My W124 M102 ('92, 100k miles) is losing coolant, about an inch every two weeks or so in the reservoir. Replaced a couple of hoses that were dripping a bit, and the reservoir cap. Now there is no visible exterior leaks from the system. Also changed coolant and flushed the system, including the heater core. Still losing.
In addition, I've noticed that the loss is greater when the car has been running in stop and go traffic, at 100 - 110 =BAC. There is almost no loss during highway trips with the T=BA steadily at 80 =BAC.
My last hope before start thinking about head gasket is that the loss is from the water pump bearing. The car has a clicking noise coming from the belt area when starting cold in the morning, which disappears completely after a minute.
I tried a mechanic's stethoscope on the pump, but the test wasn=B4t conclusive, the alternator's bearing could be the noisy one.
Aside from the coolant loss the car is almost perfect. It has good power (for a 4 cyl.), idles smoothly, no oil or fuel in coolant, no coolant in oil, and no white smoke from the exhaust. Also, all the spark plugs look fine, with a normal brown color.
Is there a better way to diagnose the pump? Should I change the pump and see?
Please advice.
Thanks MMansilla