One of the threads here made me think of something that happened a couple of years ago. It really caused a bunch of head scratching for everyone......so can you figure out what the problem was?? The situation. 4 Different mowers (type does not matter as it is a engine problem) First mower had a 26hp Briggs ELS engine 225 hours Second mower had a 26hp Briggs ELS engine 20 hours Third mower had a 25hp Kohler Engine 5 hours Forth mower had a 30hp Kohler Engine 5 hours The mowers were to run all day and night with just stops for fuel and oil checks. Every day we would go down to the same gas station and pick up fresh fuel once or twice a day. Fuel sled with 3 15gallon tanks. The first machine had been running 24 hours a day for 5 days Second for 20 hours Third and fourth were just started 5 hours before.... it happened: All 4 mowers were shut down and refueled and oil checked. When we checked the oil.......all 4 of the crankcases were full, at least a quart or more oil in the crankcases. Didn't smell of gasoline at all. Hmmmmm all 4 engines were running good so the boss just decided to drain 'em and let them run, there was a chance the rings hadn't set on the low hour ones and who knows about the high hour one. First time this had ever happened ever. Engines drained, new oil and a refill of fuel and off they go again.......5 hours later we check again........all crankcases full AGAIN. Now we know we have a problem. Called the engine companies and they came out and did all types of checks but the engines were all in perfect working order. Compression good, power good all that. Was told there wasn't anything wrong with them. So we went back to running them and just draining the oil every time we stopped......2 days later the problem just went away never to return. Can you figure out what caused the crankcases to fill up?? We did figure it out a week or so after this all happened. So lets hear what you think.