PeachParts Mercedes-Benz Forum - Is 100 c too hot, 300SD

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Quote:

Originally Posted by Skippy (Post 3169107)

I think there is some variation in thermostats, and many of them will put the car's operating temperature around 100 C. To the folks with cooler running cars, what kind of mpg are you getting? I get about 24-26 flogging it hard, and up to 30 if I'm easy on it.

I ask because I used to have an old Lincoln in which I ran 'stats from 160 to 180 F, and I lost a couple mpg with the cooler one. I also lost hot heat in the coldest part of winter, which is what prompted me to switch back to the hotter one.

I get almost identical mileage figure sin my 1878 300SD running the 71°C stat. I can knock down 26-28 driving 80-85 mph on the freeway with the A/C on and this is consistent over 40-45 tankfuls.

FWIW, my 617 runs better and feels stronger when I run it cooler. I used to run the 60°C Tropical Climate thermostat back when you could still get them.

My Supercharged 4.6 litre 2001 Mustang GT gets equal mileage with a 195°F or 180°F thermostat.

22 mpg in California, Texas, and Florida....28 mpg in Arizona, Louisiana, Mississippi & Alabama, most likely due to the differences in fuels....FWIW

Quote:

Originally Posted by resto108 (Post 3169367)

Can't let this one go. The radiator cap has nothing to do with coolant temp. it only increases the boiling point of the coolant.

It very well does, it does increase the boiling point....but what cause things to boil? Heat....if the cap isn't controlling the boiling point, thus more heat is generated thus causing higher temperatures.

Quote:

Bad radiator cap

The radiator cap does several things. It
seals the system against the outside world (main seal function)
keeps the system pressurized when needed, so as to raise the boiling point of the coolant
allows excess pressure and coolant expansion to vent to the expansion reservoir (pressure seal function)
allows coolant to return to the radiator when the engine cools down (return seal function)
As you may have gathered from the above section, the radiator cap has three seals, any of which may fail independently of the others:
The main seal is the one that seals the cap against the top of the filler neck. Just a rubber gasket that operates just like one on the lid of a pickle jar. Simple and reliable.
A failed pressure seal will allow the coolant to boil at a lower temperature, and coolant will be able to travel freely and foamily to the expansion reservoir. This will cause localized hot-spots inside the engine, which can lead to premature head warpage, and may hasten head gasket failure. It will also cause the rad coolant level to be low, just like a failed head gasket.
A failed return seal will prevent the coolant from returning to the radiator as the rad cools off, causing a vacuum that can collapse the radiator's hoses. This will prevent the coolant from circulating if the hoses don't re-expand as the engine warms up.
A bad rad cap can cause similar symptoms to a failed head gasket, so it's a cheap first step to try before bringing it in. If you replace the rad cap and you still have bubbles in the coolant (or foam in the reservoir), then suspect the head gasket.
If the engine starts to overheat at idle, or in heavy traffic, and the gauge goes down when you rev it, the coolant is probably low.

Moreover, a neglected cooling system can load up the cap with crud and corrosion, preventing proper coolant flow in and out through it. Peel the seals back with your fingernail to check for goop. If you find any, a blast with a garden hose and probing with a toothpick should clear most of it out.

But anyway, a new rad cap is less than $20. Make a habit to change it every 5 years, just in case. It's pretty important.

Overheating: Symptoms and causes

Quote:

Originally Posted by cooljjay (Post 3169417)

We should all remove our radiator caps and use the extra heat which is generated to propel our vehicles around.

Quote:

Originally Posted by Shortsguy1 (Post 3169423)

We should all remove our radiator caps and use the extra heat which is generated to propel our vehicles around.

Only if it gets me 50 mpg :P

I don't think these engines are capable of that kind of MPG....

Quote:

Originally Posted by cooljjay (Post 3169417)

This is not clear...

If the coolant is not pressurized enough the coolant will boil, which means MAXIMUM coolant temperature will be 100 C by definition. But, coolant will be lost, obviously.

By increasing pressure we increase boiling point allowing for higher coolant temperature without problem, assuming equal or increased heat power transfer into the coolant.

There is no per se problem of coolant temperature, there is fire in the engine at 1000 C so these 100's are not important. The problem is if coolant boils inside the block, it can make engine overheat as the steam doesn't transfer heat properly. We want to run as hot as possible (improved efficiency) without boiling...

FWIW all my MB I have seen seem to be set to have 80 C (thermostat opening) as the coldest temp and 100 as the highest (when fans start to really work) I would not consider overheating until the gauge goes to... the red

Quote:

Originally Posted by Volker (Post 3169544)

If the coolant is not pressurized enough the coolant will boil, which means MAXIMUM coolant temperature will be 100 C by definition.

A 50/50 mix of Zerex G-05 will boil at 226F/107C at atmospheric pressure (no radiator cap). If you pressurize it with an additional 15 psi, the boiling point goes up to 265F/128F. I think that is why the redline temp on my old car is just below (at?) 250F and on my new (?) car is 120C.

What is counterintuitive (to me, at least) is that, if you car has a tendency to run hot, you are better off running a lower concentration of coolant in your cooling system. In other words, with pure water (assume no radiator cap for simplicity), your system would boil at 100C. As mentioned above, with 50/50 coolant (again no rad cap for simplicity), your system would boil at 107C. But you are actually more likely to overheat (boil over) with 50/50 coolant. It has a higher boiling point, but its heat capacity is sufficiently less, that you car is actually more likely to get damages from excessive temps. Of course, I am not advocating running pure water because from a corrosion standpoint and freeze protection standpoint, this is not a great decision. But I just wanted to use the limiting case of pure water since it makes the numbers very familiar.