Overview:
Here I tackled the heavily soiled engine bay of a 15 year old 16v Mitsubishi Colt which I suspect hadn’t seen so much as an oily rag for years. While the finished results were far from perfect due to areas of rust and corrosion, they do still illustrate the vast improvements that can be made on even the muckiest and most neglected of engine bays with a little elbow grease!
Before:
A neglected valve cover gives the relatively low mileage engine a tired, worn out appearance.
Years worth of baked on oil overspill needed cutting through.
As did a thick layer of stubborn dust.
Every possible surface was equally caked in a thick layer of grime.
As well as being splattered with residue from a previous blown radiator incident.
Process:
Before getting to grips with any actual cleaning the first step was to cover any exposed or sensitive areas such as the alternator, the battery terminals and any other electrical connections with plastic to prevent any water from penetrating and potentially damaging the engine. 
The foam was then lightly agitated with a Vikan Long Reach soft bristled brush in addition to an EZ Detail brush, before being thoroughly rinsed off at pressure to remove the dirt it had now softened, ensuring the jet of water was kept moving at all times in order to prevent unnecessary water penetration. 
With the engine bay now pre-foamed and pre-rinsed the bulk of the cleaning could be undertaken. I liberally applied Autosmart G101 (an all purpose cleaner/degreaser) diluted 1:10 with water from top to bottom to cut through the years worth of ingrained grease, oil and grime. It was thoroughly worked in using the same two brushes as before, as well as a Valet Pro detailing brush, before being fully rinsed off to reveal a contaminant free engine bay. 
Generally, the easiest way to dress an engine bay is when it’s still wet, so directly after rinsing I sprayed a generous layer of a non-silicone, water-based dressing product in the form of Autosmart Finish to all areas (including the painted inner body panels) and spent some time thoroughly working it in with a clean soft bristled detailing brush and damp microfiber towel until a satisfactory, uniform finish had been achieved. Lastly, a few choice features such as the oil filler cap and rubber radiator hoses were then accentuated using Autosmart Highstyle, a durable weatherproof exterior dressing product which gives a lasting deep finish to the more hard wearing components, applied with a clean, dry microfibre applicator pad. 
The various plastic coverings that had been applied earlier were then carefully removed and one last buff over the entire area was undertaken to remove any remaining residue, before the car was started and left to run for a few minutes to allow the engine to fully dry out and the products to properly cure. 
After:
Full video here.