Hi David,
You may have two things going on. I can’t say if/why your pump failed, but the low pressure symptoms are exactly as expected with the wrong sender unit. You stated that you had 36 PSI with one pump running and 37.5 with both. You also stated the pressure was good with the engine not running. This is exactly what happens with the wrong sender unit. I’m betting that when the pump is reinstalled there is no change to this problem. When you get the pump reinstalled, you can easily test this. With the engine off, running either or both pumps should produce the correct fuel pressure. Then start the engine. If you have the wrong sender, the fuel pressure will read about 36 PSI at idle, then ramp up to normal (about 43 PSI) as you advance the throttle.
I have watched this happen to several aircraft manufactures over the years. Some manufacturers ended up setting their fuel pressure alarm lower as a work-around until they finally got their arms around it.
There is another common mistake by manufacturers with their first iS engine installs. The fuel system requires a high pressure bypass around the fuel selector valve on the fuel return line. If this is not installed, and you accidentally run a pump with the selector in the off position, the pump will momentarily deadhead against the selector valve and can be damaged. You might ask Texas Aircraft if this bypass is part of the system, I have seen pumps damaged in this way because no bypass was installed.
Please keep us updated on your progress.
