Coolant Temp reached over 250+ on highway.

Asked by Kristian Feb 25, 2020 at 07:14 PM about the 2001 Cadillac DeVille

Question type: Maintenance & Repair

After driving an 1.5 down the highway, I noticed my Coolant Temperature was ascending over 230 degrees, all the way over 250 degrees. It happened right near a Rest Area, where I was going anyways, so I pulled in. After I returned from the Rest Rooms, I checked for a Coolant Leak, and noticed bright green coolant was dripping lightly from my Front Passenger Side mudflap, and some underneath, leaving a good fist sized puddle of coolant.

I decided I should cancel my trip and head back home. So I waited a bit for the temperature to cool down, and journeyed on with the heat on full blast.

The temperature remained normal, and after 45 minutes I stopped for gas, and to check for leaking.  I still noticed green coolant gently leaking.

I kept driving for another 30 minutes, but this time I decided to turn the heat off and see how it did. The temperature remained fine the whole way. I pulled over to check for leaking again, and this time..there was no green fluid leaking. Just a bit of what must be dirty water from the same area.

So I kept driving the next 30 minutes home without the heat on, and the temperature was perfectly normal, between 190-196, sometimes to 201.

I really can't understand how it got up to 250 and leaked or overflowed coolant going down, but it was a normal 194 range with no leaking going back.

Several days before this I had attempted the trip, and after 1.25 hours in my ENGINE OVERHEATING LIGHT notification turned on. I pulled into a supermarket, got some coolant. A bit spurted out when I opened the cap too soon. I let it cool, added some, and kept going. After 25 minutes I was confident it was fine. But the ENGINE OVERHEATING notification came on again, on the side of the freeway. I let it cool, added more coolant, and kept going. Then I started getting a COOLANT LOW notification and decided I would just call AAA, and I got towed 2 HOURS home.

The next day I took it to the mechanic. They did a pressure test, and added dye. They said they found NO LEAK. They had no answer for what happened.

One thing I remembered is before my trip, I added 2 quarts of oil, without even checking how much I had. I just assumed I might need some. Wrong.  added WAY too much.

So with the mechanics doing the pressure test, and having no answer, I assumed that adding too much oil was the problem. I got an oil change, and tried my trip again.

This is when the temperature reaching 250 happened.

The mechanic had no answer for any of this! What is going on???

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