Newbie here.

I've tried to fix an exhaust leak twice, and a mechanic has tried a third time, and it's leaking again. :hmm: Leak is on the passenger side and sounds like it's coming from the center bottom of the exhaust manifold. Every time it's fixed, it runs quietly for a week or two then gets louder, as though the gasket is burning out. When you disassemble, you find that in fact the gasket HAS burned out.

I'VE BEEN TOLD THAT ON SOME 500s AND POSSIBLY 472s, NO GASKET IS REQUIRED. IS THIS TRUE?!

There is a gasket on the driver's side and it's nice and quiet.

Couple of other notes...


Manifold has been inspected. It is solid and mating surfaces are clean and true. Block is true with a straightedge too.
Mechanic removed heat riser butterfly and welded holes shut. I didn't want him to but he insisted this was part of the problem. It made no discernable difference.
Mechanic smeared the whole thing with high temp gasket sealer. Looks terrible (this is a show-condition vehicle) and I don't trust it.
Leak is MUCH QUIETER when car is hot.
Could this be a head gasket? Should I compression-test it?
If no gasket is required, that may be my answer right there. Can't find the info in factory shop manual. What do you think?

Thanks!

Mike
1968 Cadillac DeVille Convertible

Hello.

You heard right. These cast iron exhaust manifold and heads are made to sit flush with no gasket. If they match up to the heads and seal well, then you're good to go. If not, you may need to get them milled so they sit flush.

You mechanic gutting the valve didn't make a difference, but definitly didn't hurt anything. This is very popular when people get a custom exhaust put on these things. They gut the valves and weld the holes to prevent leakage.

High temp gasket sealer isn't going to do anything but look horrible, as you stated. If you leave it on there long enough you will notice that it will be completly burnt off, leaving no evidence it was ever there.

So, start by pulling the manifold, taking the gasket remains off (make sure you get it all) and put it on and torque it to specs. Do the same to the drivers side while you are at it. If this doesn't solve your problem, then obviously your manifolds aren't sitting flush with the heads. (This could be the reason gaskets were put on in the first place)

Hope this helps, I went through this same problem a little while back

terrible one,
Thanks for the info! I'm going to try again without the gaskets this weekend and I'll let you know how it goes.

Sounds good. I wish you the best of luck!