You don't say which Frontera or which pipes but here's a stab at it anyway.
Assuming the pipes that carry coolant (hot water) for cooling (the engine) and heating (the interior), the problem could be one of two things.
Old pipes or blown head gasket (causing the cooolant system to pressurise). To check the latter remove the collant filler cap (WHEN COLD ONLY) If the head gasket has blown you will see rythmic bubbling in the water. Two other symptoms to check for head gasket are loss of coolant and "mayonnaise" in the rocker cover.
Terry, September 2006
