I've been trying to track down this slow coolant leak for a while. He recently replaced the radiator, and I've had it in the shop two times since to track down the leak. On Friday, after failing to find any "external leaks" my mechanic finally said that it must be a head gasket leak, and to live with the slow leak.

Today, after the car had been sitting since Sunday evening, I noticed a small puddle under the car. On the passenger's side, following a drip of coolant up from the front cross member just behind the Evaporative canister, I come to a cylindrical horizontal object that the accessory drive belts run around. It is at the front of the engine (obviously) This cylindrical part is machined in two pieces and joined together. There is green coolant leaking from where the two parts join together.

In the Haynes Manual, on page 1-21, drawing 30.9 shows the Auxiliary Drivebelt Arrangement and it is on the lower right, as it would be if you faced the engine from the passengers front fender looking across the car.

What is this part? Is it reasonable that coolant would be leaking from this part?