Coolant Leak

My 300e has a coolant leak. The dealer can't seem to locate it. The first time I took the car he found an oil leak that he repaired. (I was sceptical that this was the problem, but I gave him the go ahead to put some new gaskets - $200). It did not solve the problem - it WAS a coolant leak, as told the dealer. The second trip in the dealer, he said yes there was coolant on the splash pan. He replaced a small fitting on the top of the radiator and pressure tested the system. He said he could not find a leak, but he doubted that he had the problem fixed. He was correct the problem is not fixed yet!

It seems to be temperature related. It does not leak at the dealer. - even though the first time I took the car it was cold (dropped off in the night before), and the second time the car had been running.

The dealer suspects that it is the plastic expansion tanks on the side of the radiator. I am reluctant to pour money into a new raditor when he can't even see the leak.

Does anybody have any suggestions what to do? Also has anybody had good or bad experiences getting the plastic tanks resealed to the aluminum core (I personally have had bad luck with this on a ford).

Thanks for help.