Burnt valves? I highly doubt it. The only real thing it can do is make your engine run hotter (hot gasses in the engine compartment) and possibly cause premature wear on your air filter. Other than that it'll just smell funny sometimes. If it does, just open the window to get some circulation. DO NOT turn on the heat or A/C if it starts smelling weird, it'll make it worse.
"Nitrous? So is that like viagra for your Jeep?" -mom
I think if you run an exhaust manifold leak for too long, you run the risk of warping the mating surfaces enough that if you do change it out someday, you will need to get the surface machined flat again.
How easily this will happen, i'm not really sure. I'm sure there is someone on this board with more experience than me on the subject!
[SIZE=1][URL=http://www.cardomain.com/id/koenig1]1989 Jeep Cherokee Laredo [I]"Betsy"[/I][/URL]
~147,000km, 4.0l, AW4, K&N custom cone intake,
Dark Cordovan Pearl Metallic in color, 4 Dr. w/ chrome trim.[/SIZE]
This is somewhat off topic, but I used to have an old dodge daytona and the turbo somehow rattled loose. Anyways it was blowing all that hot exhaust onto the little heat shield there. On top of that I had a leaky fuel injector that was soaking that fiberglass heat shield. I had a great time trying to use the squeegy at the gas station to get the fire out when it finally decided to burn.