Why is this happening?

Obviously you are getting exhaust backing up into this valve and melting it. You also mention replacing the checkvalves . We assume that they are installed in the right direction. Did you try to start the car without the hoses attached to the check valve to see if you are getting leakage through the checkvalve. If I recall this valve is operated by high vacuum which opens the flow from the air pump into the exhaust header. So if you are getting unburned fuel somehow backing up from the checkvalve to this valve and it is igniting that would cause the problem. Also maybe the airpump is not working correctly . Try running with the checkvalve lines open and see what you get. If they are working nothing will happen and if they are leaking you will find out by the noise.

 

I thought 86 328 doesn't use air pump.

 

One other thing you might check is that the logic on the vacuum hose controlling the cut-off valve is working correctly (by removing it and just feeling the end of the hose with a finger):

engine cold = vacuum present (which opens the cut-off valve)

engine warm = no vacuum (which leaves the cut-off valve closed)

Not sure that having the cut-off valve open when warm would cause your specific trouble (and I would think it would drive the Lambda system crazy if it was open when warm), but it's an easy thing to check and rule in or out.

 

No air pump but they still have a system to inject air into the exhaust at times (for certain markets.... maybe all with a Cat.??)

 

You must still have a bad check valve.

http://ferrarichat.com/forum/showthread.php?t=195364

Take out the check valves and blow through them in both directions. You should only be able to blow in one direction.

Hint: don't "inhale" through the tube to test both directions, or you will breath in all that exhaust dust. I know it know by self experience. :-(

 

i have a rather clandestine question to ask in this regard.
is the air injection necessary for a proper running engine?
i realize for emissions purposes it is essential, but what happens when it is removed or inoperable?
some states do not require cars over 20yrs. to pass an emissions inspection. on the other hand,
some folks desire to maintain the authenticity of their car completely.
while i'm not advocating trashing any emissions device, i was merely curious how these cars perform
without the use of anti-pollution systems.