We've received a lot of questions recently regarding the excessive consumption of oil by your smart.
Mercedes define excessive oil consumption for the suprex engine as 1 litre per 1000km. (1 litre per 625miles)
Firstly, and the most inexpensive option is the failure of the crank case breather pipe valve.
The Crankcase breather pipe contains a 1 way valve, which can also be referred to as the PCV valve (Positive Crankcase Ventilation valve). The function of this is to release positive pressure from the crankcase caused by the movement of the pistons, and also any blow-by from the piston rings.
There are 2 modes of failure of this valve:
1) The valve sticks closed, therefore it cannot vent its own pressure. The crankcase pressure will increase as the load on the engine increases and will either force oil past the piston rings into the cylinders, or down the valve guides again, into the cylinders. This will burn the oil and make the engine run hotter too.
2) The valve sticks open, therefore positive pressure from the induction is forced into the crankcase.
On normally aspirated cars this (although problematic) is not as much of a problem than it is on forced induction cars... like the smart!
With the valve stuck open it allows the boost from the turbo to be channeled into the bottom of the engine - this is very bad news! You are now potentially pushing 1.5 bar (22psi) into the engine, this will definitely force the oil past the cylinders and past the valve guides at high pressure.
What you might also find with this instance is that the dipstick will be forced out of the dipstick holder with the pressure, and you'll start finding oil spots on the rear of your car.
Assuming you haven't been driving with the faulty valve for too long, the part is inexpensive (about £15 from smart) and you can change it yourself using this guide here.
Valve guide oil seals...
What is becoming increasingly common as the smarts get older is loss of compression in one or more of the cylinders.
This can be caused by a number of things, but more likely than not it will be the failure of one of the valve guide oil seals and/or the failure of the piston rings.
The suprex engines are running a lot of power for their size and so are very stressed little engines.
It is not uncommon for the valve guide seals to start to fail which allows oil to seep into the cylinders.
If you experience blue smoke on start up, but then it clears, it is likely that this is the cause, and the failure is in its early stages.
You must get this fixed ASAP. The oil in the cylinders burns much hotter than the petrol, therefore the result of ignoring the signs can either be a valve burning out (causing it to shatter in the cylinder) or even a piston burning out, causing the same symptom. Both of these failures will require a replacement engine, whereas changing the guides can be done simply by removing the cylinder head, and this can be done leaving the engine in situ.
Another common cause of oil consumption is piston ring failure. Piston rings are sprung rings designed to seal the gap between the piston and the cylinder which can be decimals of a millimeter.
Whether through normal blow by, or by positive crankcase pressure forcing oil past them, these rings can weaken. Ultimately they can 'stick' into the piston, resulting in the seal against the cylinder being lost.
This will then allow both oil to be forced up into the cylinders and/or the compression after ignition to be forced down into the crankcase.
Both of these failures are serious and, if left unattended, can result in total engine failure.
It is recommended that when replacing the valve guide seals, the piston rings are also changed (and vice versa).
Expect to pay between £500-£800 for a rebuild of your own engine, or £800-£1500 for a replacement refurbished engine.
There are further examples of oil loss, such as turbo bearing failure - the smart turbo built by Garrett uses a floating oil bearing, so if this fails you will lose boost and your oil consumption will rise.
This however, is not a very common failure on the smart.
You may also notice some oil seepage from the base of the rocker cover by the oil filler cap. This is due to a breakdown of the mastic seal between the rocker cover and the head. If you look inside the oil filler, you will see the top of the cam chain. As this operates, it flicks oil over the inside of the rocker cover which can come through this weakened seal. There is no replacement seal available, however tightening the rocker cover slightly will reduce this slight leak.
The most visually obvious reason for excessive oil consumption is of course an oil leak! Notoriously around the base of the dipstick, or the oil filter housing. Generally though the engines are well sealed from engine oil leaks.