I believe I have to replace the head gasket on my 89 525ia @ 123,000mi. I am losing about 1 quart of coolant for every 50mi of highway driving. It does not appear to be leaking anywhere outside the vehicle either. At the same time, There is no smoke whatsoever coming from the exhaust which I have always thought was a tell-tale sign of a blown headgasket. Two questions:
1) Is there a chemical additive that I could try dumping in the radiator which might stop minor head gasket leaks?

2) Assuming that I do have to replace the head gasket, I know I have to order the set which includes all the other gaskets/seals which need to be replaced. I was wondering if there was anything else I should consider while the head was off? I think I will do the timing belt and water pump as well, is it necessary to change the timing belt tensioner?

Thanks,
Geoff