AO

^^^ What he said.

WallyP

The function of the pressure sensor is to hold the "Low Coolant" warning after the coolant gets hot. If you get a low coolant warning, and the pressure stays on the system, the warning is held even if the coolant expands enough that the low coolant switch is not tripped. If you lose pressure, the system assumes that you removed the cap to add coolant, so if the coolant level is above the minimum, no warning is triggered.

If the switch "blows", yes, you will lose pressure and coolant, and the switch will need replacing (with your choice of a new switch or a piece of pipe).

The 928 was Porsche's best car, and they went the extra mile to try to make it as close to perfect as they could. The extra sensor is just an attempt to keep you from having a low coolant warning the next time that you crank the engine from cold, after thinking that the level was OK.

dr bob

For those reading at home, Mark is joking.

Losing pressure WILL allow temps to increase, thanks to localized boiling at hot spots in the block. Vapor doesn't transfer the heat away quite as handily as liquid, so the hot spots get hotter and the problem gets worse. Finally there are bubbles passing through larger areas of the cooling system and things go downhill, including total system temperature rise. The rising temps come after the internal problems in the system. But the temps ultimately will rise, and wiyhout boiling off coolant in the resrvoir.

Ha ha!

VehiGAZ

Ok, I guess I get it now. But if there's no leak at the sensor, then nothing will change.

dprantl

Yes, you are correct. My original point was that this was happening on my car for over a year and I think that this sensor was slowly failing and releasing some pressure in the cooling system making the car run hotter than it should have. I have seen many people complaining about their later 928's running hot even though all other cooling maintenance was performed. I was suggesting that maybe this sensor is the culprit, since in the mode of partial failure it is very difficult to detect/diagnose. Usually it is only after it explodes into two pieces and you see a cloud of steam that you will know.

Dan
'91 928GT S/C 475hp/460lb.ft

JPTL

So on a '91 GT, assuming that these dash temp gauges are somewhat consistent, is running just past the last white line hotter than the norm?

VehiGAZ

Thanks for the confirmation, Dan.

My acute interest in this topic comes from the fact that at my one-and-only DE day a couple of years ago, I had overheating problems on the track (Lime Rock Park in CT). I saw the coolant temp climb to the red line and got the over-heating warning from the digi-dash during my last two sessions, at the hottest part of the day - about 100*F in the afternoon. I was running most of the course at high rpm in 3rd gear, shifting to 4th only on the front straight. I did not have my belly pan installed, and I was running with a more-or-less 50%-50% mixture of pump gas and high-octane race gas.

Now, if it were any other car, I would understand it overheating from pushing the car hard under those rather challenging conditions. But I expected the 928 to be able to take it - it's an over-engineered, high-performance Porsche after all!

Since then, I've noticed the coolant temp climb steadily to the high end of the gauge's range while driving public-road twisties in low gear and at high rpms in warm weather, but I haven't pushed it to the point of overheating again.

For these reasons, I am concerned that I have a problem in my cooling system. I will re-install the under-tray (a recent thread convinced me of that), but I guess I have to do some more high-rpm/low-gear driving on a hot day to see if it was just too hot that day at the track, or if I really have a cooling problem.

AO

On a hot day w/ AC, I tend to run a bit past the 2nd line too. I'll let you know if I notice any difference.

mcholdfast

My 89 GT does the same. It used to worry me constantly but after driving it very hard during a DE at Road Atlanta last summer for two days during mid 90 temps, I found that the temp never got any higher. I don't worry about it as much now but I'd still like to bring the temp down a bit. The 90 GT and the three S4s I've had all ran much cooler according to the temp guage. I'm anxiously awaiting Andrew's report also.

fraggle

Does anyone have some additional IR data to throw in here as reference?

VehiGAZ

Mike, were you running with the belly pan/undercarriage tray/whatever you want to call it installed in those conditions?

I wouldn't care if the temp ran to the top of the range - I just don't want it over-heating.

Manfred

On 100 degree days with the AC running I sit just above the last white line ('91GT--belly pan installed). It bothers me and I keep an eye on it but it's never gone above that level. It comes down just a small amount if I'm cruising on the highway--goes up just a hair in start and stop conditions. Never tracked it.
Dan

VehiGAZ

As many here will exclaim, we run 180*F thermostats, but often the temp will stay lower than that - the max it should hit is 180*F

dprantl

As an extra data point, my car does not have the front or rear belly pan.

Dan
'91 928GT S/C 475hp/460lb/ft

dr bob

The thermostat probably has 5-10º of range full-closed to full-open. The heat transfer in the radiator is governed by differences in engine load and radiator air temp, then airflow. Seeing an operating range that's as wide as half the space between the two upper white lines is therefore very normal. On extremely hot days with lower airflow and AC running, seeing another few needle-widths of gauge reading doesn't get me at all concerned.

----

I did pick up a few cans of the coil cleaner recommended by a couple other folks over the weekend. I didn't want to use it on the hot radiaor last eveneing when I got back from H-D. It's into the 80's here already so too hot to go outside and play today anyway. Maybe a bit later when the garage is in the evening shade, and I'll report back results.

Two of those coil cleaners will get used on the HAVC condensers here at the ranch. Summer is due to start today sometime. May try some on the 928 evap unit too.