biturbomax

Hi I'm from Italy I live in Mestre-Venice and i have a porsche 928 s4 '88 Kat.
I have a problem with the water temperature.
Its stay near the second white line of the temperature gage.
I have replaced the rad with new one (the old rad leak on the left corner on the bottom) i have replaced the water pump i have replaced the expansion tank and the cap.
About 3000 km ago i have replaced the thermostat and the rear seal and the o-ring i have replaced also the heather valve and the water pump.
One month ago i have make a journey to Rimini (about 200 km) and the temperature stay between the first white line and the second white line when i'm returned to my home i have used the heat on my car and after the temperature stay near the second white line and various position more frequently that in past but stay always more high that in past.
When i returned to my home for this reason i have changed the radiator the expansion tank and the cap i have also tested the thermostat the new one and the old one that i have conserved in my garage the new one begins to open at about 84-85 C° and open slowly the old one begins to open at about 81 C° and the old one open more quickly.
I have try to burp my refrigerant system several times.
Today i have measured the water in the expansion tank with a thermocouple and the temperature was at 84 C° i have removed the temperature gage sensor (sensor meuasure at about 18 C° 670 ohm) on the thermostat housing i have measured 88-89 C°.
Yesterday i have measured the temperature with an IR gun the top rad house was at about 82-85 C° the bottom house was at 78-80 C° and more low when the fan goes on but i want to be more sure and i must repeat the infrared misuration because i don't remember well.
When the car is at operating temperature the 2 house top and bottom are firm and the water level increase when the car is cold the water level decrease.
I have also tested the fan switch and the resistance was in the range of the workshop manual.
In attached the image of the temperature on my indicator.
I'm waiting for receive a combustion test for head gaskets and i'm waiting for a new temperature gage sensor.
Can you help me to understand because the temperature is changed ?????????????????????
Greetings.
MASSIMO.
sorry for my english.

biturbomax

this is the photo of my temperature indicator.

dprantl

If you turn the heater on with the car fully warmed up and idling, then rev the motor to ~3,000RPM, does the air get significantly hotter? If yes, see below.

Park on as steep a hill as you can find with the noise pointing up. Start the car and set the climate control to the defrost setting (bottom slider all the way to the right). Remove the radiator cap (careful if the car is hot and it is pressurized) and run the car to operating temperature (until the thermostat opens). Squeeze the upper radiator hose several times. Look into the coolant reservoir to see if the coolant level has dropped any (yes, it's hard when the car is on a hill). Put the radiator cap back on and go for a spirited drive (keep the climate control set to defrost), then park on a level surface and check the coolant level again. The idea is that once the system is free of air, the coolant level should no longer get lower.

Dan
'91 928GT S/C

Leon Speed

Hi Massimo, welcome. You say the temperature started to rise after you turned on the heater. It can be that there was rust in heat exchanger and that this is partly blocking the flow somewhere. Did you open the heater when you changed the cooling fluid? If not, this might be a good option to try. Second, there are two types of thermostats, one that opens at 75 C degrees. The other one is used as of model year '90 and opens at 83 C degrees. Do you maybe have the 83 C one?

EDIT: I believe stock is 83 C degrees. I'm not sure when 75 C degrees were introduced..

biturbomax

the old thermostast and the new thermostat are 83 C°.
In attached a photo of old thermostat.
The car have 139000 km. and i have changed the thermostat when i have make the timing belt and my mechanic has make the burp of the system and set the heat on the max.
After i have changed the heater valve in my garage the last summer when i have fixed the a/c system and the vacum system.
About one week ago i have changed the rad the expansion tank and the cap after the journey to Rimini and the problem with the temperature in open road with a fluctuating indication of temperature on the gage when i have make the return trip to my home i have used the heat and after i have noticed the variation on the temperature and more precisely when i had set heat to the max temperature decrease when i had set temperature on cold temperature raise.

biturbomax

my coolant level is okay when the car is cold and is more high when the car is hot.
in my garage i have raised up the car on the front and on the back and i have squeeze the top house and the bottom house but the level non changed.

PorKen

This is normal. My '86 quattroporte with a 83°C thermostat looked the same as this. Even on the dyno, it would not go higher than the line. On the freeway it would be lower.

I installed a 75°C thermostat, and it doesn't go higher than halfway between the lines. The oil cooler has a thermostat too, so the oil will get hot enough.

Performance is improved with the lower temperature thermostat, I've found.

Some parts places may supercede the factory 75°C '928 106 129 16' with the 83°C '928 106 129 17', so use the Behr/Wahler number '116 200 00 15'.

Jim M.

Hi Massimo, I think your system is working just as the Porsche engineers designed it to. On the US cars and I think world wide the second white line corresponds to the temp setting of the thermostat. The gauges themselves are not alway accurate and sometimes get less accurate with age. Additionally you seem to have done a good job with the maintenance of the system. I assume the coolant was changed with the cam belt change and the new thermostat. Typically you only have a problem when the needle gets into the red area.

Bill Ball

Unless I read incorrectly, the temperatures you are measuring are about the same as the thermostat. So, that sounds OK. I changed to the 75C thermosat and on cool days my water temperature stays right at the thermostat closing temp, but on hot days, climbing a hill, it can go to the second white line and a little over. In other words, the system is capacity-limited. The colder thermostat changed the behavior on cool days only in my car.

6mil928

Mine stays at the first white line on my 81.

biturbomax

this is the second time that i change the refrigerant in about 5000 km.
but I do not succeed to explain me because the temperature is changed. Always she had been to half between the first white line and the second white line. It can be that the system did not work correctly before that the temperature changed?

Bill Ball

OK, so you are saying this is a new behavior. I missed that or it wasn't clear.

You seem to have replaced everything in the system that could account for a change except the guage.

Did you do anything in particular right before the temperature behavior changed?

Jim bailey - 928 International

The S-4 and newer 928 cooling system is MUCH different than the 78-86. The later system uses a computer brain next to the passenger seat to control the temperature. It uses input from several sensors to determine what is needed and then turns on one or two fans at varing speeds to maintain the factory desired temperature. In addition the 1987-90 ALSO has a set of flaps in front of the radiator which would open and close as needed to regulate the temperature. The factory specified thermostat is the 85 C which is the temperature that Porsche designed the engine to run. Most S-4s the temperature guage seems to run higher more near that upper white line where early cars (belt driven fans ) are lower white line to midrange depending on air temperature, road speed and loads. The output stage (finned amplifiyer looking ) in front of the radiator passenger side contains two mutually independant output stage modules with seperate protective circuits each module controls a seperate cooler fan. If a fan motor fault occurs the cooling air flaps are opened 100 % and the intact fan operates at full speed while the faulty fan is switched off. A check is made aprox every 20 seconds to see if a fault persists... At engine temperatures below 74 C the front flaps are closed at 74-85 they are at 30% and 85C > they are 100 % . The fans begin to run at about 79 C but at only 8 Volts. By about 95 C they are at full power. The full description of how it works is about 25 pages in the workshop manuals.

Bill Ball

Jim brings up a good point, of course. In my car, if one of the fans is not operating, the temp does not just go to the second white line - it overheats much worse. Still, worth checking fan operation. A simple check to just see if they are operating at all is to switch on the AC - both should come on full speed immediately. Otherwise, they come on progressively, as Jim describes. Certainly, by the time the water temperature is to the second white line, both fans should be opreating fully, AC on or off.

Like many others, I've disabled the flaps by pulling the fuse. There is a **** under the rubber boot on the flap motor that will allow you to adjust them open after you pull the fuse so they will always be open. It somewhat slows warm-up on cold days.

PorKen

78-86 have a automatic temperature control system (with fins), too. It's called a 'fan clutch'.