Replace Thermostat
and Radiator Hoses
1990 928 S4

July 20, 2000

As part of the pre-race inspection, Marc Thomas found some bulges in my upper radiator hose. He recommended replacing the radiator hoses and changing the thermostat at the same time. I ordered the hoses and thermostat from DEVEK on Monday (7/17) and they arrived Wednesday.

I also decided to change all the coolant. Since I was changing the oil, I had already removed the bottom engine cover. The first step was to remove the cap from the coolant reservoir located on the passenger side of the engine compartment. Then I drained the radiator. There's a drain plug on the bottom passenger side. It's blue with a phillups head. I had a large catch basin to collect the old coolant. About 2 gallons came out of the radiator.

Next I drained the engine block. There are 2 drain plugs, one on each side of the engine. I used a 13 mm socket to remove then, and had emptied and move the catch basin to collect the coolant. I got another 1 1/2 gallons of coolant from the engine block. I then reinstalled the drain plugs (with new washers).

Next I removed the upper and lower radiator hoses. They are held on by hose clamp at each end, loosen the clamps and slip off the hose. I also removed the hose going from the thermostat to the reservoir.

The thermostat housing was now clearly accessible, it's held on with 2 13 mm bolts. I used a socket wrench on the top bolt, but needed a 13 mm box end wrench for the bottom bolt. Even after the two bolts were removed the housing stayed on the engine. I had to gently persuade it off the engine block. Now the thermostat was exposed. I had to grab it and pull if out.

Once the thermostat was out, there is a seal that should to be replaced. This seal is made of metal covered by black rubber, and it's hard to get out. I had to us some RoboGrip pliers to bend the bottom of the seat enough to get a good enough grip to pull it out. I then cleaned up the mess, and installed the new seal. Luckly, that just presses in without too much fuss. The new thermostat was a 75 degree C type, and it slide into place along with a large O-ring type gasket. [Note: On the new thermostat there was a small ball valve that goes toward the top to let air escape].

I then reattached the thermostat housing, and installed the new radiator hoses. After making sure all the bolts were secure and hose clamps were tight I added the coolant. I started with one gallon of phosphate and silicone free anti-freeze, a bottle of Red Line Water Wetter, and then 2+ gallons of purified water. I used a small funnel to help get most of the coolant into the tank. I took about 30 minutes to fill the tank and let it drain slowly into the engine and radiator, then fill it again. When it wouldn't drain any more, I started the engine and let it run for a couple of minutes. The coolant tank drained some more, so I topped it off. I repeated this process a couple of times.

The whole process took a couple of hours and cost a little over $100 for the 3 radiator hoses, thermostat kit, and coolant.

Thermostat Housing

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Copyright 2000, George A Suennen
written: 07/23/00
rev: 08/29/00