My mercedes ML 270 cdi 2003 seems to haave developed a starting problem in that when i go to start it it does not fire straight away.Have to keep turning it to the point of battery almost going flat before it fires.Have noticed this only after car has been running and engine is warmie/if been out to local supermarket ,park up then have to start again whilst engine still warm. Local indy seem to suggest poss injector problem or air getting into fuel pipes? Any thoughts? Once started ML runs fine ,not sluggish,no smoke. What would be the tell tale signs of injector problems ?