Can anyone recommend a cheap and safe homebrew engine degreaser that can be used in a spray bottle. When I say safe, I mean both for the engine and me. I would be rinsing with either another spray bottle or a water hose.
EDIT
I went to every hardware and auto parts store in my city, and literally at the last one I checked I found this abro purple power degreaser which I'm going to try. However, I think this is still an interesting question.
You will generally want to look for something that is a solvent, or is solvent-based.
A good homebrew 'degreaser' would be using washing soda (sodium carbonate) diluted in water. You can make washing soda from baking soda by heating it (baking soda) up on a stove. There are plenty of videos and guides on how to safely to this.
But in terms of effectivity, you are better off buying citrus cleaner concentrate and diluting it in water or buying degreaser bottles. Heavy-duty degreaser on store shelves often contain alkaline washing agents and a few other chemicals that make them much, much more effective than anything you could easily make at home. If you are having unsatisfactory results with degreaser that you have bought before, you may have unrealistic expectations of what degreaser can do. It does not completely dissolve heavy dirt and grime build-up, you generally need to scrub down and/or use a pressure washer afterwards.
Regarding safety, solvents that are strong enough to aid in cleaning engines will always be mildly or considerably toxic to humans. Although many people will not wear mouth/nose protection when using degreasers or solvents, it is generally a good idea as prolonged exposure and inhalation of them can be detrimental to you.
In short, look up how to make washing soda, dilute it in water and use that. Have low expectations of how well it will clean compared to store-bought degreasers and solvents, and possibly go through a more involved process of cleaning out your engine. Wear mouth/nose protection if you can.
This isn't answering your question but may answer your need (removing stubborn engine grime).
Soften the grime by rubbing engine oil into it. The engine oil doesn't have to be new; used oil works just as well.
Once the grime is softened, it becomes much easier to pull it off with a less intense solvent like brake cleaner.
Safe and Cheap? Wow, you aren't asking for much. This question got me looking at different Material Safety Data Sheets (MSDS) and I was surprised nothing is safe anymore, sigh. Here's the MSDS sheet for Procter and Gamble laundry soap for goodness sake. Here's a random purple degreaser.
My guess is that nothing that will cut oil residue is totally safe. I guess there are different ways to look at that. Things that are flammable or evaporate easily or go thru skin are way bad (i.e. anything solvent based.) So where are you drawing the line on safety?
etc... I was going to suggest laundry soap, slightly cut. But as I look at the MSDS and at JaceAce's recommendation, his vote trumps mine. He's got this one exactly correct.
Note: I would have deleted this answer, but I thought the MSDS on laundry soap and purple degreaser really supported JaceAce's answer. Consider this a lengthy comment in support of his answer.
To be honest, I wouldn't bother. I picked up some excellent purple degreaser at the local dollar store for (I seem to recall) $1.
