Pressurization of the cooling system?

[JHZR2]

My 1991 Mercedes 350sd hasn’t consumed or lost a drop of coolant since I’ve owned it.
But it doesn’t build pressure in the cooling system.

Should it, driving highway with ac on in 80-85 weather, for long distances? Mine doesn’t. Ive seen the needle creep up towards 100 (common and normal), but never over, due to a weak fan clutch. Still no pressure.

The system doesn’t have an overflow bottle, the radiator cap is on the reservoir bottle that is pressurized.

The HG is good. No concerns with anything. Shoukd I be concerned?

in reality, I’d rather keep it like this since these systems are old, and limiting pressure could be a benefit.

Thanks!

 

[michaelluscher]

It doesn't build any?
Or just not enough?
After a long hot drive, all hoses are still soft to the touch?

 

[Lubener]

Pressurizing the cooling system does nothing more than raise the boiling point of the coolant. You didn't mention if you have replaced the cap but I would if that wasn't done. Sometimes just snugging up the hose clamps may help.

 

[CharBaby]

Hi JHZR2,

[JHZR2 quote:] "The system doesn’t have an overflow bottle, the radiator cap is on the reservoir bottle that is pressurized."
CharBaby: Isn't the reservoir bottle the same as the overflow bottle? I've seen system like this(Ford/Mazda & others) that have cap-less radiators.

Does your system hold pressure after the engine is turned off?...Meaning, after the engine is off(HOT), are the radiator hoses full of pressure?

I think your cooling system is A-OK.

 

[Trav]

It should have pressure to prevent excess air in the system which can lead to hot spots and cavitation, a pressurized system is a good thing especcially with a diesel. Test it with hand pump tester if it builds pressure the cap is probably bad.

 

[RooflessVW]

Echoing Trav here

 

[bullwinkle]

It should have pressure to prevent excess air in the system which can lead to hot spots and cavitation, a pressurized system is a good thing especcially with a diesel. Test it with hand pump tester if it builds pressure the cap is probably bad.

I agree, there's a leak somewhere-or the thermostat is stuck partially open & not allowing it to reach operating temperature. The "overflow" tank is actually a degas tank, under system operating pressure-it's possible there's a hairline crack in it somewhere (above the coolant level, which would mean no visible coolant leak).

 

[SC Maintenance]

I ran lots of old farm equipment that wouldn't hold pressure - but the Radiators were huge, and the engines not very sophisticated.

How do you know its not holding any pressure?

If your worried about a leak, you could try something with a really low pressure. I think you can get caps as low as 4psi.

 

[JHZR2]

I ran lots of old farm equipment that wouldn't hold pressure - but the Radiators were huge, and the engines not very sophisticated.

How do you know its not holding any pressure?

If your worried about a leak, you could try something with a really low pressure. I think you can get caps as low as 4psi.

I can drive 100+ miles with engine at temperature, AC on, no issues, no pressure.

It doesn't build any?
Or just not enough?
After a long hot drive, all hoses are still soft to the touch?

Yes soft to the touch. To be more descriptive, if I was to remove the cap when the system was warm (never desired to try it full hot), coolant will come out the fill hole. But the hoses are soft to the touch.

I agree, there's a leak somewhere-or the thermostat is stuck partially open & not allowing it to reach operating temperature. The "overflow" tank is actually a degas tank, under system operating pressure-it's possible there's a hairline crack in it somewhere (above the coolant level, which would mean no visible coolant leak).

The engine warms up fast. No issues there. So it’s dubious that the thermostat is an issue. A leak, maybe. But I’m not seeing any signs, and perhaps more important, not seeing any loss.

It should have pressure to prevent excess air in the system which can lead to hot spots and cavitation, a pressurized system is a good thing especcially with a diesel. Test it with hand pump tester if it builds pressure the cap is probably bad.

Good idea regarding the hand tester.

My first perspective is that if it’s maintaining temperature and no pressure that’s less stressful on the system overall.

But the hot spot/cavitation thing is a concern which is part of why I asked. Low pressure and keeping cool seems intrinsically less stressful on everything. But it does affect boil temperatures. Which then again is offset by the glycol percentage…

 

[Trav]

Cavitation and hot spots are a lot more stressful. This is targeted more to larger diesels but it is also applicable to any liquid cooled diesel even those that do not use loose wet liners, the ignition pressures of a diesel creates vibrations in the blocks cooling jacket far in excess of gasoline engines. It will not damage dry liners but can erode all internal metal parts in coolant.

https://www.wanderlodgeownersgroup.com/downloads/CAVITATION%20AND%20SCAs.pdf

 

[SC Maintenance]

In addition to the cavitation, if you ever do get it hot it will boil and puke all your coolant out, then you will be in a pickle.

I would think if your system will hold pressure, your better off with it than without. Its not particularly going to hurt anything. The only reason to run without pressure is if it causes something to leak - which means its already broken.

 

[JHZR2]

In addition to the cavitation, if you ever do get it hot it will boil and puke all your coolant out, then you will be in a pickle.

I would think if your system will hold pressure, your better off with it than without. Its not particularly going to hurt anything. The only reason to run without pressure is if it causes something to leak - which means its already broken.

Yeah that’s a fine point. I have a new OE radiator cap, I would guess that’s the culprit but will have to see.

 

[JHZR2]

Cavitation and hot spots are a lot more stressful. This is targeted more to larger diesels but it is also applicable to any liquid cooled diesel even those that do not use loose wet liners, the ignition pressures of a diesel creates vibrations in the blocks cooling jacket far in excess of gasoline engines. It will not damage dry liners but can erode all internal metal parts in coolant.

https://www.wanderlodgeownersgroup.com/downloads/CAVITATION%20AND%20SCAs.pdf

Yeah I don’t want that. The 60x engines with iron block/aluminum head are more prone to HG issues though very long lived otherwise… which was part of my rationale behind keeping pressure low…. But it makes sense.

 

[zrxkawboy]

I've owned my '67 Belvedere since 1994. It has always been the same way.

 

[JHZR2]

I've owned my '67 Belvedere since 1994. It has always been the same way.

Doesn’t build pressure either? Weird.

A diesel is pretty efficient. Some say they can’t make enough heat to sustain in the cold winter.

 

[zrxkawboy]

Doesn’t build pressure either? Weird.

Yes. I remember when I first got the car, I stopped by the local radiator shop and asked the owner about it. He thought it was strange, too, so he checked it with his pressure tester and it held pressure...just didn't seem to build any on its own. A new radiator cap made no difference, so I've just never worried about it.

 

[Dadillac]

My understanding is that the water in the coolant is what causes the pressure build up. If by chance the coolant in your system is 100% coolant then there will be no pressure building. Is it possible that someone put in straight coolant?

Don

 

[JHZR2]

My understanding is that the water in the coolant is what causes the pressure build up. If by chance the coolant in your system is 100% coolant then there will be no pressure building. Is it possible that someone put in straight coolant?

Don

No. I have been doing a changeover to JD Cool Gard II and so it has gotten pretty high water percentage, then i drained the rad and block again and it is definitely 50/50. All exactly the same.

If I leave the radiator cap off the level of fluid will rise if a funnel is attached, or it will spill from the mouth. So maybe it is pressurizing, just not making the hoses hard to the touch.

 

[meep]

By “creep up to 100,” do you mean Fahrenheit or celcius? I assume you mean Celsius? If it’s Fahrenheit, it’s not getting hot enough to build any pressure.

if it’s Celsius, which would mean approaching ~200F, then there is something relieving the pressure, either from a leak or from excessive expansion. Are the hoses all good? Is there air trapped somewhere? I know you mentioned it hasn’t lost coolant, so that’s not it…

 

[Chris142]

The radiator cap is what holds the pressure in then releases pressure at a set pressure. It's either bad or is not sealing. Could be a ding there or even stretched from years of heat if it seals to plastic. Can you post pics of where the cap seals?