Thanks Dr Bob!
By the way belt tension light is a false reading. It's not even connected yet. And it only came on for a minute then it turned off by itself.

 

Most properly operating 928s will have the temp gauge read from the middle to the upper white line and hover around there.

The air pocket must have purged finally.

 

That's the normal running temperature in warm weather with the stock thermostat. The needle should creep closer to the 2/3 white line, the fans should come on, and the gauge should drop back to about here. The fans should turn off.
Going down the road, in cold weather, the air going past the engine will reduce the water temperature so low that the thermostat will never open. This is why Porsche added the "flaps" in front of the radiator in 1987 (?). The gauge, in cold weather, will read about 1/2.

 

Very good info. Couldn't have done it without everyone's help here. Thanks Greg!

 

Greg,

i have heard that story from Porsche previously but do you not find it somewhat suspect? The ambient heat loss from the engine casing is [I suspect] not going to be sufficient to maintain the requisite temperature and thus the thermostat is going to open the flow path to the radiator and thus progressively close off the internal recirculation. Quite feasible that the thermostat will cycle open and closed in really cold conditions but the best the flaps can do is cut off the cold air flow- that would keep the thermostat open longer of course.

That Porsche dropped the flaps for the GTS suggests to me they finally worked out it was a cost intensive irrelevance. I also remember some "expert" 20 years ago advising they were more of an aerodynamic aide for top end - that sounded like a crock to me as well but a bit more credible. For my late S4 in our hot environment the main dealers wired the flaps open and I was advised toremove the slats altogether to imporove the cooling a tad which it did.

Maybe we will never know the real truth?

 

Fred:

Every year, in the first few hot days, I get 15-20 cars in to check for running too hot. It's always the same story..."It usually runs in the middle". I dutifully check everything over, pull the thermostat and make sure the rear seal is good and the thermostat opens at the correct temperature, make sure there is water circulating, make sure the fan/fans are properly working, remove the sender and put it a pan of water to check the calibration.

I may toss a new hose or two on the cars (if I think they are going to fail in the hot weather.) I might install a new thermostat. I might have to replace a fan or two.....an a occasional fan amplifier. I might do a cooling system flush. Heater control valves and the short hose to the valve are common things.

I explain how the car ran all winter at the 160 degree mark, because cold air was rushing by the engine and the thermostat didn't open, explain how the thermostat works and how the fans work. Explain where the gauge reads when the thermostat opens. Explain how the high speed fans work, what to listen for, what to check to make sure both fans are running, and send the cars home. 15-20 times I repeat the same talk...I should record it and just have people sit down and watch it!

We "plan" ahead, for this stuff. Mary "stocks up" on radiators, fans, thermostats, caps, factory water hoses, etc.

 

Greg,

Not too surprising- that is entirely consistent with what I experience but consider to be "normal". The difference is my car has never seen an ambient temperature less than 15C. 20 years ago when I first entered 928 ownership folks would tell me how the 928 cooling system is "problemmatic" in hot weather and I was "advised" to use a 75C thermostat- I quickly concluded in the cool season that was nonsense and replaced it with a stock item.

The thing that limits heat transfer is the water pump and that appears to be more than adequate. In really hot weather it is the ac system that limits cooling performance and even then I have been cruising at reasonable speeds in 45C heat with the needle just before the last white line which according to my ST2 kit is running at 95C as the coolant leaves the motor. I eventually concluded that the cooling system design is quite brilliant but in our hot climate the flaps need to be removed.

I also discovered something quite interesting recently- after my ac compressor packed in I decided to do the most thorough system clening I could. I pulled the condenser and cleaned it both externally and internally. Externally there was nothing obviously clagging up the condenser but even so I carefully used my pressure washer to cleanse the unit from both sides- now when I run the motor without the ac running the temp gauge indicates it is running noticeably cooler so I put the ST2 on the car and sure enough what the gauge suggested was in fact correct. Examination of the stock maps in ST2 suggests that warm up enrichment stops at 80C so this in turn tells me that is the point where Porsche consider the motor to be fully warm. On the other hand the fan programme maxxes out when the cooled coolant hits 95C that in turn implies that Porsche considered it OK for the motor to run with a coolant temperature exiting the motor of about 101C assuming a 6 degrees C differential- which as per my interpretation is just past the last white line. Strangely enough when pushing the motor along with the ac operational in 45C ambient temps the needle moved just to the right of the last white line. That told me that Porsche knew exactly what they were doing.

Most regular cars one sees show a temperature gauge sitting at one point and seemingly never moving from that point no matter the ambient conditions or how the motor is worked. I concluded that our temperature gauges are in fact very accurate and by design are intended to show exactly what is going on rather than something just there to keep the owner "happy". The notion that they are "not that accurate" is folly as far as my expereince suggests. I eventually concluded that the motor is quite happy as long as it runs somewhere between 80C and 100C and that is what owners need to get their heads around- it surely surprised me but there we are.

I also concluded that it does not matter how much power the motor is making- the heat the cooling system needs to remove is a constant give or take a little and as I thought about it that made sense given fuel burns at the same temperature no matter how much mix one crams into the combustion chamber- of course more heat goes out with the exhaust but the boundary layer resistance on top of the pistons is what for the most part limits heat transfer into the cooling system and that is why your superb motors do not need enhanced coolling system design. Similarly the GTS does not need more cooling than the S4 motor so when they dropped the cooling flaps for the GTS that suggested to me that Porsche considered it an unnecessary folly.

 

I suggest you rethink this assertion.

 

Why?

 

The First Law of Thermodynamics comes to mind. If you want to educate yourself on the subject, I suggest seeking out the article below.

14 Rules for Improving Engine Cooling System Capability in High-Performance Automobiles
Produced by the National Automotive Radiator Association (NARSA) and by Richard F. Crook, Transpro, Inc.

Reader's Digest Version: ~1/3 of combustion energy goes to mechanical work, ~1/3 exits the exhaust and ~1/3 is transferred into the cooling system. Load on the cooling system is directly proportional to the combustion energy (power generation).

 

On a related note I spent the winter troubleshooting insane overheating issues on my 2000 Honda Insight. Bled it, drove myself nuts, wound up replacing the water pump (worn), radiator (disintegrating), coolant, and eventually replaced the head gasket. The last item fixed it, problem was gasket was leaking (even with 180/170/180 ratings in the cylinders) and the pressure was vapor locking the cooling system.

Might want to see if the coolant is under pressure when engine cold.

 

Maybe you would like to think about why a Greg Brown stroker turning out over 500 bhp does so without modification to the radiator and water pump. It most certainly involves a knowledge of thermo dynamics and heat transfer and more importantly - the knowledge of how to use them.

 

+1
The stock 928 cooling system is ridiculously over engineered, and thankfully rather simple in design.

They designed these cars to able to maintain top speed while driving through the Sahara Desert with the air conditioning running (idling in traffic under the same conditions too).

Mark Anderson, over two decades of racing a 500+hp 928 with a stock cooling system, in California.

The only reason to buy a non-stock radiator is cost and the fact that 928 International sourced an OEM quality replacement for a fraction of the price

 

Doesn't require much thought at all. In normal operation, the 500hp engine runs about the same thermally as the stock 300hp one. When it is running at its increased power capacity, it is most likely doing it for brief periods of time - bursts of acceleration. The heat capacity of the engine and cooling system is capable of absorbing this additional transient energy without issue as the cooling system was designed to handle 300hp steady state at high ambient temperature. This (300hp steady state) would be equivalent to driving throttle open at top speed for extended periods of time. If the 500hp engine can run steady state at full throttle for extended time with the stock cooling system, and I'm not suggesting it can't, then Porsche over designed the cooling system, but I can assure you that the thermal load on the cooling system will be higher by ~67%.

 

Porsche themselves said the flaps were for drag reduction when the S4 first came out. They function similarly to cowl flaps on a piston powered airplane.
Allowing air to flow through the radiator creates drag.
While that drag is negligible at any speed below around 100 mph, drag is a square function, just like kinetic energy and stopping distance.
Those flaps allowed the car to reach it's 170 mph top speed.

Most modern cars (my Cayenne included) have water temp 'gauges' that are little more than warning lights that have needles. If the temp goes up or down much, people freak out.
My work truck has a functioning temp gauge. I can see the thermostat working as the temp fluctuates around 180. I can see the temp climb when I'm using a lot of power, like going up a hill with a heavy load. The fan kicks on at around 210 or so and kicks back off at 190 or so. I can see these fluctuations.

The 928 has a 'real' temp gauge. It's pretty precise and repeatable, although not super accurate. Mine reads just above the lower white line for 'normal'. I've shot the hoses & water bridge with a temp gun and the cooling system is fine. Other's gauges read other places, typically somewhere between the two white lines. As long as the owner knows where 'normal' for their car is, they are fine.

Temperature and heat are two different concepts. While the gas burns at the same temp no matter what, the amount of heat generated (and needed to be gotten rid of) is vastly different depending on fuel burned.

We're simply driving cars that have a very capable cooling system.

They dispensed with the flaps on the GTS because the motor made more power. Plus nobody ran the car that fast anyway (don't forget that a GTS motor won't survive extended high RPM operation without oil issues).