after I had hoses and thermostat replaced. I was about 500 miles from home a few weeks ago when I developed a coolant leak. Kept coolant topped up on the ride home, and the gauge was running at about the normal 82 deg C all the way home. Brought it in to local mechanic to have hoses replaced, and thermostat was replaced at same time. When I got it back, it ran at a steady 90 deg C. Returned it to the garage, and they replaced the thermostat with another 82 deg thermostat. Same result. Mechanic confirmed that the thermostat was opening at 82 deg or so. My question: Is this something I need to revisit, or could it be a dash gauge problem? My ride has 'only' 286,600 miles on it, and I want to make it last to 500K if I can...

Thanks in advance!

__________________
Mine: 1987 MB 420 SEL - 62K (Dec 2019)
Mine:1991 MB 350SDL - 244K (Aug 2011)- Totalled 9/14/2016
Mine:1981 MB 300SD - 326K mi (July 1997)-sold
Wife's: 2008 MB C300 Sport - 92K mi (April 2009)

In my experience, 90* in the summer heat is much more typical than 82*. My 300D won't run cool as 82* even in the winter.

Could be that the coolant temp sensor is a bit off. My 300D runs about 82C. My 300 SD was hitting about 100C -- replaced the sending unit ($26) and now it registers 90C. I have shot both engines with an infrared temp gun and they both (according to this gun) are at about 80C.

__________________
John Schroader
bio burnin' 83 300D, '83 300 SD, '79 240D
"I've never met a man who was good at making excuses who was good at anything else" Ben Franklin
"You cannot permanently help a man by doing for him what he could and should do for himself" Abraham Lincoln

Thanks for the input guys.

My first thoughts were along the lines of the sending unit maybe being off.

I've owned the car for over 12 years, and before this I had never noticed the temp over 85C, even with the AC on in Alabama summers...

For $26, I'll have to give it a shot. Is it a difficult DIY project?

Don

__________________
Mine: 1987 MB 420 SEL - 62K (Dec 2019)
Mine:1991 MB 350SDL - 244K (Aug 2011)- Totalled 9/14/2016
Mine:1981 MB 300SD - 326K mi (July 1997)-sold
Wife's: 2008 MB C300 Sport - 92K mi (April 2009)

Agreed.

90°C. is perfectly normal and, in the summer, it'll probably climb toward 100°C.

Not worth the time or effort to chase "the problem".

If this condition start just after having the stat and hoses replaced I think either the stat is in backwards or upside down. The stat should have a small hole at the top so coolant passes through to bleed out air and if it is upside down the you trap air in the head and then the stat doesn't know when to open. It was 100 here saturday and my car ran in the 80s with the A/C on with no problem.

__________________
1983 300SD
200000miles

Unlike a Chevy engine, the thermostat housing in the 617 engine contains a bypass passage which completely eliminates the need for a hole (real or imagined) in the thermostat.

Thanks to a phenomenon known as humidity, a 100 degree in Nothern California is not the same as a 100 degree day in Alabama. (When I lived in CA, the air was so dry plenty of folks used swamp coolers instead of air conditioning.)

Additionally, 90 degees is so close to being "in the 80s" that the difference in not material.

What would cause the sending unit to malfunction concurrent with a thermostat and hose replacement?

what would cause a problem with the sending unit, but it sounds like it's not worth spending much energy on this issue.

Just wanted to make sure I was not potentially harming my engine with this step change in readings after 12 years....

Thanks for everyone's input.

__________________
Mine: 1987 MB 420 SEL - 62K (Dec 2019)
Mine:1991 MB 350SDL - 244K (Aug 2011)- Totalled 9/14/2016
Mine:1981 MB 300SD - 326K mi (July 1997)-sold
Wife's: 2008 MB C300 Sport - 92K mi (April 2009)

The thermostat in my car has a hole at the top and I installed it as per the MB manual.

__________________
1983 300SD
200000miles

My copy of the 617 engine manual makes no reference whatsoever to a hole in the thermostat. If yours does, I would like to see it.

My manual does, however, go in to significant detail in describing the vent passage that is designed into the thermostat housing.

Furthermore, unlike a Chevy, the coolant in the 617 engine does not flow through the bypass thermostat as it leaves the head.

I'm telling you what I did and why, take it or leave it doesn't mater to me.

__________________
1983 300SD
200000miles

Unfortunately, the "why" part just doesn't hold water. If you would spend a few minutes studying up on the 617 cooling system, I am certain that you would agree.

I start by putting a meat thermometer into the radiator anytime I have a question regarding engine coolant. It eliminates the guess regarding what temp and is the thermostat opening at the proper temp.

That might be appropriate if you could find an access port in the radiator................near the upper hose.