V50 diesel starting problem

[kimib]

Any idea. When temperature goes below 0 and I use Webasto (or integrated fuel heating system - how it's called) it's really hard to start engine. When it stars it hardly takes not more than 300-400 RPM on idle. If and when engine do not stop until or during acceleration pedal is pressed to get RPM up to 2000 for a while problem is gone. Then it run fine with idle of 800-900 RPM.

No problems if webasto is used when outside temperature is over 0
No problems if webasto is not used when outside temperature is below 0

I did check on service ECU for errors, one information of low fuel pressure and it was stated that maybe empty tank together with webasto make it happen, but it is same with full tank. And it do not matter if webasto is still running or not when starting.Only common thing is webasto usage below 0 degrees.

Any idea what can cause this, some sensor issue? Volvo service had no idea and they start discussing of battery change but I think that is not the way to solve this.

Thanks for all hints.