Correct operating temp. 300SD - PeachParts Mercedes-Benz Forum

Box-a-rox · #1 07-10-2002, 01:24 AM

I have a 1983 300SD I just got it 2 weeks ago its my first MB diesel. Any way it runs at 100*C it runs alittle higher on 100*F days but even on cool days like 65*F it just warms up to 100C and stays there. Im thinking it has the wrong stat in there shouldn't it run at 80*C?
If I change the stat what type of antifreeze do you use?
I also found a vacuum leak by the EGR the vacuum hose looks like it runs to a (thermal switch?) in the head before it goes to the vacuum block on top of the valve cover. One of the nipples on the vacuum switch snapped off. The EGR vacuum line is plugged at the EGR can I just plug it before the switch or does the vacuum switch have to be in the circuit? Looks like it only controls vacuum to the EGR.
Sorry for all the questions I don't have a manuel yet?
I sure would like to find a old style shop manuel I would rather not deal with a CD.
Where do you find manuels?
Thanks, Ken

ck42 · #2 07-10-2002, 10:16 AM

Congrats on the new ride

As for temps, mine sits right smack in the middle of 80 and 100C...which means it's 90C....suposedly.

From everything I've read and heard, 80-90C is considered *typical*.

One thing you may want to check, before you start replacing and trying to *fix* things, is that the temp guage is reporting the TRUE temp. Only way I've heard of is to take a temp reading with a known accurate device right at the plug where the temp sender resides....this should give a true engine temp.

If the guage is in fact accurate, then you can begin a sequence of troubleshooting events to find out what's causing the slighly elevated operating temp.

Off the top of my head:

Wrong mixture of coolant, ie, 100% antifreeze
wrong or malfunctioning thermostat
clogged or badly performing radiator
water pump problem

----these are not necessarily in the order they should be done----

Those are things directly related to the performance of the cooling system.

Once those are verified as working correctly and you still are running a feever, then you need to start looking at issues involving the motor. Things like a blown head gasket or whatever else causes a diesel motor to run too hot.

As for the EGR and vacuum leak: Fix the leaks. Keep the EGR line plugged at the end of the hose going into the EGR. Can't answer your question regarding the 'thermal switch' and whether you can just plug it somewhere else...sorry, that's one for the experts.

I'm by NO means a diesel or mechanical know-it-all, so please, anyone please correct any misinformation I may have given.

Good luck!

rickg · #3 07-10-2002, 12:36 PM

My '84 SD runs between 80 and 90 most of the time. In really hot weather or during a long run up a mountain pass it has climbed up towards 100. Tha's about all I can answer so far.
Good luck and congrats on the new cruiser

#4 07-10-2002, 03:18 PM

my 83 300 ran dead on 80 when I go it three years ago, but in the winter when it was 50 (florida) the car would drop and hold below the 80 mark. Figured the stat was weak so I replaced it with one from fastlane. It now runs about 85 unless I am going over 85 mph in the summer when it will go to 90. If sitting in traffic or a drive-thru for a long time with the air running and outside temps over 90 it will almost get to the 100 mark. Never goes above. Last summer however I had a rad cap fail in N. Ga. hills and it did boil over on a big climb and went to about 105c. Replaced the cap and the problem went away.
Things you can do...
Flush and refill with what you knowis correct mixture of h20 and antifreeze
use "waterwetter"
replace cap
replace t-stat
If still pushing 110c have a rad shop check it out for blockage.

RG5384 · #5 07-10-2002, 05:23 PM

i had a very similar problem, i ended up replacing the rad and it went away. If you dont flush out these systems they get all sorts of crap blocked up in the radiator, over time they will cause the radiator to function poorly.
Ryan

Elripster · #6 07-10-2002, 06:26 PM

Which antifreeze do you guys use? I just did a trip from San Diego - Sacramento- San Francisco, and back to SD. The outside temps were 90+ on much of the drive. While climbing the LA mountains w/ the AC on, she got up a little over 100°C and stayed there until I crested the top and the temp dropped as I went down hill. There's a fair amount of crap in the resevior so I'm thinking I'll flush the whole system.

Frank.

RG5384 · #7 07-10-2002, 07:09 PM

elripster: a good flush can't possibly HURT anything, the only thing it could do is help. The temp rising a little on long stretches upward is seemingly normal though. If you do decide to flush it you may want to take the resivoir out so you can spray that clean with a hose, you'd be suprised at all the crap lodged in there.
Ryan

ck42 · #8 07-10-2002, 07:41 PM

Frank...if what you're asking is which coolant wiill give the best cooling performance, the answer is...the one with the least amount of anitfreeze material in it and the most H2O.

If you really wanted to be anal about summers and keeping the motor cool, just use 100% distilled water and a bottle of water wetter. That is THE best you can do. Of course, once temps start dropping towards freezing, you wanna start diluting with the antifreeze again for freeze protection.

#9 07-10-2002, 10:32 PM

any mixture of anti-freeze and water, but don't forget that anti-freeze also raises the boiling point of the coolant to a significant degree. Running straight water introduces the risk of turning a minor temperature rise into a damaging boil over. And then there are those anti-corrosive aspects of quality anti-freeze.

ck42 · #10 07-10-2002, 10:45 PM

"And then there are those anti-corrosive aspects of quality anti-freeze."

Hence my mention of using the water-wetter...but you are correct on the increased boiling point; though, this would simply be more of a safeguard in case something went really wrong vs. an absolute detriment that would occur.

In fact, a 50/50 mixture of (ethylene glycol / Water) will only bring the unpressurized boiling point from 100C to 106C....not a HUGE difference. The big increase in boiling point comes from the pressurization of the coolant system; which brings it up about another 25-30C.

Elripster · #11 07-11-2002, 12:19 AM

Hmmmm maybe I'll do a 70/30 water/antifreeze mixture. I doesn't get very cold in San Diego. The only worry would be if I take it to go snowboarding but I can always add some more antifreeze. I'm not that anal. I'd be happy with a good all around mixture. Where does one get the water wetter? I don't suppose there are any issues with using that orange 5 year coolant?

Frank.

ck42 · #12 07-11-2002, 12:28 AM

Redline Water Wetter....in my area, the only place to find it is PepBoys; otherwise, have to order it from somewhere. Usually runs about $5/bottle. Same people that make the Redline DFC 85+

#13 07-12-2002, 07:20 PM

Ditto on the pressure comment, that is key to keeping your coolant heat transfer efficient. Before all that other stuff, or maybe after the flush which it likely needs anyway, I would check the pressure the coolant system operates at. Start with ensuring the correct cap is installed. European coolant systems generally operate at much higher pressures than American, and if a previous owner or mechanic did not know this they might have put the wrong cap on. There is a reason the Eurpoean manufacturers put on those expansion tanks...

Jeff72_220d · #14 07-12-2002, 08:42 PM

I would recommend that you use the antifreeze mixture specified by Mercedes. Their "orange" stuff is a little more expensive (as, in $10 per gallon); but, it will be completely compatible with the different metals in the cooling system.

Regarding the EGR, although not legal, disconnecting it at both (as in two of them) tube connectors BEFORE the valves (under the little black box on top of the valve cover) will eliminate a significant source of leaks, AND GIVE YOU BETTER ACCELERATION.

My '82 300SD used to run about 70 deg. C until I finally replaced the thermostat. Now, it runs just around 87-90 deg., just where it should be. More efficient combustion at the correct temperature.

stephenson · #15 07-12-2002, 10:19 PM

Or, instead of Water Wetter, and with the same effect, you could add the same amount of water.