Detailed explanation of air intrusion pathway

franklin2

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I am debating with a guy in another forum about the role the o-rings and caps play in letting air into the system and causing hard starting.

We all know the o-rings cause it when they leak air. He claims the return system has nothing to do with the fuel filter or the input to the injection pump. Like I said we all know it does, I have experienced it myself. But can anyone explain how the fuel is drained from the fuel filter and the input to the IP if the fuel leaves the injector return circuit?

I see the fuel filter head has a line running directly from it to the return caps on the pass side. But he argues that there should be a check in the filter head at that line connection, and I have also read that some of you guys just delete this line, eliminating that pathway.

I also see a return line leading from the IP. I thought aha!, this is the connection between the return system and the fuel input and fuel filter. But he argues that the return line connection from the IP comes from a upper portion of the IP and there is no way that fuel leaving that line from the IP can suck all the fuel out of the input to the IP and the fuel filter.

I told him recently I had the o-ring leaking on the heater in the filter head and he admitted this would be a problem and let the fuel drain from the filter.

I find myself having experienced this problem, and know all about how to fix it, but not knowing the details of how it exactly happens.
 

Flyfisherbob

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Thanks for asking.
I am currently trying to understand the same situation.
Placing clear lines at various locations can give a visual to the path of the air flow and source.
I also placed shut off valves on the feed & final return line to tank.
Strange things are happening with my leak.
It's the why that bugs my too.
 

Thewespaul

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There is a check valve in the 7.3 filter heads for that return line. There is also a check valve in the fitting on the ip return. The reason why o rings will cause air intrusion is because the body of the injector is apart of the return port. So if you have air getting in because of bad o rings your injector body will fill with some air. Air doesn’t compress well as we know and it takes some effort to purge all eight injectors. If one o ring is bad typically all will have some amount of air. Franklin if you have a spare injector laying around disassemble it and blow through the return port at the top of the injector here
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You will get air coming out the blue port, which is the inside of the injector body. The red port is the feed coming from the injection line.
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franklin2

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So you are saying that air is getting into the high pressure side of the system? That blows my theory as to why if these o-rings go bad, the engine will start and run for a second and then stop and then need a lot of cranking. I thought the high pressure side was good, the reason why the engine would run for a second, till the air hit the ip.

It is looking like my "ideas" of what was going on are not correct. The guy I am debating with also states that these engines should run fine and start fine with no return lines hooked to the injectors at all. He claims a brand new injector will not have hardly any fuel running out into the return line path, but as the injector wears the return line volume gets greater and greater. He claims that return fuel flow is a indicator of how worn your injectors are.

Is that what you are saying? That the combination of a injector with wear and bad o-rings will let air into the high pressure side of the injector? And that is what causes all the problems?
 

Thewespaul

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The feed side of the injector will stay primed until the pintle lifts then the air will get into the feed side, thats why it starts up then dies and needs cranking. When the pintle lifts the fuel in the injection line goes out into the injector body (path of least resistance) and air goes into the injection line since air wants to travel up. As the engine cranks it slowly purges the air out the return until it starts.
 

IDIBRONCO

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The guy I am debating with also states that these engines should run fine and start fine with no return lines hooked to the injectors at all. He claims a brand new injector will not have hardly any fuel running out into the return line path, but as the injector wears the return line volume gets greater and greater. He claims that return fuel flow is a indicator of how worn your injectors are.
Another thing to consider is this. This guy can't possibly be right about this. The return system is to return unused fuel to the tank. It doesn't matter if your injectors are brand new or not. Unused fuel is still unused fuel. I could be wrong about this, but I think that the amount of unused fuel could be more with new injectors. The way I see it, if your injectors are worn, it would take less pressure to open the pintles. Less pressure required to open would mean that a greater volume of fuel could flow through the injector since the pintles would be open longer. That's assuming that the fuel pressure from the injector pump is the same between the new and the worn injectors. You should ask him to remove his return lines and then post a video of how it runs. Also ask him to explain how used injectors return more fuel than new ones.
 

franklin2

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Another thing to consider is this. This guy can't possibly be right about this. The return system is to return unused fuel to the tank. It doesn't matter if your injectors are brand new or not. Unused fuel is still unused fuel. I could be wrong about this, but I think that the amount of unused fuel could be more with new injectors. The way I see it, if your injectors are worn, it would take less pressure to open the pintles. Less pressure required to open would mean that a greater volume of fuel could flow through the injector since the pintles would be open longer. That's assuming that the fuel pressure from the injector pump is the same between the new and the worn injectors. You should ask him to remove his return lines and then post a video of how it runs. Also ask him to explain how used injectors return more fuel than new ones.


What that guys says make sense in a way. The IP is metering the fuel, there is no unused "on purpose" fuel.

If you look at the picture below, on a brand new injector, the tolerance between the needle valve and the orange area in the picture should be very tight, and there would be very little fuel getting by into the spring area and out the leak off pipe. As the injector wears, the tolerance between these two parts is greater, so you get a greater volume of fuel into the spring area and out the return pipe.

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franklin2

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As I said, I know leaky o-rings can cause hard starting. But I can't find the "smoking gun" that explains how it happens. He said the air getting in the high pressure injection system theory is not a good one, the pressure is always high, the air could never get in around the injector.

He claims the only way the o-rings could contribute to starting problems is if your injectors are slam wore out, and the fuel in the high pressure side leaks by the pintle in the injector, and then runs out the return system and back to the tank.
 

Thewespaul

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You can’t really make a correlation between return fuel and injector wear, there’s just too many factors in the pump that will lead to more of less fuel return to just make a blanket statement that more return means more wear. If a pump has had an advance piston sleeved or oversized, the pump and injectors will return much more fuel than a rebuilt pump that hadn’t for one. Even if that statement was true, what benefits does that really give? Is he planning on measuring each injector’s return volume to determine wear? There’s just no replacement for removing the injectors and physically popping them on a tester. You get a pressure reading and visually see the nozzle’s spray pattern which is what’s actually important. Don’t really see the point of his argument with that.

The side tolerance of the pintle is designed to be able to allow fluid by, it doesn’t do any sealing, if it didn’t it wouldn’t be able to freely move. The tip (face) of the pintle is what does the sealing which I highlighted in red here
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You can see that when the pintle rises the high pressure fluid has a clear path to the injector body, and a clear path for air to mix in. If air couldn’t get into the feed side you wouldn’t have the engine start then drop until the air is purged out, that’s unequivocal evidence for air getting in, and it doesn’t matter the wear of the injector, new and old injectors will do this exact thing.

If he wants to see this in action, put an injector on the pop tester in a vertical position. Pop it until you have fuel coming out the return, then let it sit. Come back to it hours later and slowly depress the handle and you will see the pressure climb much much slower (May even take a few pumps) before full pressure is obtained. Even more evidence that air is getting into the feed side.
 
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nelstomlinson

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Wes, if we start out with the spring area of the injector filled with fuel, and the o-rings leak, doesn't the fuel stay pooled in the spring area? It seems odd that the air leaking in past the o-rings could displace that pool of fuel there. I would have guessed that the excess fuel in the annular gallery would push its way up into the spring area faster than air would intrude to the annular gallery, but obviously I'm guessing wrongly.

Also, just to be sure I'm interpreting that diagram correctly, it shows the injector popped open and spraying, and the red highlighted cone on the pintle will come down and seal against the bottom of the body, right?
 

Thewespaul

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You’re looking at it correctly, and the spring area will have fuel in it, but any leak causing air intrusion will cause priming issues, it may be that’s it a combination of several factors like someone who has the return line going from the filter may have some of the filter supply drain out the return line and that cause addition cranking.
 
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