Wait, there may be another way.
I do not know if this will work, but we used to do the Coke bottle clean out on cars before we tuned them.
If you remove the catalytic converters, you might be able to pour water into the intake while the engine is racing ( you working the throttle to make sure it does not stall ).
The water breaks apart the junk in the combustion chamber as well as around the valves.
Not sure it will work on the injected caddy, but it beats the heck out of $1500 for a valve job.
