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

 

Al,

I totally agree, and this has been my approach (keep things cool, return fuel to tank, etc.).  One point of clarification concerning the purpose of the air bleed.  The closer the pump is to the fuel supply, the less need for the bleed circuit.  Initially, my pumps were mounted on the firewall, about 8 feet from the tank.  I have since moved the boost pump closer and in the process was able to lower it significantly.  My only suggestion is that we check our fuel systems to verify that the efi pump(s) are able to re-prime if/when we run a tank dry.  That is where I found a problem with my installation.  The bleeder circuit fixed that nasty little problem. 

 

Mark