There's a total of 35 Steam achievements to earn in My Summer Car. Here's an incomplete list on how to grab them all. Currently one of the achievements (Total idiot & full idiot) cannot be unlocked.
Please feel free to fill in any missing info or correct any issues.
Remember: Any steam achievement from any game can be unlocked only once at a time and cannot be re-unlocked.
Piss in the radiator. You can piss in either the stock or the racing radiator. The radiator needs to be empty, or not completely full. An easy way to empty it is to take the lower radiator hose off, so that the coolant diminishes. After that take the radiator cap off, drink until your urine is high, aim right and watch for the green filling bar to appear. Once it does you should get the achievement. This can actually take a few tries as the hole to piss in is rather small.
Fill your septic truck up to its max capacity (10,000 litres of crap). You will have to empty 4 septic tanks and a little bit of a 5th to get all the 10,000 litres. One septic tank amounts to ~2,300/2,400 litres.
Drive the moped to Teimo's Shop and exit the vehicle. (Tip: You can take the moped with you to the store in the Satsuma, Ferndale, Gifu, Kemet even the boat. Be careful when you put the moped in the Satsuma)
Drink so much alcohol that you pass out. (about 1½ beer cases at once or several vodka shots at Pub Nappo/vodka bottle from the barn and a couple of beers). Once you pass out, you will wake up with a bad hangover in the form of shakes and wake up somewhere near the property. Keep in mind that if you pass out in the middle of the day, you will probably wake up in the middle of the night around your property and will have a hard time getting home in the dark.
Successfully pick up and drop off your drunk neighbor from the pub. You can pick him up with the Satsuma, the van or the muscle car. Exit right onto the highway, then take the first exit onto the dirt road. Continue straight until you reach a grey house and your new friend makes a comment (the house is on the right hand side of the road, right after the railroad tracks when driving towards home from Loppe). Be aware that you can knock the drunken sob out by pushing him, going near him etc. Once he is knocked out you will not be able to drop him off or get him in the vehicle. Also watch out for moose when driving in the night (watch the signs), they cross the roads and a direct high speed collision with one will lead to certain death.