Get some coolant test strips.  You can test for exhaust gases in the
coolant.

Jason

On Thu, Oct 31, 2013 at 10:13 PM, Dennis Haynes <d23haynes57@hotmail.com>wrote:

> Combustion gasses getting into the cooling system can be a cracked head or
> more commonly the heads are actually loose. Under load they are lifting off
> the cylinders and letting gasses into the cooling system. In the air cooled
> days you could actually hear loose heads. Often it sounded like an exhaust
> problem.
>