" It has been very hot" In this kind of heat, your AC is not weak or needs recharged, it's probably like everyone's car, the system just works to hard in this kind of weather The puddle under the car is normal, but the expansion tank is another story. With the heater system off your heater core is probably ok, or like you mentioned, the carpet would be wet.
Have you ever changed the CABIN FILTER ? As for the smell, that's where I would start 1st, It's behind the glove box. I would suggest testing the radiator cap under pressure, Autozone will probably let you use their tester. JMHO/good luck.
David g.

 

Cabin filter seemed ok - it's only a year old - no smell and it was pretty clean. I will look at the cap thought - thanks.

 

Good advice from Southwind on checking the radiator cap.
Other common leaking points are heater hoses and the two main radiator hoses and where the plastic radiator top meets the metal fin area.
If the expansion tank was empty, make sure you 'burp' the cooling system after refilling it.
The smell going away when you turn the AC off could be a BIG hint for the cause and hopefully someone in this forum will have the answer.
Can you smell it if you turn the AC off and the heat on?

Buffalo4

 

thanks - I will take another look there. I ran the heat for about 15 minutes today and no smell - and heat worked. oil appears ok (but a little light) and no smell. there was some moisture at the tailpipe. both make me think head gasket.

I don't have a compression gauge or that would be my next look. I'm calling a shop tomorrow.

 

heater matrix failure would cause the symptoms you describe in your first post. It can also happen if you have windshield washer antifreeze left in that system which gets into the hvac inlet after cleaning windshield

 

thanks....looks like a big job to remove the dash. I'm going to start with my local guy and see if he can confirm or rule it out. The head gasket almost seems easier. I doubt he will want to take on either job.

 

Moisture in the tail pipe is normal, esp until the engine has been at operating temp for quite awhile.
No smell with the heater on and a smell when the AC is on is interesting.
Try running the AC in the recir mode. Could it be a different odor other than coolant?
If you remove the radiator cap (engine cool) and run the engine, you can look for a stream of bubbles coming out the filler neck which could indicate a head gasket problem.
Still, I have no reason to believe that a head gasket leak would cause a coolant smell only when you are using the AC, unless you only get outside air in with the AC and inside air in with the heater.
I have no other ideas.
Buffalo4
PS: Remember the KISS principle.

 

Thank you sir - just what I needed. Car is here with me today, I'm going to let it run for a bit with the AC on recirc (or MAX from my GM days)....I'll report back.

 

coolant didn't really bubble - but rose up pretty quickly....no smell in or out after running AC on recirc for about 20 minutes. so maybe I'm crazy and looking for a problem that isn't there. I just can't explain why it would be low almost a 1/3 gallon.

 

Have your radiator cap tested or just replace it and/or look for some hints of coolant leakage near the top of the radiator where it joins the plastic top tank. That is one of the main areas to leak.
Since there is no smell on recirc, it seems like it is coming from the intake section in the cowl.
Possibly as Rocky suggested, it may be from the windshield washer fluid.
Keep monitoring the coolant level.
Buffalo4

 

Thanks appreciate you working through this with me. I'll order a new cap - it's only $20 for the honda part.

 

I'd check the coolant reservoir for a leak.

 

I was just wondering if like Buffalo4 stated, were you running your AC inside (rec) air, or outside air. Since you have an empty overflow jug, and if you had AC on outside rather than (rec), it is possible that you were smelling a leak under the hood somewhere.
I rarely use outside air on AC/Heat for the simple reason that you pick up all the sulfur/diesel smell from traffic. Just a thought, your coolant leak must be found.
David g.

 

I punted and took it to the shop. I could not find a visible leak under the hood. Hopefully they can find it.

 

Since it only smells when it is on outside air, perhaps sniffing around under the hood, when the engine is hot, may find the cause. That should eliminate the heater core and the evaporator.
At least you aren't losing sleep over a possible head gasket leak.
Buffalo4
PS:If you ever do discover the cause, please post back.

 

Thanks again for all the help. I figure the leak will eventually get worse (if there is one).

Last question (promise) - does the evaporator use only Freon (no coolant)? just looking at the hoses it appears they come direct from the compressor. I only ask because I hear a loud hiss near the compressor when the engine is off and cooling down. I originally thought this was the source of the leak.

 

Freon only, the hissing sound is pressure bleed off in the closed system. Let us know after you get the new radiator cap on. If it's leaking pressure while running, it will push coolant from the burp tank. JMHO and good luck.
David g.

 

A hiss near the compressor is fairly normal after shutting off the engine if you have been running the AC. I believe it is just the pressure equalizing between the high side and the low side of the compressor circuit.
And, as Southwind says, the compressor only used R-134a refrigerant and no coolant, such as antifreeze.
Buffalo4