Hello, if is silicon put ecu in acetone, don't put the ecu connector in acetone because acetone ruin plastic.
DO NOT MAKE USE OF ACETONE IN CLOSE SPACE LIKE YOUR GARAGE OR HOME IS DANGEROUS.
In the end you will have to find a metalic aluminium case or a new plastic case if you want to use ecu again.
Who makes the ECU?Most the ECUs I've run into from Mitsubishi and Denso actually use polyurethane potting. The best solvent I've found for this is MEK (methyl ethyl ketone) which can be found anywhere they sell paint supplies.
(Edit) if it is polyurethane the case will be marked PUR somewhere.
You can soak the ecu in it for days with no damage to the ECU plastics or electronics. Carve off as much of the big chunks of potting as you can with a knife. Then soak it in MEK for a couple hours. Use a small screwdriver or knife to carefully scrape away the dissolved chunks. Repeat. When you get close to the circuit board use an old tooth brush to clean the remaining potting from the circuit board.It usually takes several days. Soak for a couple hours, work on it for 10 mins, soak for a couple more hours etc. It is not the fastest method but it works well and gives you a very clean board without tearing off any small surface mount parts or damaging cpu pins.When done wash with a lot of water and blow dry or dry in the sun.BTW do this outside. The MEK fumes are not only very flammable but very noxious.