A couple of possibilities. First, my 602 and 603 cars do hit 100C idling with the A/C on occasionally, the viscous fan clutch doesn't engage until around 100C, check the FSM, I believe it is around 96C? The electric fan should be running on low with the A/C compressor running, have you verified this? If not, a very common problem is the speed resistor behind the L headlamp, cracks and fails (ceramic), fairly cheap. Next is the high-speed of the same electric fan, I'm not sure the temperature that's supposed to turn it on, I believe it's 105C but again you might want to check.

Last, the temp gauge is a voltmeter, if the battery is weak, it might not maintain 12v at idle with the blowers and stuff running, turn the headlamps on/off to see if it moves the needle. The '87 124 charging system doesn't put much out at idle and you're running partially from the battery if you're idling with much electrical equipment on. Check the grounds also.

Last, 100C isn't bad for the engine if it's full of circulating, pressurized coolant. If the coolant isn't pressurized, it's boiling in the head, if it isn't circulating or not full, there can be large temperature variations in the head and can cause warpage and even cracking. Verify that the fans, rad. cap, and water pump are working properly and don't worry too much about 100C as long as it doesn't climb much higher.

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Gone to the dark side

- Jeff