Barry Chan

Ever since i restored the car 3 years ago, i have had the 7 lb low pressure cap, in the winter the temp is around the first white mark in city and below the white mark on highway, car was never overheated even in the summer, out of curiosity i went and got a 10 lb cap last week and found the temp slightly higher almost to the middle of the 2 white mark in city driving and just below the middle on highway? I went back to the 7 lb, true enough the temp is back to what it was before the 10 lb cap, anyone else experience this at all or similar?
Barry

Tom. M

Perhaps the "air bubble" around the temp gauge sender is bigger (and not as hot as the coolant) with the 7 lb cap...and at 10lbs..it shrinks and the sender can actually give you a proper coolant temp????

unscrew the temp gauge sender to see if there is trapped air?

Just a SWAG...

depami

Isn't 14 PSI the norm? Or 1 bar (which is about 14.5 PSI)?

Randy V

That sounds right. Higher system pressure results in higher operating temperature which translates to increased power efficiency.


Huh?

jpitman2

How can increasing the boiling point temp (by a higher pressure cap) produce more heat to lift the temperature ??? If the 7lb cap was adequately preventing coolant boiling in your environment, which is all that a pressure cap is supposed to do, why would you change it? If you moved to a hotter environment, it would probably become necessary to fit the proper cap.
I also dont see how an air bubble at the sensor could cause misreadings - how long after the coolant reached stable temp do you think the air bubble could stay cooler ? its surrounded closely on all sides by hot coolant and hot metal. During warm up it might show low values, but after its all heat soaked and stable, I doubt it.
jp 83 Euro S AT 54k

Speedtoys

The higher pressure carries a higher boiling point..and thats where the cap regulates the energy in the system as a whole.

At 7psi over Standard Atmpshpheric pressure, sea level, is about 231d.
At 10psi over, it is 6d higher.

I would not be surprised to find 6d a significant shift in temp gauge needle position.

jpitman2

I understand the relationship between pressure and boiling point, BUT ITS NOT BOILING.
When the coolant is at a stable temp, the heat being produced by the engine is being balanced by heat shed to the air at that ambient temp, by means of circulation and the radiator. If the radiator and pump were overspecced for the job, the stable temp should hover around the thermostat setting. If under specced, the stable temp would vary with ambient temps, to varying degrees.
But, IMHO, as long as the coolant temp never gets near boiling (ie not close enough for localized steam bubbles to form in the hottest spots), the pressure of the cap should not be relevant.
I agree that 6d change in temp should be observable in our gauges. Sadly its not true in most modern cars - my 2008 Suby gauge never moves once its up to stable temp, but my obd-ii reader has shown a range of 83-96C can occur. And cap pressures are rising all the time - our Mazda 3 has a 2bar cap.

I wonder if the OP was (un)lucky enough to have different ambient conditions when the oddity was observed?
jp 83 Euro S AT 54k

Speedtoys

The coolant is most assuredly boiling around the cylinders. Tiiiiny bubbleeesss...



Youre moving more coolant out & into the system with a lower pressure cap, than a higher one, and you can support a higher temperature with it.

jpitman2

mmmmm
'moving more coolant out into the system with a lower pressure cap' - What does this mean, noting that the pump is circulating coolant all the time? Out to where - away from the liners to the radiator?

The ACTUAL (overall heat energy)temp of the system CANNOT change, unless some heat is added (more fuel used), or lost (change in ambient temp of the air, more radiator capacity added, more airflow added etc).

What might be happening is that the temp gradient (from hot cylinders to temp sensor) is being flattened by the higher pressure cap reducing local boiling on the cylinder walls, and therefore conducting more heat to the coolant....
jp 83 Euro S AT 54k

Speedtoys

To the overflow tank?

Either way..im holding to "holding a higher temperature before cavitaion" which will begin to eject coolant as vapor is 20x more volume than liquid.

If he was conducting LESS heat to the coolant with less pressure, he'd be building up heat as the cylinders then ran away from what the water was pulling from them...although the 'magic' of HIGH pressure caps and soap (water wetter, etc) does a lot to prevent hot spots from cavitation as well.

Tom. M

Adding a cap with a higher pressure capacity will only affect the boiling point of the coolant. Like stated earlier..going from a 7 to 10 lb cap may increase the boiling point by 6 degrees....that said..
Unless the coolant was at the max heating capacity of the fluid at 7 lbs.....adding a cap that holds 10 lbs...should have absolutely no effect on the temp of the coolant...as that is derived from the engine load (add heat) and the efficiency of the rad (minus heat) and the coolant flow.

Of course if he stressed the engine more (ran the A/C..or ran on a hot day under load)..then adding a higher pressure cap may be enough to keep the fluid from boiling off..

Based on his readings...he was no where near the heat retaining capacity of the coolant (I think the lower line represents maybe 80degrees?.. Odd that he is able to reproduce the "cooler" readings with changing the cap.. Any fluid thermodynamics experts out there?

Barry Chan

3 yrs ago when i was reading the forum, OEM 14 lb cap for our car and some said low pressure cap help ease the stress etc... but more reading with some said higher pressure cap is better.
I did the comparison on 60 degree outside temp both days with a 45 minutes test.(didn't want to double the pressure so i use a 10 lb)
So it doesn't matter what pressure cap to use as long as no boiling over?

depami

If boiling does exist a lower pressure cap will allow it to “puke out” sooner and possibly keep other things from “blowing out”. A higher pressure cap may increase chances of a “blow out”. With a standard 1 bar, or even a 2 bar cap, a “blow out” will still only occur if a problem exists such as stuck T-stat, rotted hose, toasted heater valve, etc. But, boiling should never occur in a properly functioning system.

Are there ever any puddles or drips?

danglerb

Anybody put a pressure gauge on their 928 and actually measure pressure during normal operation?

Different parts of the engine are at different temperatures, the point of antifreeze and a pressure cap is to prevent boiling at the HOTTEST places because steam does a lousy job of cooling, so boiling allows the hot places to get much hotter.

terry gt

When was the last time the T-stat and the GASKET behind it was changed ? could be why the pressure changes the temp.