Careful-careful-careful. Those "engine flush" compounds you can find at the auto parts store can do a tremendous amount of damage.

I have long maintained that your engine is not a toilet (Chevrolet Vega, Cadillac HT4100 and V8-6-4, and Ford 3.0 owners excepted) and therefore does not need flushing except under specific and rare conditions as a corrective measure. Everyone's got a pet theory on how best to flush a crudded-up engine without pulling the pan. Some methods are harmless but ineffective. Some methods are potentially harmful. Some methods are harmless and effective. For best results, pick one of that last kind.

I do not believe there is any such thing as a safe, effective and fast engine flush procedure. You can pick any two of these three: Safe and effective (but not fast), safe and fast (but not effective), effective and fast (but not safe). The risks fall into two categories:
Softening/damaging engine gaskets and seals so they don't seal well any more
Sweeping large amounts of dirt and crud from its resting place into the oil filter, which plugs and goes into bypass mode, sending the crаp directly to the bearings and quickly failing the engine. I watched this happen to a Chev 305 once. It was quite a spectacle.

Those "5-minute engine flush" compounds mostly contain Butyl Cellosolve,
which is a specialized solvent that's very good at one particular task: Cleaning the mayonnaise out of a crankcase that's had coolant in it due to a faulty head (etc.) gasket. Their use in any other situation is risky.

My own engine flush recipe is a delicious blend of Marvel Mystery Oil (very light weight and good at dissolving gums and sludges), Kroil (best penetrant on the planet), ATF (detergent/dispersant with good lubricity), and Berryman B12 ChemTool (good at dissolving crud too tough for Marvel Mystery Oil). My procedure involves warming up the engine, draining the oil, changing the oil filter, replacing the drain plug (!), and pouring in the soup. For a 5-quart crankcase, I usually start with 1/1/2/1 (Marvel/Kroil/ATF/B12). Then start and run the engine in the driveway at around 1200 to 1700 RPM with no sudden acceleration and no load applied, for 15 minutes.

Shut down, drain (really let it drain, walk away for 45 minutes), change the filter again, repeat with new soup for 30 to 45 minutes depending how gross the first batch of soup was when it was drained and how quickly the second batch of soup cruds up. Check the dipstick periodically.

If the 2nd batch of soup comes out coalmine black and full of chunks, run in another batch of soup (and another new filter!) and repeat until chips, chunks and tar stop coming out when you drain it. Then it will also be a good idea to service the oil pressure relief valve to make sure it's doing its job, as described in these two threads: thread 1, thread 2.

You'll note the filter is replaced before any attempt is made to introduce a flushing agent into the crankcase, and the filter is replaced again every time you drain a batch of flushing soup. Without doing this, you run the very real risk of inundating the filter, which will go into bypass mode and send all the loosened-up crud directly to the bearings and other critical parts: Goodbye, engine, it was nice gnawing you.

I've gotten amazing amounts of corruption and trash out of engines using this recipe and method. Other methods and other recipes may work better
for other people with other cars. The Kroil people (Kano Labs) have an engine cleanout "soup" of their own they call Kreen; I've had great results with pretty much every one of their products I ever tried, but I don't have enough direct experience with Kreen to comment. And as always, be advised that if the engine is really tired and whipped, even a safe flush can cause additional problems in the form of "new" leaks.

The van is now in my garage but I had to put the project on hold again. I am getting the engine back together soon.

I acquired a 67 A-108 van in the spring of 2019. I got it running a couple months later by just rebuilding the carb and replacing the fuel pump(ran it off a gas can). One day I ran it for over an hour, running coolant system cleaner through the system. Life got in the way the the van was put on hold.

The engine allegedly has about 150,000 miles on it. Assuming(correctly) that the timing chain was the original and that I'd replace it, this spring I pulled the timing chain cover and noticed the engine was gunked up with dried oil crud.





I removed the oil pan. There was a course grained, I guess dried up old oil, that was in the pan. It had clogged up the intake screen. I'm confident there's nothing to flush the engine that would turn that stuff back to a semi liquid state. I'm also confident that once this stuff started flaking off it would clog the pickup screen again. I also wouldn't want that stuff going through the oil pump, if it made it that far.



I cleaned all that up. Then I removed the valve cover. I had already started to clean it up before I took this photo.







There was no way I was going to run the engine again until I got most of that gunk out of there for fear of clogging the oil pick up screen again and possibly causing oil starvation. When I ran it for that hour months earlier, I hadn't thought about crud in the pan and I didn't have an oil pressure gauge and I didn't know if the sending unit was functional but...it idled nice and smooth for that time so I'm pretty sure there was oil pressure.
With the oil pan still off, I scraped the big chunks off. I finagled a small hose inside a wet vac hose to get down in the lifter valley to suck up the chunks. That took awhile as the chunks kept clogging the small hose. I used Kerosene with a parts brush to clean up the head and lifter valley. It took awhile to get all the crud out through the drain holes in the lifter valley but I got it done! All that fell to the ground. The hardest part was cleaning the tops of the lifters.







The van is now in my garage but I had to put the project on hold again. I am getting the engine back together soon.