My mechanic recommends I do an emission system clean out.  They use Puri-Tic 240 in the fuel tank, crankcase and through a vacuum line to clean the valves.  I already put marvel in the tank and crankcase.  Can I inject it in a vacuum line to really clean the valves?  The vehicle is a 98 Grand Voyager, but we also have a 98 Mazda B2300.

I’m not familar with Puri-Tic 240, but I suspect it is a highly concentrate polyether amine fuel additive that will clean baked-on carbon deposits in 20 - 30 minutes when injected through the vacuum line or fuel rail.  Marvel Mystery Oil is not a polyether amine based additive and will not do the same job.  However, since you already use Marvel Mystery Oil in your crankcase and fuel, the deposit level should not be that extensive as to warrant the Puri-Tic 240 treatment.  Instead, use a 1-tank clean-up fuel additive such as Marvel Complete Fuel System Cleaner, Stock No MM02.  This product is a polyether amine fuel additive.  This product will remove the hard baked-on carbon deposits over the course of one fuel tank of gasoline.  Use this product about every 3000 to 5000 miles.  Continue to use the Marvel Mystery Oil at every fill-up in between treatments with Marvel Complete Fuel System Cleaner.