khalloudy

Since I did my last service and changed the coolant etc, I decided to replace the thermostat considering it was on the car for a long time… probably 15 years (though no issues) as preventive maintenance.

what I have noticed since is that the car will warm up normally but run a little warmer. If I drive it hard it the needle will move up higher then when i calm the driving a little it will come down a few needle width lower. Also as it runs hotter, it will suddenly drop a few needles lower suddenly… not gradually on its own.

I am just wondering if my new thermostat is faulty, sticking or for some reason misinstalled…

Any thoughts?

Speedtoys

Or...your old one was sticking open, and this one is modulating just -fine-.

With my cooling system and gauge wiring end-to-end in factory shape, my gauge moves around a lot over 70 degrees F ambient temp outside...but it never gets HOT. Ever..

Petza914

Always a good idea to test a new thermostat in a pot of water on the stove with a thermometer to make sure it opens at the correct temperature before installing in the car.

khalloudy

Good call on both comments. The car seems to be running fine, but different than with the older thermostat. It definitely runs warmer so not sure where the delta is. One of the things I know for sure is that I changed the coolant/water mixture from 50:50 to 30:70 as it never gets that cold here; not sure if it makes a difference.

Mrmerlin

are the head gaskets original?

khalloudy

Nops, head gaskets were replaced in 2004 or so and I do not show any loss of coolant or oil/coolant mixing.

Speedtoys

That wont make a difference.

Get a temp gun, or plug in the ST2, it's like;y _just fine_, and this 'stat probly operates "more properly" than the old one.

A lot of people believe these a 0/1 binary units, the BEGIN to open at a temperature, and become fully open at another. I would not expect two different ones _years_ apart to operate exactly the same.

928NOOBIE

How old is the expansion tank pressure cap. If older than 5 years would be a good idea to replace it. Pay more than 10 dollars.

Not saying thats going to change your temp gauge difference you describe; just good practice when you think about the punishing environment they are in and the work they do.

I'm in Speedtoys' camp the new thermostat will work differently than the old one if for no other reason than its new and your old one was do we know how old?

When you replaced the thermostat did you also replace the rear seal? it is a separate item and doesn't come standard with the thermostat.

There are posts in various threads about the rotten condition some of the rear seals are in because they were never changed. If you didn't do that then you only did half the job; especially if this is your first time going into the thermostat area. It costs peanuts. There is no reason I can think of not to include it with a thermostat replacement.

khalloudy

Hi buddy,

I did change all the seals and gaskets in the thermostat housing while at it. Quite an easy job to be honest. Maybe I should open up the housing again and check/test the new thermostat with hot water. The expansion tank cap is about 20 years old now give or take as I think I might have changed it around 2004 along with a new tank.

Speedtoys

If it aint leaking, leave it.

Validate if there's _even a problem_ at all first.

You would be surprised how -far- a functional guage circuit moves the needle with 1 degree celcius change.

FredR

.

If there was a problem with the thermostat chances are you would know about it. Providing you purchased a quality made item the chances of an issue are slim to non existant. Testing them to check for correct opening temperature is always a good idea.

One of the worst pieces of 928 advice I ever received was to use a 75C thermostat- made no difference during summer but during the cool season a definite no no.

The most reliable method of monitoring the cooling system performance as Jeff suggested is to use ST2 if one has one - this monitors the output channels from the temp 2 sensor.

What you see on the dash panel gauge is a comletely separate channel with its own sender unit, wiring elements, cluster comonents and finally the gauge unit itself. Instantaneous step changes of temperature just do not happen and if you see such it will be because of a measuring system glitsch. Mine recently started under reading- chances are the sender is goosed.

If you see the temperature going up over the course of a minute or whatever and then the alarm goes off chances are the water pump has thrown the impeller [and you shut down immediately].

khalloudy

Hi Fred,

Thank you for your input. The new thermostat is from Porsche and has been in my box parts for a long time in its nylon box (probably like 10 years :-D) but new nonetheless. Did not test it, but I wonder it is properly seated; will open the housing to check.

It mostly moves in an analog fashion running halfway. However when I drive it hard it will creep up towards the second white line and accordingly when I calm the driving it moves back down in an analog manner, though sometimes it just drops a couple of needles width down. Both the fact that it can drop suddenly sometimes and that it can creep up under hard driving caught my attention to further check.

Unfortunately I do not have a ST2. Luckily it never moved up in the fashion to go above the 2nd white line or threw the alarm. Again, I am also running a wrong mix of coolant to water ratio so maybe also that is contributing to something.

Speedtoys

Remember, the 2nd white like is still NORMAL. Very much normal, its only about 91c AT the white line, if I recall. I imagine getting INTO the red, is closer to 96c..then you're getting there.

But what you'll find is the needle will go to or very near the top white line, then stop. The gauge moving is just signaling where equilibrium is. More load makes more head (combustion) and more RPM makes more heat (A fantastic amount of ring friction)..the needle temp is a result.

Also as the coolant gets hotter, the radiator is able to shed heat better than when it was cooler. The higher temperature delta is at work here..so..the warmer the coolant gets, the slower (at some point) it will continue to get warmer.

The red light comes on at _115c_. That is "hot".

FredR

Khaled,

I do not see anything in your notes that warrants intervention or the slightest concern for that matter. What you see on the dash panel seemingly does vary from example to example and the position of the gauge should be read in comparative terms and if you suddenly see the needle mving onwards and uowards compared to where it normally sits that is the time to be worried.

Both my late S4 and my current 928 showed exactly the same behavioural characteristics. Whereas both ran with the same motor, the indicator loop infrastructure in part differs. My cooling system is well sorted in that it has a radiator with slightly larger capacity than the stock item [but that is not critical or needed], the GTS variant also dumped the slats- that allows more air to flow through the radiator, I used to run with REdline water wetter- that also helps improve heat pick up and rejection and have always run with a 30/70 coolant mix which is the ideal ratio for a hot climate- any leaner than that and one risks the coolant protective chemistry being compromised. The latter is probably the single most effective way to improve cooling.

Control of the system is based on the temperature of the coolant leaving the radiator and that typically sits at about 90C. The radiator typically takes out about 8 degrees C worth of heat. The fan speed is controlled to operate in the range 85C to 95C as measured in the radiator outlet tank. In practice my system invariably controls engine temperature within a very narrow range and typically runs at 98C which corresponds to a couple of needle widths before the last white line and that line is meant to convey a temperature of 100C. That the lines are coloured white signifies normal operation. Beyond that there is then a red line and that signfies danger. I am not sure what temperature the redline region represents but I suspect it is meant to be about 110C and then slightly above that the alarm annuunciates on the digi dash panel..

Thus as long as the needle hovers around the last white line all is well.