I was wondering if a thermostat is mostly just to keep the coolant from going to the radiator until the engine is at operating temperature, or if it opens and closes frequently.

I just noticed when I pull a trailer I get up to 90* and that's only about 5 or 10 minutes at 60MPH. I know my thermostat is a little hotter than the normal one because the PO replaced it so that his coolant would be hotter for his veggie conversion. If I drop back down to the normal thermostat, will the car stay cooler while I tow my trailer?

(my normal operating temp is about 85* with this thermostat. the PO says that before he changed the thermostat it only ever got up to about 65*, but I am not sure if he is credible on that)

any advice? Summer is coming and i don't want to get caught with my pants down.

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1985 MB 300D/T

Fell off the WVO bandwagon when people started trying to charge $2.50 a gallon for dirty vegetable oil.

The correct thermostat is 80C. As I recall, it fully opens at about 95C. The engine should warm up to 80C within a few minutes regardless of the ambient temperature (mine stays at 80C in sub-zero temperatures). Running cooler than 80C is not desirable. In the summer they should stay around 80-95C under normal conditions, and you may temporarily get up to 100 or 105C under heavy load conditions. You do not want to approach 120C.

Installing a cooler thermostat should not affect the maximum summer temperature, because once it's fully open it will allow the maximum flow through the radiator. I wouldn't worry about 90C operating temperatures.

85C is where you want to be at full operating temp. A little higher is normal when climbing hills or pulling a trailer. It sounds to me that you have the correct thermostat. Yes the thermostat controls flow through the radiator but it's not like a switch. It gradually opens and closes depending on the coolant temperature.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

cool thanks. I thought a running temp of 60 seemed a little low, even in the winter.

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1985 MB 300D/T

Fell off the WVO bandwagon when people started trying to charge $2.50 a gallon for dirty vegetable oil.

Mine did that when I first bought it, turned out to be a bad thermostat.

Running cool can also hurt economy. My MPG jumped up to 27mpg this week (from 22 average) after I put in a new t-stat and fresh coolant last month.

MB put out different T stats for different regions across the globe, for instance, here they tried 78C and then went to 72C for OM616 turbo as many objected to 90c operating temps, my personal take is an engine running at 80c constant as in the Japanese cars runs the happiest and longest, also keeps the oil happy and well within viscocity range.

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99 Gurkha with OM616 IDI turbo

2015 Gurkha with OM616 DI turbo

2014 Rexton W with OM612 VGT