V6 boiling at intake manifold?

When the 66 Dauntless is up to temp there is a sound like someone hitting the intake manifold with a small hammar and I can feel the knocks at the heater hose where it comes out of the manifold. It continues for a few minutes after I shut down.

I removed the radiator and heater core - pressure and flow tested ok at a radiator shop. New thermostat, fanbelt, hoses. Rad cap checks good (tried another one too). I blead air by removing the heater hose at the core when I filled it up with coolant.

I figured it was the heat riser not opening and overheating the intake (Chris sent me a spacer to replace it but thats not done yet) so I wired the riser open and took off the carb and blocked the passages. No change.

The V6 starts easy, warms up normally, holds temp, idles nice, drives nicely but this noise doesn't sound healthy.

What is it?

Tom

 

What does the temp gauge say? Sure sounds like an air knock to me. There has to be some air space in there somewhere for the heater hose to be pulsing I would think. With the engine running and the radiator cap removed and the water topped up, you should be able to see the bubbles in the coolant and just keep watching it until there are no more.

 

I tried a cheap mechanical gauge and it says about 180-190. The stock gauge reads at about 1/4 up. I have done the cap off and look routine but no luck.

Is now the time to try the voodoo of drilling a hole in the thermostat? I thought the bypass hose could take care of that.

At this point I just want to be sure I don't destroy the engine. None of the GM products I have worked on before ever baffled me like this.

Thanks
Tom

 

If you take off the cap and run it for a while, you should bleed all the air. If you have bled all the air this way, you may have a localized boil ; Perhaps the water pump impellor is in backwards?

 

Thanks
Will give it another go.
The waterpump is original with 31k miles so its never been messed but it is the one part I didn't change so possibly its bad.

I'll give bleeding it another shot and see if it can be isolated.

Tom