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Coolant Reservoir doesn't drain back into Radiator

Why would coolant flow from the radiator to the overflow tank when hot but not return to the tank when cool? To answer this question you might should know the following:

- We’ve tried two new rad caps.

- Overflow hose flows nicely. Clamps are good.

- Radiator is pretty new, and pressure tests perfectly.

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There has to be a vacuum between the overflow tank and radiator. A hole in the hose will prevent a vacuum from occuring. Also, a radiator cap that does not seal will prevent flowback when the radiator cools.

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Is the coolant system emptying itself? What is your concern?

You probably have some sort of air leak on the coolant overflow hose. The expanding coolant flows out but when it contracts as it cools, air gets sucked back into the radiator instead of coolant.
Also, make sure the coolant overflow tube actually reaches the bottom of the coolant overflow tank.


What Other Symptoms Are Present ? Is The Car’s Cabin Heat Output Good ? Any Abnormal Temperature Indications ? Any Steamy Exhaust ? Any Coolant Consumption ?

As was discussed there needs to be a vacuum created by a “closed loop” cooling system. I hate to suggest that a failed head gasket, one possibility, will open a closed system.

A cooling system pressure test may be needed to see if the system holds pressure over time and help find the leak.



What year Trooper?

Cabin heat output is great.
No Steam in the exhaust.
After topping the radiator off, driving maybe thirty miles in the heat, and returning to idle, the gauge will begin to read hot. After allowing the engine to cool, I open the radiator, and the last two times the coolant has been low by about 1 quart. The level in the overflow bottle gets high and stays that way until i siphon it out.

Fill the radiator, Install a cooling system pressure tester and pump it up to around 10 psi. Start the engine and rev it up and down, watching the pressure gauge carefully. If the gauge jumps up a little every time you rev the engine, you have a blown head gasket or cracked head…

cracked or broken reservoir. or hose at the reservoir end is loose. is the hose going into the reservoir a one piece, extending all the way to the bottom of the reservoir? or is the hose cracked or split at the top end of the reservoir?

It could also be a leak anywhere in the system that allows air from the source of the leak. Get a pressure test of the coolant system just in case that is the cause.

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Forgive me for shouting…HEAD GASKET(S).

Been there and done that on my '04 Civic that had very similar symptoms.

A cylinder pressure check found 2 cylinders leaking to the water jacket.

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I had the same problem with a 1993 Oldsmobile 88. The hose between the radiator and the recovery tank appeared to be fine, but would collapse when the car was shut off and the vacuum in the radiator tried to suck back the coolant. The hose acted as a check valve. The repair was so inexpensive and simple that either a caveman or I could do it. I did the job since I coulnd’t find a cave man.


Did you solve the problem? I have a same problem for 95 Acura Legend. I replaced a radiator, water pump, thermostat (twice)…some mechanics said it’s HEAD GASKETs, some said ECU.

Along with this coolant doesn’t return to the radiator, also the REV is jumping up & down, and D4 light will start blinking.

What would be a solution for this old car? (to solve problems not dumping it)

I have the same problem. Did you fix it?

One of the things that has not been mentioned is that the radiator cap serves a dual function. It maintains a pressure when the radiator is hot, but also has a secondary valve that is blocked when the radiator is pressurizes, but opens when the radiator pulls vacuum to let the coolant from the reservoir go back to the radiator. If the coolant is dirty, the secondary valve can get plugged. I have a 2003 Venture where I had to clean the secondary valve which is in the middle of the pressure seal stem.

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How many times should he have to replace the radiator cap? After these basic efforts it is reasonable to believe the head gasket was leaking.

It is likely that this vehicle went to the scrap yard years ago, how many Isuzu Troopers do you see lately?

Does anyone know how to unblock the hose valve on the radiator side? I have a full coolant tank, but empty radiator, because it’s blocked. The hose is fine. But the part the hose attaches to is a small metal hose going into the top of the radiator, not far below the cap. I can’t blow air into that part. It seems this is where the blockage is.
My car is already overheating, and I want to fix the head gasket with seal-up, but im worried that even if i do, it will still blow a hole in there as the pressure cannot escape into the coolant overflow.

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You can easily replace the hose, clean the passages, but my guess is you have a leak in the coolant system that does not allow negative pressure to draw the coolant from the reservoir. Have a pressure test done, could be bad radiator cap, garbage on the place where the cap locks onto the radiator leak, or a loss of pressure due to a coolant system leak, hoses, head gasket, heater core, clamps etc.

So you don’t think the little spout that the overflow hose attaches onto is blocked? I can not stick a piece of wire in it, but it also turns at an almost 90 degree angle to go into the top of the radiator tunnel. I also cant blow into it. So I assume that its blocked with rust. Shouldn’t I be able to blow air through that spout into the top of the radiator tunnel? Or is there some kind of valve that only allows a flow at a certain temp? It doesnt look there is any space for anything other than it being a little hole that goes down into the radiator tunnel. That is blocked.

Sure start there