Overheating Puzzle

[oregonian2]

So here’s the puzzle.
This is a very reliable 1970 280SL (4 spd.).
Last night I did a routine fluid and hose check. Everything was fine. This morning (sunny, temp. ca. 60F) after driving about a mile, I notice the temperature gauge was reading above its normal 180 and continued to climb quickly as I went on. I pulled over and popped the hood. No leaks, no funny smells, no steam anywhere. Since the expansion tank was still cool to the touch, I took off the cap and heard the usual vacuum pop. Coolant level normal, so I replaced the cap and drove on, but as temperature continued to rise closer to the red mark, I stopped again, and did some window shopping while trying to decide whether to head home, go on to my usual shop, or call for a tow. When I got back to the car after about twenty minute, the temperature had dropped back to ca. 180. Still no signs of any leaks. When I started again, though, the temperature again began to rise. Since I was less than a mile from the shop, I decided to try to drive there, which I did, watching the needle slowly climb again. A few blocks away, I was able to throw in the clutch and coast a bit (no traffic Sunday morning in a smallish town), and as I pulled into the shop I noticed that the gauge had returned almost to normal! Decided to drive around the block to see what would happen, and as I did, the needle continued down until it reached its normal spot and stayed there. I drove home (about three miles). Everything looked, sounded, and felt fine, and the temperature gauge was normal as if nothing had happened.
So…what happened? Is it possible that I didn’t secure the cap tightly last night? Would that matter? Is it possible that I was never really overheating, but something was going on with the gauge? I should also say that the gauge was replaced a few years ago with a new one from the Classic Center and has never caused a problem. Fan clutch has also been replaced, coolant flushed and renewed 5,000 miles ago.
Any thoughts? Thanks!

[Jordan]

I'm sure the experts will chime in but it almost sounds like your thermostat was stuck closed and finally opened.

[Cees Klumper]

Jordan/Marcus gets my vote. Although perhaps it was the gauge, since it doesn't sound like the engine actually got that hot. Definitely keep an eye on it, I would get one of those infrared thermometers to check out the temperature next time it happens.

[oregonian2]

Just to follow up a week later: the problem hasn't returned, so the diagnosis of a (temporarily) stuck thermostat is looking pretty good. I'll keep an eye out, though. Good tip about checking the engine temp., too. Thanks a bunch!

[teahead]

Or you had an air bubble.

Be sure your clutch fan is working consistently.

Keep an eye out on your coolant level too.

And of course, don't use the green stuff coolant (I gotta flush that crap out of there asap!).

[Tom in seattle]

Maybe a $5 investment in a new thermostat would give you peace of mind.

[Tom in seattle]

What is the green stuff and it’s problem in our motors?

[teahead]

What is the green stuff and it’s problem in our motors?

Mercedes demands a certain type of coolant, which the typical green stuff for older US cars usually requires.

Folks on the board says to use MB coolant, or GX-5 (?).  I plan on using Pentofrost NF.  Non-toxic and also good for Benzes.

[russelljones48]

here's a pointer to an excellent article on cooling systems and their components.  as you'll read the best "coolant" when judged by it's ability to dissipate heat is good old H2O - however, it has some shortcomings - not the least of them being it's tendency to freeze and expand at it's freezing point.  This article is one that I keep a PDF of: https://www.hotrod.com/articles/ccrp-0707-high-performance-cooling-system/

[wwheeler]

The green stuff is coolant, that in the past, was always green and for use in cars with mostly cast iron engine components. As you will notice nowadays, there are more coolant colors than in a rainbow. So identifying coolant by color will no longer work.

The reason Mercedes and many European cars require a different coolant is because there were more aluminum parts used in their engines for our cars. I believe the green stuff is too alkaline and will cause corrosion with the aluminum parts over time. G05 and the Mercedes brand coolant are more neutral and will not cause the aluminum parts too corrode if changed at regular intervals. And even with that, you need to keep track of the coolant's Ph to be sure it is within a proper range. Otherwise, you will suffer damage to the aluminium components.