Nothing in my owners manual describes what the max temperature might be on a 85 degree warm summer day at freeway speed driving. I was driving with the A/C on at the speed limit on the longest Hwy 80 continuous uphill grade and my temp gauge was pegged on the last marker prior to the horizontal red stripe and It never went above that. Is that acceptable or do I have to investigate the reason for that reading.? By the way, I have my 1989 S4 radiator shutters disengaged in the open position.
This is a common question. The answer is as long as the engine coolant temperature indicator needle isn't in the red, you are OK. I live in Arizona where the temps can be above 110 F / 43 C. My 1987 S4 with air conditioning turned up to max often indicates in the upper 2/3rs range and sometimes nears the red but never gets into the zone. What is as important as the engine coolant temperature indication is the rate of change ... if you see it rising quickly, that is an indication something may be wrong (coolant leak) .... if decreasing the throttle opening facilitates a drop in the rate of increase, keep doing that.
I recall once driving my 928 GTS through Death Valley during the summer. The temperature was new 120 F / 49 C and the air conditioner was blowing as hard as it could. As I was driving up a long climb I kept an eye on the engine temperature indicator. I reduced speed/throttle opening to keep the coolant temperature indicator within 2 needle widths of the red zone and all was OK - yes, use your throttle as the engine temperature moderator - downshift if needed.
To be sure, use an IR heat sensing gun and verify that the temperature isn't too high, aim at the thermostat housing. It should be below 210F / 98C.
The 1987 and newer 928s have two electric cooling fans. Both need to be functioning for the A/C to work properly. The easiest way to check is to open the hood, start the engine and turn on the A/C. Verify that both fans are running. If only one is working, get the failed fan fixed or you can suffer over heating and excessive high-side refrigerant pressure which can blow hoses and over stress the system.
The 1987 and newer 928 fans run at different speeds as needed to properly cool the engine. When the engine is cool, the fans will not run at all. You may find that if the coolant temperature reads a little high, turning on the A/C may help as it forces the radiator fans to run at a higher speed.