coolant level drop no evidence of leak

I just purchased a 2002 C4S with 30K miles...just noticing that the coolant level in the reservoir is dropping about a pint or so every day or so...probably around 50-70 miles worth of driving...no evidence of a leak under the car...no smoke...no smell of anti freeze...I assume the leaking is only occuring under pressure and that it is leaking onto something hot and evaporating. The weather has been particularly hot as well...and I wonder if having the a/c on in the car has any affect. Notwithstanding the drop in level, the car temp is running absolutely perfect. Any thoughts?

You should never drive it below min mark. If it is low enough you risk introducing air pockets which can cause engine cracks (and much larger problems). It is possible there is air in your system and since you've added more coolant now it is able to purge this air out, thus requiring more coolant. However eventually it should "fill up" and you will be done filling it. Or it is possible you have a leak somewhere, in the tank or a hose. When the car gets hot enough the bleeder valve opens by itself thus allowing the system to bleed itself to a certain degree. Or on a cold car you can manually pull up the bleeder valve to ensure it gets bled (although there are more exact procedures on doing this properly - you should search/read up on the subject). If you have a vacuum system (such as uview airlift) you can test for leaks (it should hold 25psi, if not there is a leak and hairline cracks should become visible) and also use it to refill the system properly without any air. Or take it to a shop that has a vacuum system.

Bad reservoir caps are well documented sources of coolant loss. Check your cap number, and if it ends in less than 03, replace it. I think they're up to 04 now. Cheap and easy.

check your oil dipstick if it doesnt look like a chocolate milkshake- highly possible you have an air pocket that keeps getting less...in which case you should not drive the car...you should have the air removed!!

Check the bottom of your reservoir, use your finger to wipe it Therea may be a hair line crack, which only leaks out during high pressure, and it dried right away because of the high temp of your engine.

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I agree. Our coolant tanks are notorious weak links in the cooling system as are the coolant caps. If your tank is yellow it's probably an original.

Thanks for the advice....will check and update.

My car lost coolant for months before the crack in the tank was big enough to leak all the way down to the ground. Prior to that I think it was leaking right on top of the exhaust and boiling away before it ever made it's way down. I had an Indy shop do a pressure test and they came up with zip. Get on the ground and look up under the drivers side tail pipe for stains and evidence of leaking coolant.

Methodically, check: 1. Coolant Expansion tank cap. I replaced this after my coolant started dissapearing. But no luck 2. Coolant Expansion tank itself. I replaced this following 1. It was cracked, but coolant still was dissapearing somwhere, but at a much slower rate 3. Had to replace both radiators. Coolant never hit the ground, but the plastic endcaps/connections were leaking ever so slowly. Now it's ok.

Easy quick thing to do is make sure the caps seal doesn't have imperfections, dry rott, cracked or caked on stuff between rubber and plastic. Dirty rubber will let gases escape even under low pressures.

Just topped up the level a little this morning...to the middle of the arrow by MIN on the reservoir...I drove around town for around 10-15 minutes to the point where the car was fully warmed and the fan came on. When I got home, I just checked the level and was a little surprised that it was all the way up at the lip of the top half of the reservoir...just below MAX...I wonder if this is just how this car operates. I was told that some 996s do purge at a level where you would think it was low. Anyone else have this situation. I am wondering if it is not a leak at all...

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My experience with a properly operating cooling system is with the A/C on the engine coolant runs cooler than it would otherwise. The reason is the A/C turns on the fans and these run continuously. The fans running continuously while the A/C is on keeps the coolant temp lower and at a more consistent temp level than relying upon the fans to come on when the coolant temp climbs to 212F. Sincerely, Macster.

What's strange is that the temp gauge stays right in the 160 range...even when I sat in over an hour of heavy stop & go traffic recently....the only movement is the level of the coolant in the reservoir....as I listed above, I noticed that after running for a short time...a/c off, the level in the reservoir expanded from in the lower portion of the arrow by the MIN designation to the top of the lower portion of the tank right below the lip of the cover of the tank, by the MAX designation. I am wondering if the operating level of fluid on my car is lower than the MIN level and that when it is operating at temperature, the fluid is evaporating out of the bleeder valve?

It's an expansion tank. When the car is hot the coolant level should be near max. When cold near min. Thats the normal range.

Change, water pump, thermostat, expansion tank and cap and hoses around the engine. VUALA! Thats what i did ... haha

Get a used oil analysis kit ( Blackstone labs or?) send off a sample of your oil. High Sodium content indicates coolant in oil.

The area of coolant leak leaves residue behind, that is the good leak indicator otherwise have the cooling system is pressure tested. Could just simply be the water pump going bad, happened on my other cars where a leak is not present under idle. I would also change the oil for a piece of mind and look for 'water' droplets since you just bought the car. I assume a PPI inspection was not done before you bought 996. Good Luck.

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The in-dash temp gage may not be that sensitive. They are generally dampened. Unless you are monitoring coolant temp using an OBD data viewer/code reader connected to the OBD2 port and comparing the actual coolant value to the in-dash gage's needle location you don't know what the temp really is. Also, if the car is sitting idling with the A/C on this will keep the radiator fans on all the time and the coolant temp will remain rather low and stable. Even if the A/C is off the fans come on (at 212F is my info) and run until the coolant temp drops to 205F. This rise and fall of temperature may not register all that much movement at the in-dash temp gage. The coolant level should be near the max line with the engine up to temp. When the engine cools down the coolant level can drop. Sincerely, Macster.

the water temp and the air in the cooling system temp might be differant too..if there is an air pocket near the sensor what are you measuring

Thanks for all the advice. Will follow up as I find out more.