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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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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My m104 engined 124 car did this, though perhaps not to the degree you describe. My assumption (guess?) is the thermostat is a bit slow to open when it has hot water on one side (engine) and cold on the other (radiator hose). I always saw the temp guage rise a bit above the normal point then settle back down. For example, the engine always sat at 82c in cooler weather; it might rise to about 90c from a cold start, then drop to 82c and stay there.

Mike D and others. I believe I fixed my running hot problem. I replaced the thermostat 2 day ago and it has run hot every time it went from cold to hot. So I decided to put the old thermostat back in. This is what I found and it is going to be hard to explain but I will give it a try. On the 84 and 85 and probably others but not the generation 2 420 & 560 there is a small hose hooked to the top of the thermostat housing that goes to the radiator and reservoir. I believe it has 2 functions. One is to bleed air out of the thermostat housing and to provide water flow to the top of the thermostat and the temp sensor. The connection that the hose hooks to at the thermostat housing has a check valve in it so it can't pull air back in when the engine cools off. That fitting was plugged up when I checked it. I pulled the housing off a spare 500 engine I have. That is when I found it had a check valve in it. I believe it being plug up prevented any water from flowing through the housing so the temp sensor and thermostat didn't operate as they should. When the housing finally got hot enough for the thermostat to open, it and the sensor got very hot water very quickly causing the temp to rise quickly and then fall back quickly. Once the thermostat was open, even a little, every thing worked fine. The generation 2 engines are different. MB redesigned the thermostat system to a more conventional system so they wouldn't have this problem. I just drove the car again, starting from an engine temp of about 70 deg F. The temp went up to 200 deg F and settled back down to between 188 to 190. I think that is about normal operation.
I don't like to do several things at one time to fix a problem but in this case, it made more sense to do them all at once. This is what I did. I replaced the housing because of the blocked port. I put the old thermostat back in because it didn't fail nearly as often. I replaced the temp sensor because I dropped the old housing when I tried to get the sensor out and broke it. All of the (new) parts were used. We'll see how it does tomorrow morning.

Sorry for the long post but I hope it will help someone else.

Paul

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84 500 SEL (307,xxx miles)

Let us know if it does the trick. That'll be one more entry on my "Things to do when I have more time than money to do stuff" list.

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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

Well, back to the drawing board. It did it again this morning. A little different. This time it was 40 F when we left the house. Got on the interstate in about 10 min. Temp went up normally to about 185 F and stabilized for about 10 miles. I was so happy, I got off the interstate. I was going up the exit ramp coasting. When I got to the top to turn left, I had a green light so didn't have to stop. 2 or 3 seconds before I started the turn, the temp shot up to 216 F. Buy the time I completed the turn and got straightened out, the temp went back down to 190 and every thing was fine. I have a hard time believing the temperature of an engine can change that much that quick. 185 to 216 to 195 in no more than 6 seconds. Remember, I just changed the thermostat and temp sensor 1 day ago. Once it does this, it won't do it again until the engine cools off. It has never run hot. Never looses any coolant.
Anyone have any ideas? Pleaseeeeeeee Help.

Paul

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84 500 SEL (307,xxx miles)

OK, thanks for posting the results. I think I'm going to move this from my "Things to try, not that I think it'll do any good but what the heck" list to the "unexplained quirks of this car" list.

Other than the mild sense of panic I get when it occurs I have never had a problem with the cooling system. Oh well, "If it ain't broke....", I guess.

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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

Well another day in the running hot 500. This morning, it was 56 deg ambient when we left. Went the same route we had been before. 10 min to interstate, about 10 min on interstate at 65 mph. Temp started up. We timed it with a stop watch and started timing when the temp went over 200 F until it went back down to 200 F. 16 seconds. It topped out at 216. Then stabilized at around 185. Talked with a friend who works at a Mercedes/BMW independent shop. He said they had seen this before on a few cars and just decided it was normal for some of these older cars (Mine will be 30 years old this coming November). Well, I am not giving up that easy. I didn't do it a year ago and shouldn't do it now. My son had a problem just like this on a 86 (?) Peterbuilt long nose with a Cummings engine It had 2 thermostats, one for bypass and the other for flow through radiator. Turned out there was a part number change in the bypass thermostat. The new thermostat didn't have a bleed hole that would allow some hot water to get around it to heat up the working part of the valve. He made a small hole in it, and fixed the problem. He found out later that Cummings found out about the problem and fixed it by putting the hole back in it. Well, I just drilled a hole in my thermostat so some hot water could heat the element quicker. Worst thing that can happen is it will take longer for the engine to warm up. I think the spike in temperature is between the time the thermostat starts to open and the bypass is closed. I have gotten my R&R time down to 35 min.
I'll let you know either way.
If anyone has access to the MB parts manual, could you please check to see if there was a PN change in the thermostat for a 1984 500SEL in the last few years.
Thanks

Paul

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84 500 SEL (307,xxx miles)

I read an article by diesel giant about drilling holes in the thermostat to make the engine run cooler. I talked with my son again about his Peterbuilt truck with the same "run hot during warn-up cycle". He drilled a hole in the bypass thermostat and that fixed it. So, I drilled 2 3/16 in. holes in my thermostat and it worked. Yesterday afternoon and again this morning I drove the same route that has made it fail every time. Now it goes up to 185 +/- 2 deg and stays there.
I know that the MB design engineers know a lot more than I know. If something doesn't work correctly, fix it back to the way it was designed. Then I got to thinking, this car is almost 30 years old. The thermostats I have been putting in it are maybe only 1 year old. Not being able to buy a 30 year old thermostat, I may be forced to make what is available work. I know that there was a change in the thermostat housing in the 84 116 & 117 engine and another change in 86. So the 84 and 85 are different from the 81 to 83 and different from the 86 to 91. Why did they change them in 84 and again in 86 unless they had a problem? Why would they keep a thermostat in stock for 2 (116 & 117) engines only made for 2 years if something else will work almost as well? Give me your thoughts.

Paul

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84 500 SEL (307,xxx miles)

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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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)