Manifolds

Timmy,

You stated the manifolds had not been replaced but the risers have been. Your engine is probably plumbed to cool the manifolds with the closed loop antifreeze/water. The risers are cooled with raw water. There should be a SS block off plate and gaskets blocking the water passages between the riser and the manifold. A leak here can cause part of your problem, or the bubbles in the heat exchanger could be combustion pressure leaking into water jacket, ie crack, or what if there is a rust hole in the exhaust manifold between the exhaust passage and a closed cooling passage. Exhaust could push into the cooling system/heat exchanger with the cap off, causing the air bubbles. Inspect those manifolds and riser gaskets!

Greg