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  #1  
Old 08-15-2007, 08:02 PM
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500sl overheat/loss of oil pressure

My 1992 500SL did an odd thing last night. During some intermittent stop and go and idling I noticed suddenly that the temp gauge was pinned at the top and then the oil pressure suddenly dropped to ZERO and the red warning light came on. Then the pressure gauge would suddenly read pressure, then drop to zero then pressure, etc. I was a couple of blocks from my house and the car was running fine so I drove it the rest of the way home and parked it. I could hear the coolant gurgling but no bad smells or anything. The next morning I honestly expected the engine to be seized (or some other bad $10k problem), howvwer the car started right up. I noticed the oil pressure gauge pinned at the top though which was new. Since I was due for an oil change anyway I changed the oil (it was dirty but no coolant in it or metal particles) and refilled it with a new filter. The coolant level was a tiny bit low, but no oil in it and pretty clean. After the oil change I started the car and the temp went up faster than normal to around 100C as indicated on the gauge. The engine was still running fine. I noticed the bottom radiator hose was ice cold although the top was hot, so I assumed a bad thermostat, water pump, or plugged radiator. I replaced the thermostat and kitchen tested the old one. The old one was not opening even at the boiling point of a pot of water. The car seems to be running fine now although the temp gauge seems higher than I remember it (running between the 80 and 100C indications.) Anyone have any opinions on any of this and can you tell me what the proper indicated temp range should be on the gauge. The ambient air temp today BTW was around 85 degrees F. Thanks!
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  #2  
Old 08-15-2007, 09:52 PM
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It sounds as if you corrected the problem by replacing the thermostat, but is the vehicle running at 100 C with the A/C on?

And do you know if the thermostats opening point is the same as the old one (when it functioned of course)?

This may be the cause as to the drastic change that caught your attention. Cause if it was the water pump I would think you would notice the change in temps over a period of time as the part degrades in efficiency. Not a sudden change as you mention.

I know that my 420 runs at 60 C normally and 80 to 90 C with the A/C on if this helps.
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Old 08-15-2007, 09:58 PM
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Quote:
Originally Posted by wscheffer View Post
It sounds as if you corrected the problem by replacing the thermostat, but is the vehicle running at 100 C with the A/C on?

And do you know if the thermostats opening point is the same as the old one (when it functioned of course)?

This may be the cause as to the drastic change that caught your attention. Cause if it was the water pump I would think you would notice the change in temps over a period of time as the part degrades in efficiency. Not a sudden change as you mention.

I know that my 420 runs at 60 C normally and 80 to 90 C with the A/C on if this helps.
Just took it out for a long test drive with the a/c on. Temp held through all ranges of driving at between the 80-100C indicator lines on the gauge. In stop and go after a good warm up it settled in at just 100C. The thermostat was identical in brand and spec to the one I pulled out. I'm wondering if the old thermostat was faulty and actually stuck open for awhile. The car seems to hit operating temperature significantly faster now than it ever did before, and then runs warmer than it used to. I also noticed the passenger side fender seemed pretty hot when I stopped, which I don't recall ever noticing before. I'm wondering if maybe I could have an exhaust leak heating things up over there. Seems unlikely though as the car is not any louder and not running badly in any way.
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Old 08-15-2007, 10:03 PM
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Originally Posted by gdmarques@cox.n View Post
I also noticed the passenger side fender seemed pretty hot when I stopped, which I don't recall ever noticing before. .
Coolent over flow is at the passenger side front fender. Is your coolent level at the arrow on the expansion tank?

The vehicle is still running on the hot side and it sounds like it could be the clutches in the viscousfan are beginning to fail.
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Last edited by wscheffer; 08-15-2007 at 10:18 PM.
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  #5  
Old 08-17-2007, 08:02 PM
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Another new thermostat

You had me wondering about the thermostat I put in the car. I checked the box it came in and sure enough the box said 87C, not 80C! So I went to the dealer and bought an OEM thermostat (which surprisingly was cheaper than Autozone at $35US instead of $42.) Of course when I took the replacement thermostat out of the car it was marked 80C! So I put in the OEM part anyways and returned the first one and got my $42 back. Surprisingly with the OEM part the car is running cooler at around 85-90C as indicated on the gauge. I suppose its possible that I simply had some air in the system and it took some running to work its way out.

I did notice a metal ring lying inside the overflow tank when I was siphoning off some fluid for the thermostat shuffle. I read another post on here that said the crimped part of the cover ring had come off, and I'm wondering if this is what it is. Its too big to pull up through the opening, so it had to come from inside. Possible I'm losing pressure around the neck, but I dont see any fluid seepage or smell coolant.
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