300SD running hot... - PeachParts Mercedes-Benz Forum

dcotejr · #1 08-09-2009, 04:17 PM

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!

tangofox007 · #2 08-09-2009, 05:19 PM

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.

John Schroader · #3 08-10-2009, 10:32 AM

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.

dcotejr · #4 08-10-2009, 02:04 PM

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

tangofox007 · #5 08-10-2009, 03:02 PM

Quote:

Originally Posted by dcotejr

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

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

dcotejr · #6 08-10-2009, 03:09 PM

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.

Brian Carlton · #7 08-10-2009, 02:17 PM

Quote:

Originally Posted by tangofox007

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.

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".

Phil · #8 08-10-2009, 02:31 PM

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.

tangofox007 · #9 08-10-2009, 02:52 PM

Quote:

Originally Posted by Phil

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.

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.

Quote:

Originally Posted by Phil

It was 100 here saturday and my car ran in the 80s with the A/C on with no problem.

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.

Ian White · #10 01-13-2010, 01:26 AM

Don,

Did you get this resolved?

Ian

81Wagon · #11 06-18-2010, 06:51 PM

I noticed recently that sometimes at idle my auxiliary fan would come on even though the temp gauge only read ~85. Figuring at least one of the two sensors must be wrong, I went ahead and replaced both of them (as far as I know, they were originals). I also replaced the thermostat while I was at it.

Now my gauge reads 95-98 degrees consistently when the car is warmed up. That would certainly explain why the aux fan came on from time to time (it should trigger at about 100, right?).

I've since flushed the coolant system. It appeared to be in good shape already - no sludge and no rust that I could tell really. It hasn't fixed the problem.

I assume it isn't the fan, either, since it's still running at 95 degrees on the freeway with plenty of airflow. Either the new thermostat isn't opening soon enough (I didn't think to test it before I installed it) or....? Any other ideas? Will driving it at that temperature all the time do any harm?