Quote:
Originally Posted by Bimmer-Bob
Yeah, I agree. But why does it start right up in the morning? Would the air bleed out overnight? And if so, why?
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Heat can play a big role when you're dealing with air in the system. The IP gets quite hot when the engine is running (which is why the fuel heater is kinda pointless unless you live in Siberia) and any air that's in there will expand and displace fuel. Air is lighter than diesel, so it tends to collect up at the top of the fuel rail and any high points in the system when the engine shuts off (the air tends to be in a foam when the engine is running). When it's hot and expanded, it displaces more fuel, you start trying to pump that through the system and you get temporarily air bound. Wait overnight and the air either burps out of the system, or contracts to the point that diesel largely fills the voids and voila - easy starting. Obviously if you have massive air leaks you'll simply air up the system and always have hard starting, but small leaks can cause weird issues.
K-Jet gas engines have a similar problem due to vapor-lock. They have a spring-loaded pressure accumulator in the fuel system to try to keep the fuel system under pressure to minimize hard starting from vapor lock. Different reasons and mechanics, but similar concept and issues.