1974 Cadillac 472 eng EGR valve problems

[TJ Hopland]

75 was the first year for cats so this should not have one.     I was thinking of the backfire.   I have seen backfires shred mufflers.   If its the type that has (or had) the 'packing' in it what could have happened is the backfire blew apart the internals so now the packing is floating around sort of loose inside.   That's about the equivalent to having something build a nest in there.   

[Jeff Rosansky CLC #28373]

A good healthy backfire could have blown some crap loose.  I'd bet there is a layer of soot lining the exhaust system.  could have blown some loose and restricting something.
Jeff

[R Schroeder]

Wasn't sure what the first year was for the cats.
A vacuum gage on the engine sure should tell him something too.
Roy

[Scot Minesinger]

Yes, good point will check exhaust, the muffler is original.

[Jeff Rosansky CLC #28373]

I think the original muffler would be a concern.  The baffling inside could have broken loose and be partially blocking the muffler.  On the airplanes you need to check the exhaust baffling during the inspection for this reason.  You also need to look at it when there is a backfire-- Now the muffler is right at the engine so not the same as when it is in the rear, but the same principle.
Jeff

[Scot Minesinger]

75 was the first year of the cc, so 74 exhaust is similar as 61-74, single system with muffler and resonator.

[cadillactim]

Scot

If memory serves me right, you can check for exhaust restriction with vacuum gauge. After car is warmed up, put engine under load (foot on brake, rev engine to about 2000 rpm and hold it there). Watch vacuum gauge. Restriction will cause needle to steadily fall, since engine can't "breathe."

Tim