So here at ClassicMercedes.co.uk we are well aware of the dangers of trying to start a car up after an extended lay up. We would consider an extended period being over 12 months.
The first thing to check would be the main battery! Unless it's been maintained, we would recommend a new battery to be fitted.
The second thing to check would be fuel quality in the fuel tank. Be it diesel or petrol, laying up a vehicle can play havoc on a fueling system so it's crucial to test the contents in the tank. With petrol cars, we have seen the fuel turn stale, and we have recently also noticed tanks forming rust inside which is not treatable.
On diesels, the fuel can turn oily, and block the screen filter.
In both cases, we would highly recommend a fuel tank flush, and new fuel refilled. With the older KJet petrol engines cars, the twin fuel pumps can cause issues when stale fuel has contaminated the lines. We consistently find at least one of the fuel pumps seized. This can give the vehicles a fuel surging problem if they manage to start at all.
The third thing to check is the oil level in the engine. An oil change will not do any harm at this point.
Always turn the engine clockwise using a breaker bar/ ratched at least 10 turns before attempting to use the ignition switch and starter motor. If the engine is extremely difficult to turn over (petrol) then there may be some issues you want to deal with before you try and crank it over and cause some potential damage. A diesel engine will generally be more difficult to turn over due to the higher compression.
If the above steps are followed and all points to the engine seeming ok, fire it up!
If the engine doesnt do anything, the ignition switch to starter circuit must be tested to endure the starter is recieving it's start voltage. Sometimes aftermarket immobilizers can interfere with this signal.
Many models can be hot triggered to get the starter motor to turn the engine over. The engine won't start but it will give you a clear indication as to if the starter motor works or not.