I have just purchased my second 1991-350, my first was an SD, my recently
purchased is an SDL with only 76,000 miles. The car is in mint condition.
My first cars temperature during in town driving on hot days (90-95F) ranged
100- 120 C, I was never comfortable with this, I replaced radiator and
thermostat and helped this somewhat. My current SDL is up to the same old
tricks, the owners manual states than in town severe condition driving can
approach the red mark.
I would appreciate any feedback on this subject.
Thanks in Advance,
They do that. Being in the red is ok. STAYING in the red for a while is
not. Having an aluminum head you only get to do this once, if it
overheats, it's dead, Jim, unlike the 5 cyl motors that can overheat
Flush the cooling system and use redline water wetter. And keep
an eye on the temp gauge.
Need Mercedes parts? http://parts.mbz.org
Richard Sexton | Mercedes stuff: http://mbz.org
-I have had a 91 350 SDL for some years now (from 106k to 182 k). They have
numerous clever ways of overheating.
- AC charge low. This seems to cause excessive heat discharge by the ac in
front of the radiator.
- Use of MB coolant. I think this stuff leaves deposits in the cooling
system. Citric acid cleaning helps.
- I have heard partial plugging of the mufflers can cause the engine to run
- Inoperable aux fans (one comes on when at 110 the other when the ac
pressures are high)
- check the temp sensor of course
- low oil level is not good
- fan clutch failure - not as uncommon as some think
- fan belt failure
- Idler pulley or dampers worn out. (this and the ac charge made the biggest
difference for me)
In North Carolina mine will often move up over 100 in city traffic but comes
back down to 85 on once traffic flows steadily. One exception is driving in
the mountains. Sustained heavy acceleration has caused it to go up to 110
(before some of the fixes mentioned above).
AND BE CERTAIN TO DISABLE THE EGR VALVE. Mine was passing black sludge to
the intake. This is not good for pistons.
As Richard said extreme high temp is sure engine failure.
Just to throw in my two cents. It seems that most 350SDLs vary their
temp around the operation range. I disagree that operating above the
upper white line of the operating range should be considered
acceptable. Operating in red zone will not cause immediate failure, but
it is best addressed when the temp starts getting above the operation
I replaced fan clutch, water pump (water pump was bad though and
flowing backwards sometimes), several thermostats all to no real avail.
The two things that really messed up our engine was 1) a burned through
head gasket probably caused by running in the red zone for too long and
2) a clogged exhaust. A new exhaust system on our car dropped the
operational temperature about 25 degrees C right back into the
I've had the AC system low and empty and for us it never really
affected the operating temp, but I can absolutely believe that it will.
I've never understood why that happens but it seems to.
In other words, do what Chris said, and don't discount the exhaust. A
clogged exhaust can cause bad heating up and low power.
It's a quirk of this car. Oily crap comes out of the cylinders and
builds up in the can in the midpipe causing an obstruction in the pipe
and there is a power loss and the engine runs hot.
It goes right up there with the bent crank arms, which are also a quirk
of this car. Personally, once the burnt through head gasket was
replaced, I've pretty much turned up the radio and ignored a great deal
of what is going on with the 350SDL.
Ummm, I have no good answer for that. We found it out by replacing the
exhaust stem from stern because it was cracking, and the side effect
was an engine that ran much cooler as well as more powerfully.
Sorry that I can't help. I just know that this is what happened to us,
and I have read similar experiences from others over the years who have
had the same thing happen. Have you tried searching this groupd on the
subject? Something useful might pop out at you. Best of luck - H
You mention disabling the EGR valve, how is this accomplished? I thought of
replacing it, a new one is $142 at part suppliers on the internet. I have a
small accumulation of oil around the cap where oil added to engine, I
immediately thought of EGR as suspect.
It is possible your aux fan is not working when AC is on... I would first
buy a infrared thermometer and aim it at the brass sensor to get an accurate
On older cars, the temperature gauge do go bad and can be off as much as 20
The temperature sensor is easily seen... they all have one or two prongs...
one is definitely blue plastic with two prongs... this is for AC. Sometime
you will have one prong with read coloring... this also related.
Best bet is go to the dealer and they will tell you which controls which.
The single black one is for temperature gauge... sometime this is replaced
by 4 prongs round black sensor.
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