Hello to all!

Well, yesterday I pulled a bonehead move.
I was driving along the interstate when I saw that the coolant light on the dash was on very faintly. I pulled over at a rest area, pulled out my coolant from the trunk (about a quart left in the jug), and like an idiot (using a towel 'cause I knew it was hot) unscrewed the coolant reservior cap and proceeded to boil over ALOT of coolant. Luckely, I didn't burn myself.
I went into the rest area and filled the jug with one quart of coolant with tap water and poured it into the reservior. I took over a gallon and when I got home I noticed that it was 'boiling' in the reservior due to overfill. Since the incident, I've driven about 40 miles and the temp guage never got above approx. 90 degrees.

Questions:
The bulk of the coolant in the car is tap water. Is this safe?
Is this the perfect time for a coolant flush? I've never performed this service in 150K miles.

Nick

Don't know why this isn't in my bio but the car is a 1991 300E with 200K on it

Sounds like the coolant was about 210*F when you took the cap off, and then boiled when you removed the pressure.

If it was boiling in the tank with the cap on you might have other issues...

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

Yes it would be good to flush radiator.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

I have a 1991 300E as well. Was having same issues. Ended up being a head gasket issue. I replaced my head gasket and a few weeks later the engine failed at 200K. Had to replace the engine and now have no issues with the coolant. Do you have any oil in your coolant? It is common in this year and model. There is a small space between the oil passage and coolant passage on the back passenger side of the car between the bottom of the case and head where the gasket breaks down. When I took mine off you could clearly see the gasket failure there. Also, my 3rd cylinder back from the front had a bad coolant leak going into the cylinder. It would have probably held up fine but I was afraid to tighten the head bolts anymore than we had. I should have went back in after running it and retightened them.

No, there's no oil in the coolant. My head gasket and valves were done about 100K miles ago.
After the incident I did a complete radiator and block flush, changed the thermostat and some hoses, and replaced the green coolant in the car with a 50/50 mix of Zerox G05 Gold and distilled water. She's been running fine ever since.
I think that there was a misunderstanding in my thread when I posted it. The car had never overheated on me in my 12 years of ownership. On the day in question, the coolant was a little low as indicated by the coolant light on the dash and, like an idiot, I unscrewed the coolant reservior cap when the car was at operating temperature. Everything boiled out...quickly. All I had on hand was about a quart of green coolant and tap water and for the next 2 days the car never overheated but came close.

Nick

You should be fine then. As long as you are not losing coolant any other time. See if it happens again and try to narrow it down.

Good time to drain coolant and refill.Doing thermostat would be great .There is a red drain screw at the bottom of the radiator .Using long screwdriver turn screw but don't remove.drain ,tighten,refill.
If the coolant light glows when the bottle is not low then the switch at the coolant tank is going bad.cheap and easy
Good luck
Rich

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88 300E White, sold 9-04
88 190E Black 2.3 sold
90 300CE White, Grey sold 12-24-04
92 400E White,Grey Sold 5-06
90 300CE 91K Signal Red,Black Sold 2007
1989 300CE Smoke silver,Tan Sold 1-08
1993 300E 3.2 M104 motor
1991 300CE White,Grey
1994 E420 beautiful car,gone to new home
1994 E320 Smoke an Parchment.Beautiful car