Ok wife's 2003 300m. Two years ago it over heated and turned out to be thermostat that swelled up to point of not opening. Anyway been driving it I bet close to 25k since then with no issues. Monday she calls me says it's in the red n chimmed it was over heating. Hauled it home put half to 3/4 gallon antifreeze in it. Been running it around here and it's not getting above half on gauge. Ran car with air on high for half hour to point fans kicked on high. Checked oil no water in it. Appears no oil in water. Fans turn on. Top hose is hot. Bottom hose is cooler so water pump and thermostat I'd say are working. Checked temp on upper and it's like 200 when fans are on high and lower is like 125. Hose coming out of overflow tank is like 170. No leaks that I can see on radiator. Doesn't leave any signs of leaks in garage or underside of car. No white steam or smoke our tail pipe. Doesn't have antifreeze smell inside car or wet floor so I'd say heater core is ok. I'm stumped without doing leak down test I don't know. Any other ideas? Been watching fluid levels and haven't gone down. I got the overflow tank marked and it stayed on that mark since did the thermostat before. What am I missing?

What motor?

Sorry. Forgot that. 3.5. I also ran it with cap off watching the fluid level in the tank. It would rise up to about inch or less from top then be sucked down below fill line then minute or so later rise back up. It would just do that. Walked away for a second and it over flowed out of the tank. That mean anything?

faulty thermostate again....

You lost coolant, correct? If it's like the older 3.5s time to do a water pump and timing belt...

It over heated on her while she was in a drive through. Last few days here have been higher humidity & upper 80's. When it boiled over with out the cap on it it had been running a good 5-10 minutes just idling.

The stat I would say is good if it's flowing through the radiator? Monday she left her store drove in city traffic about 3-4 miles to get our son at school left there drove mile to a dairy queen when it over heated. If there was even a minor crack or leaky head gasket that it slowly ate the antifreeze would a person still see some type of traces out the exhaust?

I figured the other time with the stat problem I put in antifreeze that the local napa parts store told me was universal to mix with exhisting antifreeze that was still left in there after I had a hose go bad on her. The hose issue was a good 2 half years ago. Sprung a leak so I put new hose on it and emptied it to put fresh fluid in. Napa didn't have the orange on hand but said this stuff is compatible if you are putting fresh in and there is some still in block you will be fine. Was like 4 months or so later the stat left us stranded. That time thought head or head gaskets but did leak down test with no signs of leaks. Tried other fans not the problem. Flushed out radiator nothing. So pulled stat out and as soon as I looked at it knew had the problem. The whole inside was so swelled up it was not even movable to let anything through. The signs then were the car was in drive through in Wisconsin dells. All of a sudden temp warning chimed and it was in the red. Limped it to dance competition about 2 miles away by driving it til temp gauge came up shutting it off coasting till couldn't coast them let it cool off and repeated that till we got to completion meet. Let it really cool off started it back up ran for few minutes and temp peaked out between half & red in matter of a minute-2. Had to have dad drive all way there to pick us up and hauled it home. Long day left the dells at 5:30 made it back home by 3 am!

Drove car to town about 20 mile round trip. Got back to the shop and had to put about a quart in it to top it off butvit never went above half on gauge. Dad Talked to a mechanic he works with at dealership. He said if it was head gasket or head and not showing up in oil it should be showing white smoke out tail pipe and on start up more than likely have a miss until that plug dried up. Car doesn't. He is leaning more towards water pump or heater core. Said to put a rag in by oil filter to catch any leaking out weap hole and take it out for a good long drive. There looks like signs of antifreeze on the oil filter but honestly can't rule out its from the over flow tank over flowing the other day when I had it running with cap off. Took it out for a 10 mile drive but didn't show any signs on the rag so I'm going to try to drive it farther. He also said could be heater core. To me if I'm loosing this much fluid I think I'd smell it in the car or floor would be wet. There is no wet signs on the underside of the car.

You might borrow a rad pump up kit & get it good & hot & pump it up to 15lbs & see if it holds.

Stuffed the rag drove it a ways nothing. Pulled the plugs none looked like they had bees wet. Did leak down test and couldn't get any signs out exhaust or bubbles in radiator. Drove it another 30+ miles nothing. No loss in the fluid level. So any one have any other idea? I'm lost.I

I have a '98 Neon (daughters car). It had a slow AF leak. Looked and looked and couldn't find it. Topped it up each month for 6 months then one day, right in front of the garage, the radiator let go like old faithful. Pulled it in and pulled the radiator. It had a leak near a corner that was not visible installed. Salt corrosion most likely. A new radiator from radiators.com fixed it up and its still running strong 4 years later.

temp goes up in a drive through.... roll the windows down and put the heat on high and the fan! (...and don't give me that "she's got a baby in the car crap either, it aint gonna kill em!) If you can get the temp to stabilize or go down by doing this it usually points to a restricted radiator. .....and in my experience 3.5's do NOT take kindly to being overheated, and your part of the story of coolant suddenly spewing from the overflow may back up my thoughts of head/head gasket.

You Sir ......er I mean the car has a head gasket gone or head cracked . I had the same problem with my 97 LHS. done the water pump timing belt thing. Would be perfect for a while then next trip somewhere start spewing out the coolant . My personal opinion of the 3.5 is that its a very fragile engine and as said before once it gets hot , its all down hill from there.
You had the plugs out , I bet one plug was "whiter" than the other nicely tanned plugs. sure sign of antifreeze.
I could never smell anything from the tail pipe or any other of the normal signs other than the regular burp when looking down the rad .
If you do the belt an water pump thing and find no issues , its just as well to pull the heads , get them pressure checked and install new gaskets or.....get rid of the trouble .