928 overheat at idle


I am at wits end and need help for solving an overheat at idling.
1980 928, 16v 77000 mi, driven <3000/yr.
After a rigorous drive, min 1/2 hr @76F ambient temps, and reaching normal engine operating temps of 195F per Auto Meter electric temp gauge sensor installed upper hose, no overheating. Once left at idle for 5 to 8 minutes, Aux A/C fan on at ~195+, clutch fan on and apparently functioning normally, both temp meters elevate to warning light & Auto meter reads 212+, at which time I shut engine down to prevent excessive overheating. After elevated temp ~210, getting car back on road with air flow, coolant temp comes down. Seems like an air flow problem ?? Original radiator replaced in 1995 @ Browns with Behr for complained of "overheating". Coolant previously changed 1998, SHAME ON ME !.
Current Attempted fixes completed:
1.) new thermostat changed 83 deg; new 13psa coolant cap.
2.) coolant flushed/changed (Zerex G 05) and pressure tested @13 psa; burped coolant via cycling heater valve 1/2 doz. times; coolant level checked.
3.) Co2 gas test.
4.) timing @ 23deg advance.
5.) radiator flow check, via "hand on radiator fan side grill" hot checking for uneven temp.
6.) Fan clutch filled with two bottles of Toyota 10,000 silicon oil per various Rennlist theads.
7.) timing belt checked and properly adjusted
8.) new 16" Spal Aux pusher fan, 83 temp switch, activates @~195 F coolant temp.
What have I missed, I'm at wits end and open for opinions.
....BAD fan clutch ?
....Water pump failing ?
....Radiator clogged?
Ed Sanders
When you did your thermostat, did you replace the seal behind the thermostat, (this is the smaller one that goes into the water jacket)? That seal disintegrated on me causing my S4 to run REALLY HOT.
The thermostat sets the minimum temp of the engine after warm-up. Once open; heat transfers to the radiator, subject to water and airflow. If flow is inadequate and the thermostat is fully open, the problem is the water pump, or radiator restrictions. Check radiator first, theyíre more common, easier and cheaper to fix.

Airflow also varies, at high temp, at idle, the fans provide airflow. That setting comes from the thermostatic clutch on the belt-drive fan and from the temp switch and AC operation for the electric fan on pre S4ís. This all interacts so a car that overheats at low speeds but is OK at high speed often has airflow problems. Due to a fan clutch, electric fan, or thermo switch, or both. A car that overheats at high speed often has water flow problems, usually a restricted radiator. A car that overheats right after startup has water problems, a thermostat stuck shut, an inop water pump, a plugged radiator, or no coolant.

Lower temp thermostats let the engine run cooler, if thereís radiator capacity. If your car overheats, a thermostat only helps if it was stuck or plugged and is restricting coolant flow. When the thermostat opens, it expands backwards, against a gasket behind the thermostat, closing the passage and forcing the water through the radiator. The gasket deteriorates and the thermostat canít completely close the passage, so some of the coolant recycles through the engine, getting even hotter. A low-temp thermostat doesn't flow more; it flows sooner. Fan operation must be coordinated with the thermostat to stay cool. Match the thermostat temp to the fan switch, so the fan doesn't run before the thermostat opens.

Flush the debris out that gets packed in the condenser and radiator, blocking airflow, with a hose from the front, but donít bend the fins. Redline Water Wetter made it almost 3 needle widths cooler. I've had good results running the Prestone cooling system cleaner through the engine. Follow the directions about removing all the old coolant first. Drain and flush the cleaner and water mixture out of the engine, remember to pull the block plugs to get all the sand, rocks and other debris out of the water galleys. Install new seal washers and anti-seize on the block plugs. Torque to 25 ft/lbs., be careful not to strip the block. Use just enough antifreeze for the freeze protection needed, use distilled water to reduce mineral scaling.

I have never found the burping process very effective in getting the air out of the system. Air pockets are common after a cooling system service. Iíve released large belches of air after filling the overflow tank, then running up to operating temp with the system open. The gauge read cool because it was reading and pumping air, yet the level looked OK. Remove the water temp sensor located in the cooling crossover manifold at the front of the engine. Fill the system from here and reinstall the sensor. Remove the radiator end of the vent hose until you get a small stream. This is the highest point in the coolant system, and will get rid of the air bubble. My car runs MUCH cooler when filling it this way. Start the car and watch the level.

Hope this helps.
Jim Mayzurk
93 GTS 5-spd