Cleaning a sitting motor?

[MercMontMars]

Hi there, I’ve got a rather random question and want to see if anyone’s done this.. but is there something I can put in my engines to clean them out while they sit? 
 

For a little context, my 1939 Chrysler will not be running for a while but the oil in it has turned to black sludge and I want that as broken up as possible so I can dump it when the time comes to start the car (after fresh oil of course.) I also have a project 59’ Buick Invicta sitting around and that motor is in similar shape, won’t be started for quite a while. I did a bit of research on using diesel but many sources say that’s a big no no. Any suggestions?

[Rusty_OToole]

If it is that bad, take off the oil pan and scrape out the sludge. While you are in there make sure the oil pump screen is intact and clean.

If I did not do that, I would not do anything other than change the oil and filter. With a normal old engine, you might want to change the oil every thousand miles the first one or two changes.

It is bad practice to try and clean out an engine with solvent or diesel. You can do more harm than good.

[m-mman]

Gasoline and oil are of course carbon based.  The carbon molecules are modified and adapted to make the gas and oil you are familiar with.  Clear liquids. 
 

When the gas and oil is inside the engine it is heated, cooled, squeezed, splashed, burned and contaminated with stuff.  
 

This action can change the complex carbon molecules into elemental carbon.  (Coal, charcoal, etc).  Solid, insoluble and sticky. The sticky sludge that is found stuck to valve covers, cast iron surfaces AND oil passages. 
 

Carbon that is still in solution can be removed by draining the oil.  Carbon that is stuck to the inside of an engine you don’t want to disturb.  If it is stuck there, hopefully it will stay there.  
 

If you try chemicals or solvents to “loosen it up” you will not get it back in solution such that it can be drained but rather you will unstick the solid muck and then it WILL plug up oil passages. 
 

If the engine runs well, leave it. If you know or suspect that there is a lot of sludge (“coal particles”) then the only safe reliable way to remove it is to pull the oil pan, valve covers, etc and physically scrape it out and clean everything up. 
 

Yup a horrible messy job but there is no other way.  The dirtier it is (historically poor maintenance) then you wind up doing intensive tear downs.  
 

And if you are doing that much tear down, then you “might as well rebuild it” and a simple cleaning becomes an expensive project and down the rabbit hole you go.