I would add to all the suggestions for Sea foam etc: If you car has a
catalytic converter, whatever you pour through the engine exits through
the converter, and can clog it.

yes, that is one reason why I pull the exhaust manifolds off before
attempting to clean these passages.


One thing I would probably do if it were mine, is add a water injection
system at the intake. Nothing like steam for removing deposits, and it
will not hurt the motor, and would probably help it. Of course water
injection is added to reduce detonation and thereby improve power. A side
benefit is it cleans combustion chambers and passages like nothing else.

Holley used to make a nice kit. I still happen to have one, much to my
recent surprise. There are probably newer and better ones out there by
now.

I don't think it would matter one iota - remember, these passages are NOT in
the intake or exhaust circuit, they are just a separate set of "twisty
little passages, all alike" that carry air into the exhaust when the engine
is first started. I think all that putting water into the intake air would
do is to cause corrosion in the magnesium alloy intake manifold and seize up
the vari-ram that adjusts the mainfold tuning as a function of engine
RPM --- and that would be a "really bad thing".