Help with coolant issue

A few days ago the a/c shut off while sitting in traffic. This has happened in the past when the air temps get up in the high 90's or low triple digits but this time the air temp was mid 80's. The a/c will usually start working again a few minutes (I assume when the fans kick on and cool down the engine) although this time I shut off the a/c as I was worried there was a problem. The water temp gauge was at the right edge of the "0" - it is usually in the center of the "0". When I got home I noticed a strong smell of coolant. I inspected the engine bay but did not see any signs of wet coolant although there was some residue on the coolant tank cap and below the tank where it appears fluid had dripped down on the headers. The coolant level had dropped from the high mark to the low mark. Once the engine cooled off I topped of the coolant by adding about a quart of virgin unicorn blood. After reading a few threads on here I decided I would start by replacing the cap - which I did. After the drive home the following day I still noticed the smell of coolant so inspected again and see that the expansion valve on the coolant tank is extended while the engine is hot. The temp gauge was still mid "0" but looks like there was small loss of coolant. The a/c worked fine. Is it normal for the valve to be extended when the engine is at operating temps? I am also concerned that the fans may not be operating properly as the rear fan only comes on for a minute after shutting down the car. I believe that the fan used to run much longer than that last summer. I replaced the water pump last fall and have had no issues with the cooling system (including two weekends of DE) until now. I would appreciate any suggestions on how to proceed. Temps are only going to get hotter very soon so want to resolve this quickly. Maybe time to consider the third radiator?

did you also replace the t-stat?

Yes, it's normal for the stem to pop up whenever the coolant is hot. What water pump was used? Genuine, OEM? What year is your car? If you have Durametric, you can activate the radiator fans for both low and high speed.

Ahsai is right on...coolant hot = normal for bleeder valve to be in the up position. Now for a couple of tests in the garage. Start up engine and turn on air conditioner. Both fans, left and right should come on low. Doesn't matter the temperature, they both should run. Is that cap on right and tight? There is a lot of dried coolant around the top of the expansion tank. Check it and re-seat tightly. Take some clear clean water and clean that up in the driveway. It may be that your expansion tank has a leak and the coolant is leaking down on the headers and evaporating. Next step would be to pressure test your coolant system.

Agreed with what has been suggested. I had a similar issues a few weeks back. Changed my water pump, put new coolant in, it was about 40F outside, drove to FL, drove it around when it was warm out, coolant expansion tank cracked, which was less than a year old. I then replaced the expansion tank with a new OEM one, and I believe my "new" expansion tank cap had something to do with it, but don't know for sure. I replaced it also with a new one. Everything is working fine again.

Is the relief valve working correctly? Does it lift and open when temps get too high. If it doesn't then something has 'got to give' in the cooling system and the weakest seems to be the cap or the tank. If your temps are rising for no good reason then consider checking for crap on the radiator grills and condenser. DIY is quite straight forward on this job. If I were you I would do that first before replacing anything. You don't want the system to ruin another cap, or coolant tank. Check the relief vale is working firstly. https://www.youtube.com/watch?v=Um-h2aUJHY4

Thanks for the quick responses. The car is a 2004 C2 40th AE with 80K on the odometer. The engine was replaced at some point (with a factory motor) but have not been able to track down any records of when that occurred. Water pump was OEM replacement. This also makes me think the engine was replaced early in the car's life as I would not expect a water pump to fail with low miles. The relief valve seems to operate correctly - it does drop down when the engine cools off. I replaced the cap on Tuesday and tightened very snug. I will clean out the residue around the fill opening to check for any future leaks - thanks for the tip. I did pull the front bumper a couple of months ago and cleaned out all the crap. I have not replaced the t-stat. Don't know the age of the current one as I bought the car a year ago. Maybe all is OK now and I am just smelling some residual coolant. Also possible that the cap just failed and the new one will resolve the issue. I will monitor the coolant level closely for now. Thanks for the great input! +996 to the power of this forum!

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FWIW, I had a recent issue (low coolant icon in dashboard>>>car spewed coolant>>>low coolant). After troubleshooting by my indy, bad t-stat was the culprit

After another day of driving the issue escalated as I was starting to see dripping fluid. I was pretty sure the tank was leaking so called my local indy to get a new tank. Kudos to Michael Cawley of European Auto Technics - I called him to order the part but he insisted I bring the car to his shop so he could diagnose the leak. I arrived an hour later (on a busy Friday afternoon) and he had me drive the car into the shop where he immediately started checking the tank and hoses for leaks. He found a small crack in the tank that would leak intermittently as well as finding another large crack in the primary supply hose. The hose was not leaking at this point but obviously would have failed in the near future. He ordered the parts for me, had them at his shop by 5:00 pm on the same day, gave me a few pointers on changing out the tank, and sent me on my way. No charge for the diagnosis. This is the second time he has gone over and above to help me with DIY repairs. Outstanding customer service! Changing the tank and hose was an adventure. Getting the old parts out were the most challenging, including taking a shower in coolant when the edge of the crawler caught the edge of the drain pan and flipped it over on me. At least the engine was cool when this happened. Also had to lower the engine quite a bit to get the necessary clearance on the tank. New tank and hose went back in without much loss of blood and a surprising lack of vocal displeasure. About 3 hours to get it all apart and 30 minutes to put it back together.

Great news!