My 7 series just woudnt start, if it starts it shuts down in less than 30 sec

what leak was being repaired exactly?

Could be the sensor has been disconnected or broken, should this be the case the car will start, but as a precaution shuts down to save the engine.

^ +1 I'd say yes on a sensor, possibly the coolant temperature sender switch. Since a coolant leak was fix. Or there is still air in the system which is causing it to overheat prematurely thus shutting down. Though you did not mention what the engine (coolant) temp' meter is reading (if anything) when the car is running.

check to see if your heater is blowing heat (once engine is semi warm). If not, there is air in your cooling system and needs the air "burped" out. No water but air in your heater core, thus no heat.

Thanks for clarifying! PS. Bleeding the air out of the system is difficult unless the car is running!

A nice trick to bleeding BMW cooling systems is as follows, check the heater valve block (electric device on the lhf inner gaurd with hoses in and out) on higher models (ie cars with split or independant heating) there are two joined cylinders in this block. in this config, turn on the ignition and set the heater to hot with mid to high fan. these valves act as a pump and if left for 10 - 15 min (may want a batt charger) will flush through the entire cooling system and remove most the air pockets. all without running the car

What I usually do, is let the engine get warm or hot enough (if there's air in the cooling system, that wont take long) to make the thermostat open to let the upper radiator hose flow into the block. Then I loosen the radiator cap to the first notch (with a rag, since it's hot) and that will immediately burp the air out. Then I let it cool enough so I can take the cap completely off to it top off with coolant. Another way, if it is easily accessible, is loosen the temperature sender (usually the size of a small spark plug on intake manifold) enough until it's close to the last threads. You'll see air bubbles come out around the sender, or it will spurt out steam. Don't tighten it back in until you see a steady flow of coolant.

I was rechecking the title and you list as a 98 "750Li" but then mention a "740iL". Which are different engines. 5.0 liter V12 and 4.0 liter V8. But needless to say these vehicles should have a "self bleeding" radiator expansion tank that has a "Y" hose going to it. There should be no need for taking any action to bleed the system off air. If it is getting hot with 30 seconds (a few minutes would be more accurate) , then it either has no coolant or the water pump has failed. I forgot to ask: What DTC or diagnostic trouble codes is it pulling from the OBDII code reader??? That would be the first thing to check. Then go from there.

Thought I'd add this as well. Either MAF/AFM or PCV. Or maybe the mechanic unplugged your MAF and forgot to reconnect. I've had similar issues with customers, doesn't start. It was a disconnected wire harness. http://www.aa1car.com/library/maf_sensors.htm http://www.meeknet.co.uk/E32/PCV/Index.htm

The problem is a bad CPS sensor Camshaft Position Sensor. Very easy to change.The sensor is located on the passenger side head in the front. This problem is often mistaken as a bad Crankshaft Position Sensor because the abbreviations that a appear on most engine diagnostic tools are the same (cps)