Just saw your second post. I suspect that last thing I described was the issue. Once the pump starts moving fuel, that air will be out of the lines in a couple of seconds, even if they were completely void of fuel.
Let me elaborate on that bowl in the tank. The pump is able to pump one to three gallons of gas per minute; way more than the engine will ever need. The gas in the line has to reach a high enough pressure to push open the spring-loaded pressure regulator, then that gas goes right back into the tank where it keeps the bowl full. That restriction limits the volume a little, but there will still be a good gallon per minute being circulated. Only a very tiny fraction of that gas will be tapped off to go through the injectors into the engine.
Now think of the molecule of gas sitting in the injector, just waiting to pass the valve in the nozzle. There's two forces acting on it. One is the fuel pressure pushing on it and the other is intake manifold vacuum pulling on it. The Engine Computer knows manifold vacuum from the MAP sensor, and it is programmed to know what fuel pressure should be. Based on those two things, it has a starting point for calculating fuel needs. During coasting, especially coasting down from highway speeds, manifold vacuum goes way up. That increases that force acting on the molecule of gas, and that would result in an excessively-rich mixture and very high emissions. To prevent that, there's a vacuum hose connected to the pressure regulator. That vacuum helps pull the regulator's valve open easier, so fuel pressure drops a corresponding amount. Vacuum pulls harder on the molecule of gas, and fuel pressure pushes less. The net difference between the two forces stays the same. If you were to drive with a fuel pressure gauge attached and clipped under the wiper arm so you could watch it, as I have done multiple times, you'd actually see that pressure go up during acceleration, and drop during coasting.
The point of this sad story is during coasting when fuel pressure drops significantly, it is much easier for the gas to push through the regulator and go back to the tank. With less restriction imposed by that valve, volume goes way up. That is when you can't get enough volume through the plugged strainer, and pressure drops even more, to the point the engine won't run.
While chasing this problem on my '88 Grand Caravan the second time, it only acted up on really hot days, and only when dragging a tandem-axle enclosed trailer that's bigger and heavier than the van. I could see the fuel pressure gradually drop from 50 psi to as low as 15 psi, which is when the engine would start to sputter and surge. It ran fine yet as low as 20 psi. That surprised me because GM engines are well-known to not run well and often not even start if fuel pressure is just five pounds too low.
By lifting the accelerator pedal for a fraction of a second, fuel pressure would pop right back up to 50 psi, then it took about 20 seconds to drop to 20 psi. By watching the gauge, I was able to nurse it home 50 miles with that trailer. A second new strainer solved that problem.
This same problem occurred with two older carbureted engines, but since those didn't use a pressure regulator or return line, the symptoms was different. They just acted like they were running out of gas, so they ran better at real low speeds.
Happy to hear you solved it. Please come back to see us again.
Dec 17, 2020 at 8:36 AM
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