Its been a hard sell for me, but I am now thoroughly convinced of its usefulness.
Its value is mostly in rough idle conditions but can do miracles in carboned up motors. A very similar job can be done inexpensively with a can of X66p and a length of Vacuum line. X66p is an AC Delco product. It comes in a pop top can. The instructions are on the can, but basically you warm up the motor and then pull off a central vacuum line (one going into the common plenum)and add the length of hose into the can. Run the motor while sucking the elixer through the line; don't allow the flow rate to kill the motor. It should take a few minutes to empty the can. Shut-off the motor and let it sit 15-20 minutes. Then start the thing and find a deserted road to clear the mosquitoes from. Seriously, don't drive it down main street they may call the EPA on you.
The Motovac unit also cleans the injector fuel rail and inlet screen by a closed loop external fuel system. The concept here is to pick up the dirt by flowing the solution over the top of the injector. The dirt is often insoluble and won't pass through the inlet screen but this process seems to work. It also seems to work much better on Domestic injectors than on Bosch injectors. This is a conclusion based upon many stories told. We see so few injector problems that we don't use it.
Bosch Master, ASE Master, L1
33 years MB technician