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Old 01-25-2009, 09:19 AM
Join Date: Nov 2008
Location: Florida
Posts: 39
Bad thermostat?

The engine in my '88 560SL seems to warm up very slowly. Once it gets up to 80, it holds temperature well. Does that indicate that my thermostat is stuck in the open position. Would the temperature sensor tell the ECM that the engine is cold and cause the injector pulses to widen and consume more fuel? The reason I think that this might be the case is that a friend bought a new truck which never came up to temp. When the dealer replaced the thermostat, the fuel mileage increased appreciably!
Thanks for your thoughts.
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Old 01-25-2009, 09:43 AM
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Join Date: Jan 2007
Location: ontario canada
Posts: 263
My '86 runs at a steady 80. I think that is normal.
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Old 01-25-2009, 05:55 PM
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Join Date: Jul 2002
Location: Irvine, CA
Posts: 33
My '88sl also runs at 80
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Old 01-26-2009, 02:09 AM
coleyjf's Avatar
Old dog - new tricks
Join Date: May 2005
Location: East Midlands of the UK
Posts: 264
bad stat

Hi Dexter

I think your stat is poorly, the top temp is about right at 80 but my '88 300SL heats up to temp in about 2 miles.

The simple way to test the actual stat is to take it out and put it in a saucepan of water on the stove and heat it up - you should see it open as it gets hot! although you need to take it out to do it (and you might get some earache from any non-carloving female partners)

You can sometimes tell whether the water is flowing through the stat wrongly by feeling which water hoses are getting warm when you start the car from cold though I'm not sure of the "Water flow" on an "8" (mine's a 6 cylinder and I think the cooling system pipes are laid out differently)


Keep your Motor runnin'

Jim's '88 300SL
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Old 01-26-2009, 08:59 AM
Werner W's Avatar
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Join Date: Oct 2008
Location: Hudson Valley NY
Posts: 73
For the price of a thermostat, I think if I went through the work of taking an old one out I would just go ahead and put a new one in.
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Old 01-26-2009, 09:16 AM
Join Date: Nov 2008
Location: Florida
Posts: 39
speed of warm up

Thanks for your comments. I am quite knowledgeable about many automotive subjects, so I know about testing t-stats etc. I just thought it unusual that the engine should take so long to come up to temperature (three times as long as my Town Car). I also know that one of the inputs an ECM is looking for in an American car is engine temperature. In fact, a Chevy has to get to 190 before the system will go into 'closed-loop'; a mode that fine tunes the fuel feed which depends on all the input sensors. If the Chevy system sees a 170 degree engine temperature, it interprets that as being cold and fattens up the mixture (mild choke) and consequently returns poorer fuel economy. I don't know anything about MB EFI and was curious as to whether its functioning was basically the same as the American systems??
Today, American systems operate at temperatures of 195 and up. Supposedly, engines operate more efficiently at temperatures around 200.
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Old 01-26-2009, 03:10 PM
meltedpanda's Avatar
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Join Date: Mar 2003
Location: Central Ky
Posts: 6,086
yea mine heats up in 2 -3 miles and then rock steady, Tstat is easy R/R , just give it a go
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