Originally Posted by jesilvas
I don't see the differences.
What caused the issue? You mentioned low fuel and filter being above the pump. Many vehicles can have those two conditions met with no issue, what is it about yours that caused no priming?
You are most certainly correct in that many vehicles do meet those criteria for the potential drain back. But, as I stated, the manufacturers of most of those vehicles, took into consideration that if, the tank was lower than the fuel pump and, the fuel level was low in the tank, there would be no drain back.
Or, in many cases, there is an electric pump, in the tank so, even if there is drain-back, the electric pump automatically primes the system (gas or diesel) and pushes the air out of the line and replaces it with fuel, all the way to the main pump or, the injectors or, whatever's next in line.
But, let's take my case. You have a HEUI pump which, is sitting at the top of the engine. There's a smaller transfer pump, located inside the base of the HEUI pump. If tracing this backwards, you then have a fuel line, from that transfer pump, that goes DOWNHILL to the one and only fuel filter that is located in the back of the coach, approximately 1/3rd the height of the face of the radiator, measured from the bottom. On the other side of the fuel filter is the main fuel line, that leads from that filter body, all the way to the top of the tank.
The pickup for that fuel line in the tank, is of course, close to the bottom of the tank.
If, and this is the critical part, the entire fuel line system, the fuel filter body, the fuel filter, and the fuel line to the transfer pump, is AIRLESS, or, completely full of fuel, then there's no problem and, the engine starts and runs just fine. The transfer pump, has just enough pulling power to keep the line(s) full of fuel to meet the HEUI's demand for fuel to run the engine.
All is well.
But, picture this:
You have a completely full, fuel line system. But, the tank is only 1/3 full. And, the coach, is sitting in a down hill position (front lower than the rear).
Now, remove that filter and guess what happens? It's obvious. The introduction of air or, another way of looking at is, breaking the VACUMME that is held in check as long as no breaks in the system are introduced, will cause GRAVITY to allow that fuel to flow back into the tank, all the way from the filter body due to the fact that, the height of the fuel in the tank, is lower than the filter body and, you've opened the door, by removing the filter to allow it to drain back.
It's kind of like putting your finger on a straw in a drink. As long as your finger is on the tip of that straw, the fluid will not drain back into the drink, no matter how high you raise the straw. But, remove that finger and, back down into the drink goes the fluid. There are no check valves or, fuel pumps in the tank to stop this action.
That is why so many on here just jump in and suggest, "ALL you have to do is, turn the key on and cycle it several times, to prime the system". They think that there is automatically an electric pump in the tank. Well, can't tell you how many times I've suggested that ALL FUEL SYSTEMS ARE NOT THE SAME!
As you can read, I'm not the only one who's had this scenario happen to them. There's been many. So, hence, this is why I installed the fuel shut-off valves. They're there to stop the flow of fuel or, drain back, no matter how you want to describe it, while the filter is off the body. There will be a small (tiny) air gap that's created when the filter is changed. But, there's still enough fuel in the system, i.e. both fuel lines, and, the filter itself, that there will be no issues when the valves are used, the filter is pre-filled prior to installation, no matter what amount of fuel is in the tank and, no matter what angle the motor home is sitting at.
Hope this explains the situation and issue.