn’t reach the proper operating temperature, do you think cooling system is going operate properly?


Meanwhile, is this another time when a radiator cap might need to be replaced? A leaky cap will allow the coolant to boil away, won’t maintain the vacuum to pull coolant from the reservoir, and will end up with what the OP described.

  1. what the heck has this to do with the question at hand? Or to my posts?
  2. no, it will not. The radiator cap may operate properly, but the engine won’t build enough heat for the coolant to exceed the radiator cap pressure and there’ll be no movement of the coolant to the reservoir. And the coolant will boil at too low a temperature and the engine will overheat.

What will happen with the T-stat stuck open, assuming the radiator cap is operating properly, is that there’ll be no movement of coolant from the engine & radiator (the cooling system) to the reservoir and no drawing of coolant back into the engine. The coolant in the system will expand as the engine warms up to whatever temperature it reaches, but the pressure created will be maintained by the radiator cap’s ability to hold pressure in the engine and radiator. As the coolant cools and contracts after the engine is shut down, the pressure will drop, but not to a level lower than that of the coolant in the reservoir (at ambient). Since the pressure in the engine & radiator won’t drop below the pressure in the reservoir, there’ll be no drawing of coolant back into the engine & radiator.

In summary, the engine and radiator will always be at the same pressure. If it gets greater than 15-16psi above the pressure of the reservoir, the radiator cap allows it to release some pressure (coolant) into the reservoir which will always be at ambient. The engine will operate normally with the pressure higher than the reservoir. As the coolant cools and contracts, the radiator cap allows it to freely draw coolant back into the engine to prevent a pressure lower than that of the reservoir from developing.

Truth is, it is only if the pressure in the engine/radiator (the system) drops below that of the reservoir (ambient) that coolant is drawn back into the engine. If that doesn’t happen, everything remains stable. If the night gets cold, the coolant in the system will contract and will draw coolant back into the engine.

Another exception is rubber.

Also, water actually starts expanding when it gets below 39.2 degF.

Hey tester, I just purchased a coolant machine like yours posted in another post. Mine was taken apart when I bought it for a whopping 30.00…lol do you still have your manual or can you take pictures of how your hoses hook up on the inside? Any help would be greatly appreciated.

Replacing the radiator cap allowed the coolant to move to the radiator properly but I was still getting no heat
Purged the air, still no heat
Replaced the theromstat and purged the air, now i’m getting overheating on the engine (and lukewarm heat from the vents, guessing from the engine running hot), going to check the theromstat as the gasket wasn’t as snug as it probably should be.

Side note: I’m getting no coolant leak from what I can tell and no smell of coolant while in the vehicle so fairly certain it isn’t a heater core issue.