740 & 760 Very hard cold start in a 740 b230f.
Since December, I've been experiencing a very hard cold start on my 740. Cold, as in more than 12 hours.
This is how it would go: I would start the car in the morning, i can feel its very hard to crank but once it comes to life the RPM is very low (below 500) and fluctuates quite heavily. After 3 - 4 seconds of this, the car either stalls and i have to crank it again, or the RPM jumps up to 1.5k and drops back down to around 950 on a cold idle and the car runs fine after that. When the engine is even slightly warm, say after 5 hours of not running, it will start first time. But any longer than that, and its very hard to start.
Out of suggestion, I've replaced the fuel filter with an OE one, replaced the fuel pump check valve, and put in some injector cleaner in a full fuel tank. The external fuel pump is working correctly and has been checked.
But what i have noticed is after i've put the fuel injector cleaner in and put some oil flush for an oil change, this started occurring about a week later. I don't know if this is a coincidence, but maybe the cleaner loosened some gunk up and caused a build up?
My mechanic checked the fuel pressure regulator and that's not the problem. He suggested replacing the fuel line from the filter to the fuel rail, but will this really fix it? The line is not frayed or anything and seems unlikely.