Al,

What happens if the lines in the engine bay and fuel pumps get hot enough for the fuel to boil inside them?  Are the pumps still going to produce enough pressure to function normally?  My guess is no.   Paul said his coolant temp was 210+.  He had been doing extended ground runs followed by some taxiing, which would allow for lots of heat soak and little cooling air.  I imagine the fuel pumps and lines were pretty warm.  Maybe warm enough to cause vapor lock.      

Mark;

 

I agree that the issue of potential vapor lock warrants careful attention; and that a downstream bleed may be a necessary precaution depending on the system design and pump location.  I’d think that if you have fuel boiling in the line and pump you have already made a mistake in the configuration, and in that extreme case the small bleed line isn’t likely to solve the problem.

 

IMO; it is important that there be minimum pressure drop upstream from the pump – no more than can be overcome by the pressure head bringing the fuel there; minimum heating of the fuel before getting to the pump; and, if at all possible, that the pump be at a low point where air can move back up stream.  Because of its low surface tension, fuel will flow past a bubble in a -6 (3/8”) line, so there can be self priming.

If the pump is on the engine side of the firewall, it should be shielded and insulated; the feed line length on the engine side of the firewall should be kept short and insulated (at least firesleeved).  If the sump or header tank is less than couple of gallons, fuel returned from the pressure regulator should go back to the main tank or through a cooler.  With these precautions the line and pump will be kept cool by the flow of fuel.

 

Under the circumstances that Paul describes, vapor lock would have to be considered a possibility.

 

Al

 

 

 

Do you have a bleed circuit between your pump and fuel pressure

regulator? Do you have heat shields on your fuel lines? My guess is that

you may have experienced vapor lock.  Without a bleed circuit, the fuel

pump can't produce enough pressure to overcome the fuel pressure

regulator

 

This should only be an issue if the pump is not located at the lowest point in the line from the tank.  As long as there is liquid at the pump inlet – no problem.

 

Al