Radiator FULL, but reservoir nearly EMPTY after adding coolant

I noticed recently the coolant reservoir was nearly empty in my 2008 Civic. Checked radiator, and could just see the coolant at the bottom of the neck, which I understand to be normal. Temp gauge reading normal.

Thought I had a possible leak, so I added some coolant to the reservoir last night and thought I’d see if it drops gain. Drove about 8 miles (most trips I take are pretty short).

This morning, checked and reservoir again nearly empty. BUT when I checked the radiator, there was coolant almost to the TOP of the neck.

So it SEEMS like coolant is flowing into the radiator, but maybe not flowing back? Idk. Trying to identify possible issues before I take it in. Any ideas? My knowledge of cars is pretty low, so maybe this is normal and I just don’t understand how the coolant system works.

Also trying to assess if this is a “don’t drive your car until you get checked” sort of thing, or if it is ok for <10 mi trips.

The radiator cap controls coolant leaving and returning to the reservoir.

Try replacing the cap.


Interesting, ok. A bad radiator cap would mean loss of pressure, which would lead to the car wanting to pull all the coolant into the radiator?

The pressure cap is on the expansion tank.

If the cap is weak, it can allow coolant to escape as a vapor.


The radiator should bee completely full, reservoir is usually around half. There normally marks on the reservoir indicating cold and hot levels.
Follow Testor’s advice, replace the pressure cap, add the appropriate coolant to the reservoir to the “cold” mark. Then drive the car. Level should stabilize after a couple of drive cycles.

Ok. I think I may have just been low on coolant then. I think I misunderstood what a “full radiator” meant - I didn’t realize it meant that the fluid should be near the top.
I actually pulled my reservoir out and there was fluid in there actually just below the min level. It was just hard to see because it was so dirty. So I think I just need a little more.

If coolant recently replaced and full, entrapped air can eventually work its way out as you drive, which will cause coolant level to drop in expansion tank. As long as radiator is full, there’s no immediate coolant emergency. After about 100 miles of driving any entrapped out has had a chance to escape, so then (after replacing cap per suggestion above) mark two lines using a Sharpie (felt tip pen) on the expansion tank for the coolant level when cold, and when hot. Coolant should more or less stay at those levels for at least 1000 miles/3 month before needing any topping off. If needed to top off more frequently than that, suggest to post back here.

If the radiator is not full coolant will not flow back to the res but simply stay in the radiator. Fill the radiator and res and try again. Leaks in res are not unheard of either.


Also make sure the heater is set on Hot. That opens the valve between the engine and the heater core so coolant is making a complete circuit. That can help expel air that might be in there, as well as assure the whole system is filled with coolant.

Top up the radiator and reservoir after a complete warmup and cooldown. Check this at least twice.

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Does reservoir have fill line? If so, fill it to that line.