Radiator Fluid Disappearing like a Magic Trick

About a month ago, I smelled a very faint smell of radiator fluid inside the car while the air was on. This happened twice, once on the highway when RPM’s were running fairly high/engine temp was slightly higher than usual, and once on sidestreets when I was driving at high RPM’s/in lower gears than normal (manual transmission). When I stopped the car it cooled down to normal level immediately and smell went away. I added coolant to the reservoir.
Then I noticed nothing for about a month, so I figured everything was OK.
Last week, my engine was running hot (usually is at 50% on the temp gauge, it was at 75%, where 100% is red zone), driving at low speeds. About 5 minutes before that, I had the heat on but noticed the air blowing in the car was not heated even though the engine was warmed up (I assume due to lack of radiator fluid flow through the engine). The car did not overheat, because I pulled over and added water, and later added some coolant. During that incident I did not notice any smell inside the car. At the end of the week, I noticed the car running slightly hot and checked the reservoir. It was low–most of the coolant I added had disappeared in just one week.

Usual things to check:
-radiator and all hoses are new
-no leaks under the car
-no white exhaust/smoke or smell outside

Other things about my car: It is a 1997 Honda Civic. Has never had any major problems, and it’s running smoothly, but it does burn oil. The first time I noticed the smell, it was just after I had driven on the highway with slightly low oil level (still on the dipstick, but slightly below the bottom dot. Not too far on the highway–about 20 minutes). Not sure if that could be related.

If you smelled it inside of the cabin then you need to worry about the heater core leaking. But it can also leak elsewhere without leaving puddles. Slow leaks, leaks only while running, leaks that leak onto other things and partially evaporate and turn to grime before they drip anywhere. Even though the hoses are new don’t ignore them, particularly where they clamp onto their connectors.

Having the cooling system pressure tested by a shop would be a good idea. If you want to do more yourself before a shop get the car running up to temp, get it up on ramps or something and - carefully - have a good look around.

If there are no visible leaks, and there is no evidence in the motor oil of coolant contamination (the oil would essentially look like a chocolate milkshake), then the only likely source of the problem is a leaking heater core. And, if you are smelling coolant in the car, that would point to a leaking heater core.

When heater cores begin to leak, it is possible to have all of the leakage vaporized by the HVAC system, which is both the good news and the bad news. It is good in that it is not leaking on your leg, but it is very bad in that vaporized coolant is very dangerous to breathe. Trust me–I found out the hard way…

One thing that concerns me is that you don’t mention ever opening the radiator cap. If you are only adding coolant to the overflow reservoir, it is entirely possible that the cooling system is chronically low on coolant which would explain the lack of heat under some circumstances. However, the really serious effect of chronically low coolant is that you could easily wind up destroying the aluminum engine from overheating it. So, in addition to opening the radiator cap (when the engine is stone cold) and checking the coolant level, I hope that you have a mechanic assess the situation a.s.a.p., lest you wind up with pneumonia from inhaling vaporized coolant.

If it turns out that you need a new heater core, then you will have to decide whether to spend some big bucks on repairing this 17 year old car. While the cost can vary from one area of the country to another, and from one mechanic to another, I wouldn’t be surprised if it cost over $600 to replace the heater core on a Civic. The heater core itself is not expensive, but you have to remove the entire dashboard in order to access it, and the labor costs will be…very high.

I really only smelled the radiator fluid inside the car that one time, and I’ve been paying a LOT of attention. Is that really the only explanation? Could there be a leak somewhere near the input of the air system that could explain that very faint smell? (On my car, the air input is near the radiator and reservoir). I just feel like that’s not it since I only smelled it that one time…
Questions about the heater core:
-if it’s the heater core, it would smell only when heat was on, not A/C or vent, right?
-does it give out gradually over time? If so, is there anything I can do to preserve it?
-in my understanding, you can smell radiator fluid at very low concentrations in the air. If I’m not smelling anything, even with heat on, is it possible it’s still leaking/I’m putting myself at risk without knowing it?
I did check the radiator cap, it was low but there was fluid in it (as I was filling the reservoir). I have been keeping an eye on the temp of the car, and it has not been running high, so I think I’m OK on not melting the engine. Thanks, though.

"if it’s the heater core, it would smell only when heat was on, not A/C or vent, right?"
That depends on the HVAC system. Many modern systems constantly blend cooled air with heated air in order to maintain the cabin temperature, and as a result, there is coolant flowing through the heater core even if you don’t have the controls set to “heat”.

"does it give out gradually over time?"
After 17 years, many cars will wind up with a leaking heater core–particularly if the coolant wasn’t changed every 3 yrs. and/or if the proper coolant/water ratio wasn’t maintained.

"If so, is there anything I can do to preserve it?"
Unfortunately, no

"in my understanding, you can smell radiator fluid at very low concentrations in the air. If I’m not smelling anything, even with heat on, is it possible it’s still leaking/I’m putting myself at risk without knowing it? "
Yes, it is possible. One indication of whether you have a coolant leak into the HVAC system is a recurring film on the inside of the glass. If you have that, then you DO have a leaking heater core. If you don’t have it, then the possibility is much less likely, but it is not impossible.

It concerns me that the temp only goes up at high rpm or load, Some head gasket leaks are like that.
If you go to a good radiator shop they can pressure test for leaks and check your coolant for evidence of exhaust gas.

Hi Leveycat. About your 97 honda civic loosing rad fluid. I feel sure it is your heater core, several years ago I had a 88 ford Taurus & the heater core went ,they wanted $ 600.00 to install new rad core So what I did & I would say the same for you. Your honda is 17 years old. Just buy a $ 2.00 tube of that grey looking rad stop leak & you might be good for the life of your older honda. Mine really leaked bad & all of a sudden I could barely see the road as it just coated the inside of my windshield . I added one tube of that sealer ,drove around the block a few times & no more leak. I drove it about another 900 miles & traded it in on a honda accord, & it still didn,t leak.

I agree with @"oldtimer 11"‌ . Sounds like the head gasket.

Coolant odor can appear in the cabin even if the leak source is under the hood. The path to the cabin is via the vent holes underneath the windshield. Still it would be wise to lift up the carpeting near the firewall to see if there is any dampness. That’s where the leak would go if the heater core or a hose going to it was leaking.

Are you able to tell us how much coolant you have to add per mile? If not, start taking notes, as that will be an important clue for the mechanic who eventually fixes this. Good comments above about the head gasket or heater core. Could be something more simple though, like a hose is leaking, radiator leaking, water pump is leaking, freeze plug, engine drain plug, etc. A small leak might not produce enough fluid to show up underneath the car, it would evaporate first. If I had this problem myself I’d remove as much stuff as possible from the engine compartment so I could get in there for a good look-see with a flashlight, see if I could spot a leak someone. If I couldn’t a cooling system pressure test would be next.

Before you get too carried away, look at the heater hose that comes out of the head just below the distributor. If the distributor O-ring starts leaking oil onto the hose, it will swell up and eventually leak at the clamp. If this is the case, replace the hose and the O-ring for the distributor.

Replacing the O-ring is not a difficult or time consuming job and there is NO reason what so ever to replace the entire distributor as a lot of mechanics will suggest. The hardest part is getting the correct size O-ring unless you go directly to a Honda dealer.

The radiator cap was broken. It has a part that senses the pressure, and lets out fluid when the pressure gets too high, which explains why I didn’t see any on the driveway–it only leaked when I was driving. Easy peasy $30 repair, most of that for diagnostics. I LOVE CIVICS!

Thanks for the update!

Glad to hear the solution was cheap!