Why do I have high fuel pressure...

[patrickt]

Originally Posted by steve meltzer

Probably won't get to this for a week or so, but will post the data. thanx so much. All of this makes sense to me. thanx again. s

OK, so we have:

I've owned the car for nearly 15 years and it's always run about 6 PSI at idle with either the mechanical or the mechanical + electric.

Perfectly normal; as it should be.

On the road, same until about 3K RPM when, under load, the mechanical pump falters, dropping the pressure to 2-3 making the car stumble. Switching on the electric pump solves that problem and the pressure returns quickly to 6 and remains there, as long as the electric pump is on. Been that way forever, but now....

Hmmmmm, maybe the mechanical pump is collapsing the rubber fuel line coming in to it, thus causing the output pressure to drop to 2 to 3. Then when you switch the electric pump on it opens the collapsed rubber line back up and the pressure is restored. That would do it... and it would be a damn tricky gremlin to find.

... and now

The car idles and cruises at 9 PSI, e-pump off. But still stumbled until I flipped on the e-pump and then it ran great.

Either the gauge is wrong or the mechanical pump is now broken. If you have a little Go-Pro camera, it would be interesting to see if the rubber supply line, either right by the fuel pump or maybe back at the tank itself, is collapsing on load when the electric pump is off. But, regardless, we'll figure it out. There's not that many things it could be.