I have a strange problem I would like to get some ideas on. My 84 500SEL runs about 188 to 192 F in normal driving. It may go to 200 in hot weather and sitting in traffic. Now the strange problem. Car sits overnight, outside temp 40 to 50 deg F. Start, drive a mile or two then get on an interstate or highway going 50 to 65 mph. Temp will go to somewhere around 215 to 220 deg and with in 2 or 3 seconds will start back down and be 185 or so with in 10 seconds. Doesn't do it every time. doesn't do it if the speed is under 50 mph for the first 15 min or so. I changed the thermostat yesterday and it did it this morning. The old thermostat only had about 10K miles and 18 months on it. New thermostat is a Behr, don't know about old one.
I don't think this is a serious problem, but would like to fix it. This has only happened 4 or 5 times in the last 8 months.
Any ideas?

Paul

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84 500 SEL (307,xxx miles)

Had you worked on the cooling system before it originally started doing that? It sounds like maybe air in the system.

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1983 380SL
1995 C280
1995 S500 coupe
1990 Mazda Miata
2004 Suzuki Hayabusa

Water pump and thermostat installed August 2011. First time this problem happened was June 2012, 8000 miles later. I have not had to add coolant. The first time it happened we were on our way home from Maine to NC and it only happened one time on that trip. Next time was when we left home going to the Outer Banks, NC, 380 miles one way. Didn't happen again on that trip. We can't remember the other times but it was always after an overnight or more sit. Sometimes this car may set for a week or more and not do it. The only thing in common is it is always early morning before 8 AM and on an interstate or highway within 15 min or so. I believe an air pocket (a very good idea) would have worked it's way out by now. I am starting to think it could be a gauge problem but it doesn't act like one. I have never looked at the gauge just before it happens, but the temperature drops over a period of 5 to 10 seconds, not instantly. I have an over temp, low oil pressure, low or high voltage, low fuel level alarm that gets my attention.

Paul

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84 500 SEL (307,xxx miles)

My '85 SEL does the same thing. Never have figured it out. It's been doing it for two years. It gives me a mild cardio-check every time I look down and see the needle go rising but it's always dropped back to normal in the same fashion you described. Flushes, new T-stat and fresh coolant has made no change in its behavior. Beats me.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Thanks Mike, I think that makes me feel better. If I figure the problem out I will be sure to let you know what I find.

Paul

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84 500 SEL (307,xxx miles)

Coolant temp sensor?

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1971 280SL ROADSTER
1988 300CE TWIN TURBO WIDEBODY
1994 E320 CABRIOLET
1999 C43 AMG
2005 G55K AMG
2008 CLK63 AMG BLACK SERIES

We just replaced the T-stat on my dad's 420 and it does not exhibit this behavior....it sounds almost like everything is heating up so quick that the t-stat is not opening fast enough....is it a geniune Behr/Febi Germany T-stat?

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

pawoSD. The 420 is very different from mine. On the 500 the thermostat is in the housing on top of the waterpump. On the 420 it is on the side of the water pump. That change was made in the 86 my. The thermostat that came out and the one that went in were both Behr made in Germany. In my testing I installed an aftermarket gauge in the front of the intake. It clearly showed that the engine temp was going higher than the original gauge was reading. The original gauge sensor is in the same housing as the t-stat. The original gauge would start going up and at the same time the add on gauge would start going down. They would meet and go down together to a normal reading. This told me the t-stat and sensor was not seeing the real engine temp when the t-stat was closed. Drilling the 2 holes made the 2 gauges read the same (well close) from cold to normal temp of around 185 with no spikes. If there is something wrong with the system that I just covered up, I would love to know what it is and fix it. However, for now every thing is working as it should and I feel safe driving it. The only drawback I can think of with the holes in the t-stat is that it may not warm up as quick. So far, I can't tell any difference in when I get heat. The normal highway temp is the same as before.

Paul

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84 500 SEL (307,xxx miles)