"As an answer to all those that think breaking off a lot of crud does damage a friend an I did a test at local junk yard, with the help of the owner. We found a 1979 Toyota PU with a 20R 4-cylinder engine that looked like it was shot at and missed and $%^& at and hit. It smoked so bad it could be used by the Marines for beach assaults. We removed the valve cover and you could not see any detail, like a complete valve spring. the crud was both soft like tar and hard and shiny like glass. The dip stick shows a black spot on the end, but not up to the add mark.
Now here's what we did. We found several oil filters off other engines and cleaned them out with carb cleaner. Nothing but the best for our test. Then we put two quarts of 5w-30 Mobil 1 and two quarts of carb cleaner out of a 5 gal pail into the engine. We started up the engine and ran the snot out of it. It sputtered and smoked and with each passing minute, running at about 3 grand in neutral, it started running better. As the oil filter would get plugged up, we would change it and add a 50/50 mixture of Mobil 1 and carb cleaner. We reved it up and let it idle. Then we drove it around the canyon behind the junk yard, up and down the hills and never spared the horses. We beat this truck like a dog every Saturday morning for a month and each time it ran better and each time it plugged up a filter, and we unplugged it and put it back on. Then we swithched from Mobil1/carb cleaner to Mobil1/Marvel Mystrry Oil. Every one took a shot at driving it. After a couple of weeks, it got driven during the week, too. In six weeks of brutal driving we put about 1k miles on the truck. We had to replace the spark plugs twice, using NGK's both times, nothing but the best for our truck. We also cleaned the carb and pcv a couple of times and put in a new air filter and finally a new oil filter, again nothing but the best. Today the truck runs great, compression is 1=165 2=170 3=165 4=160, thats up about 10 psi per hole, with no thickners or other stuff. If breaking off crud would damage the engine, we would have seen something. We took the pan off twice, first time we scraped it out, second time it was pretty clean. Oil pressure, with a mechanical gauge is 65 psi at 3k in neutral with, finally all Mobil 1 5w-30 oil. A lot of people watched our ASTM level official test and even took turns driving the truck around. It now runs without smoking, knocking or clicking, starts right up and idles smoothly and about 800rpm, and revs like crazy without missing and it pulls the hill behind the junk yard in 4th gear lugging now (5-speed) which on the first day took a struggle in 2nd, buzzing it's brains out. Not very scientific but we had fun. The owner now used the truck for a parts truck, and finally sold it for money to someone down the street. Our testing was so much fun we are going to look for a new myth to bust."
