There is no check valve on the return side. I know because I looked for one. I had a severe problem losing prime. If I shut down and left the engine for any amount of time, it spun forever to start. More than once I ran my batteries down trying to start the engine. This clearly was not satisfactory and I searched for a solution. I had been told that there should have been a check valve at the back of the cylinder head. I found none.
I realized that if I turned off the fuel valve immediately after shutting down, the fuel valve at the rear of the secondary filter, the engine would not lose prime. One day I decided to buy and install a check valve at the back of the head. I was at a Freightliner dealer in Texas. The fellow handed me a check valve and I asked if I could carry it out to my truck to see if it would install behind the head. He said, "Okay."
There was no way. The threads were all too big for the return lines at the back of the head. However, I realized that the threads and the flare fitting were perfect for the fuel line coming out of the fuel pump and easy to reach. I reasoned that if the fuel shut-off valve after the secondary filter sufficed to prevent the engine from losing prime, then a one-way check valve installed immediately before the secondary filter should produce the same effect. I installed the check valve then and there and never had another problem losing prime.
Again, there is no check valve at the back of the cylinder head, only the orifice fitting that provides proper fuel pressure in the fuel rail. But that fuel pressure is dynamic. It should only exist when the engine is running. The fuel pump puts out fuel pressure and the orifice allows a measured amount of fuel to escape back to the tanks via the return lines. Once the engine is shut down, all pressure after the secondary filter should disappear.
That is why I ask, "What exists inside the cylinder head that stops the escape of fuel pressure at the secondary filter after the engine is shut down?"
rough running, about to sputter out
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There is high pressure in the secondary filter after the engine is shut down. Why?
Why doesn't that pressure simply bleed off into the cylinder head, and back to the tanks through the return lines?
Again, the return lines are clear—flowing—not plugged. I checked this at idle speed.
There is no check valve at the back of the cylinder head—none.
Why the pressure in the secondary filter?Last edited: Nov 22, 2019
Also, why am I getting symptoms of fuel starvation when there is obviously plenty of fuel pressure at the secondary filter?
If the fuel starvation problem were due to fouled fuel, why do the filters unspin with nothing but clean looking fuel in them? When poured out, the fuel in the filters looks as good as it came out of the pump.
What is causing the injectors to appear to be starved for fuel if the fuel in the filters is always clean—free of water or any other contaminants?Last edited: Nov 22, 2019
They should be replaced, period.
However, with regard to my present problem, even if they were leaking down, we're talking about a condition that exists when the engine is stopped—no longer running.
Even if the injectors were leaking down, what could possibly exist in the cylinder head that would allow pressure to exist in the secondary filter? If the injectors were leaking down, they would not cause pressure build-up in the secondary filter. Any pressure should be released through the back of the cylinder head, into the return lines, where there is no check valve.
Again, even if there were combustion gas in the fuel, once the engine is shut down, all pressure should bleed off—through the back of the cylinder head—into the return lines. There should be nothing in the cylinder head to retain the large pressure that escapes from the secondary filter when I unspin it.
Why is that pressure there? I emphasize that the pressure is there when the engine is stopped. What exists between the output of the secondary fuel filter and the return outlet at the back of the cylinder head that can prevent the bleed-off of pressure?
It's obvious that pressure can't bleed off from the input side of the secondary filter. There is a one-way check valve at the input side of the secondary filter.
What is stopping the bleed-off on the output side of the secondary filter—when the engine is stopped?
Remember that the engine is also exhibiting symptoms of fuel starvation.
How can the injectors be starving for fuel when there is an abundance of fuel pressure in the secondary filter?
There shouldn't be an excess of pressure in the cylinder head, not when return line fuel flows so freely.
No, the pressure is in the filter, not in the cylinder head.
What gives?Last edited: Nov 22, 2019
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