BMW E90 models have had a few problems with head bolts breaking. The external head bolts can break, sometimes causing an oil leak, sometimes not. If you have oil leak down the left side of your engine, check that this bolt is intact before condemning other parts. Repairing these head bolts can be done in a day by an experienced person.
When the internal head bolts fail, it can be a much bigger deal. The internal head bolts can cause engine overheating and internal cooling system leaks. Repairing these head bolts are quite challenging and may be better left to a professional. Camshaft removal is required and this is a daunting task.
In this tech article, I will go over how to inspect the cylinder head bolts to check if they are broken.
Remove engine covers. See our tech article on engine cover removing.
The head bolt I see broken most often, is the one right behind the oil filter housing (green arrow). The oil filter housing is shown removed in this photo to give you a good view of the bolt. When this bolt breaks there isn't always an oil leak associated with it. I would suggest inspecting each time you replace your engine oil. To replace this bolt, you will have to move the intake manifold out of the way to, see our tech article on intake manifold replacing. Then, what is left of the bolt will just spin out. I haven't had any that were stuck and would not come out by hand. Use a pick to rotate bolt until it is out enough to remove by hand. Install and torque new bolt and reassemble engine.
If you find the external head bolt is broken, I suggest removing the valve cover (see our tech article on valve cover replacing) and inspecting the remaining head bolts. This can either give you piece of mind or inform you of a needed major repair. The green arrow in this photo points to where a head bolt should be, it is broken and the head is missing. The yellow arrow points to the intake camshaft actuator for a point of a reference. When this head bolt is found broken, the repair procedure difficulty increases to the professional level. The camshaft has to be removed and multiple special tools are required to do this. Some of which are, camshaft timing tools, camshaft journal alignment tool. If you find this bolt broken and there are no other symptoms, have the bolt replaced as soon as possible. This head bolt can also be associated with engine overheating and internal cooling system leaks. Keep this in mind when dealing with these issues.