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Potential fueling issue?

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Oh, knowledgeable and experienced car builders. Finally got out on the road for real to a supposedly British car gathering that was mostly Porsche's about 65 miles away. Car ran and handled great on the way there. 
About 10 miles from home the car sputtered a few times then died. Some situation information, died near the top of a long uphill grade, air temperature was easily 100*F. ECU threw no codes. Fuel gauge read about 1/2 a tank. I had brought about 2.5 gallons of fuel along not knowing if the gauge is accurate or what type of mileage I would get. Pushed the car uphill to a pullout (brought a neighbor along with me for just this sort of event), put the fuel in the tank. It started, died, started and took us the rest of the way home with no further problems.

Got home, took everything apart thinking a leak someplace had given me poor MPG. None found. Swirl pot was warm.
This morning pumped the fuel completely out of the tank and almost filled a 5 gallon container. With empty tank fuel gauge read EMPTY. So I was not just out of gas.

Next theory is LP or HP pump froze or vapor locked? Standard GBS configuration. Facet cube LP pump in back. 75 micron brass inlet filet, 40 micron filter on the outlet due to the understanding that pumps push better than pull. Gravity feed to the pump from the tank. Curifer copper piping to/from engine bay swirl pot. Walbro HP pump through 40 micron filter to stock 1.8L MX-5 fuel rail. Modified fuel rail end (used to have pulse dampener) fitting to connect to a pressure regulator and return to swirl pot.
Boot has a pretty complete aluminum floor with carpet on top. Tonneau cover over that. Black diffuser below boot. So air circulation is not great for the LP pump. LP pump is rated -20*F to 180*F but install instructions say to install mount below 140*F.

Problem is I'm not sure how to test working pumps for failure. Have ordered a 4 probe thermocouple to be able to measure temperatures beneath boot and in engine bay.
Plan is next trip out no boot tonneau, carpet, floor. When I took spare fuel container out of the boot I may have just let all the heat out so car ran again. But of course it could have been the HP pump also? There are air vents in the bonnet sides, perhaps more are needed?
All thoughts welcome! Moving to the UK may help since it is usually cooler there but with global warming...?

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Could be many things but I would start with the fuel system.

First off I would check that the regulator is working correctly by pressure testing it, should hold pressure up to that stated in data, mine is 3.5 bar but yours may be lower. If it is either much lower or higher you have found the problem - lower and it is not allowing enough fuel to feed the injectors (sounds promising when your engine failed during high load operation) - higher and you will be over pressuring injectors (however you would have noticed poor tick-over and probably carbon deposits on the spark plugs).

If regulator ok try the fuel pump by putting pressure guage in fuel line (blocking off regulator and return fuel line), again refer to manufacturers data about maximum pressure, again mine is high at 8 to 10 bar but I suspect yours should be around 5 bar. If low pressure your not getting enough fuel and either the pump is faulty or you have a restriction in the fuel line (blocked pipes, filters, kinks in fuel line etc.).

You may have to do some messing around with fuel pipes but having a pressure test point in the fuel line is useful in these cases and getting the pressure testing kits is fairly cheap on-line. Check out these before venturing into the world of electrickery.

Best of luck, Jim

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We had a couple of instances at the Huntingdon weekend where a couple of cars suffered poor starting and miss firing. This was on the run in the afternoon (saturday) and put down to fuel evaporation due to the high temp.  Have a run out in the evening when its a bit cooler and see what happens.

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