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Coolant not returning to radiator from expansion tank

Last week during a hot/long day on the road, my engine temp gauge started fluctuating between 1/2 and almost 7/8 on the gauge. The weird thing was that it was fairly stable most of the time, but every 10 minutes or so, it would heat up, and spend bout 15 seconds at the high mark before dropping down again. It didn’t seem to be related to rpm, A/C operation, or driving intensity. When I got home, and let it cool down, I checked the coolant level. I couldn’t see any coolant in the radiator, but the expansion tank was nearly full. I guess the low coolant level can explain the symptoms above, but I can’t figure out what is happening to the coolant.

I took the car to a mechanic and had a pressure test, which didn’t show any leakage. I had a new thermostat and radiator cap installed. They flushed the system, and tested the hose between the expansion tank and radiator. After driving the car home, the coolant in the radiator had already dropped to the point where I couldn’t see it, and the expansion tank was 3/4 full. So, I guess the $350 I just spent was in vain. Now, I am planning to keep track of the levels of both reservoirs (and refilling the radiator of course) to see if the fluid is leaking, or if it is just not returning from the expansion tank. Hopefully this bit of knowledge should help locate the problem.

The successful pressure test leads me to believe that it isn’t leaking externally. However, I’m wondering if there is a small enough leak that wouldn’t be detected by the pressure test, but is significant enough that it would allow air in when the engine cools instead of pulling the fluid back from the expansion tank.

Anybody see anything like this before?

Make, Model, Year?
Some Problems Are Vehicle Specific. Also, Perhaps Somebody With A Similar Vehicle Has Had Similar Problems.

Was the engine hot (operating temperature) when the mechanic pressure tested it? Some coolant leaks are temperature sensitive.

Any residue/staining forming on or below the water pump weep hole?


I had this problem on a 1993 Oldsmobile 88 I once owned. I traced it down to the hose between the radiator and expansion tank. It acted like a check valve. The coolant would be forced into the tank, but the vacuum created when the radiator cooled could not pull the coolant into the radiator. $2 worth of tubing and clamps and 10 minutes labor fixed the problem.

Start saving for a new head gasket.

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There are basically two types of coolant recovery tanks. One type has a tube molded into the side of the tank. On this type, the hose from the radiator is connected to a port on the tank itself.

The other type has a hose that hangs down from the overflow cap and the hose from the radiator connects to a port on the top of the cap.

Problems could be a crack in the molded tube above the coolant level in the tank. If the crack is on the inside, then any drainage would go back into the reservoir and not spill out on the ground, but it would break the vacuum that would draw coolant back into the radiator during the cooldown.

The hose could have small cracks at either end that would prevent a good seal. A new hose is cheap. The hose that hangs down from the cap (if it is that type) could have fallen into the tank.

If the level dropped in the radiator while driving, before cooling down, then I’m afraid that @insightful has some bad news for you.

Did the mechanic pressure test the new radiator cap?

I’ve gotten bad new radiator caps in the past.

I once had to take my pressure tester to the parts store with me before I found a radiator cap that held the proper pressure.


I also agree with @insightful if a new radiator cap didn’t fix your problem.

I had this same problem with my 2000 Explorer. A clamp on both ends of the coolant recovery hose fixed it. I assume the hose was sucking air instead of drawing back in coolant.

I’m with @Insightful; that you should start saving for a head gasket job.

Never continuously drive a vehicle where the temp gauge is peaking that high.
Once, you may get away with it just needing coolant, but to let it keep peaking is
just throwing your money out the window.

At that point calling for a tow…or at least checking the radiator (carefully) level would have been in order as soon as you found a place to pull over. Not the next exit 3 miles away, not home or work…but right there. A minute in the red is enough to do major damage and warp a head.

I even filled mine with creek water once. I figured a few hours flushing the oysters, a little fish poo, and a muskrat were better than having to do a head gasket job.
Damn, those oysters will give you lead in your pencil.


Just wanted to add my experience. I had same problem of overflow tank not returning to rad… doomsdayers were all concluding head gasket. I didn’t cry, although I think they wanted me to. I first changed out the cap with no results. Then, I changed the hose from the rad to the overflow and the hose in the overflow. That solved my problem. The hoses didn’t show any signs of cracks, holes, or leaks…even when I took them off and blew into them, they seemed to be leak free. I’m assuming where they connected to the rad it was just stretched out and no longer had an air tight seal. So much for the end of the world…One new radiator cap, and 3 feet of new hose…less than 15 bucks. Thanks for all of your advice guys. On to the next problem!