Air bubbles in clear fuel return line

i have air bubbles in my fuel return line, the clear one that goes back into the IP. Usually you don’t see them until you tap the start of the line down near the injector at the front driver side. It has screw down clamps on the hose, tightened those to see if it would help and the bubbles didn’t go away. What was weird was, when I started the truck, it started with no problems, but the bubbles went from the connection with the IP, back down to the injector. The truck ran for about 10 minutes, didn’t bog or throttle at all, then I killed it. Bubbles again came up the line when I tapped it at the base connection with the injector. Can anyone tell me where it sounds like I might have a leak? Someone definitely replaced the return lines at one point in time, but I don’t know when. Probably going to buy a return line kit, to see if that fixes the issue but it wouldn’t hurt to get some input on it. I’m somewhat mechanically inclined but I’m fairly new to mechanic work, definitely new to diesels and the 7.3. Thanks.

Year, 2x4 or 4x4, auto or manual tranny, stock fuel pump or electric, has fuel filter been changed, mileage, etc.

riphip said:

Year, 2x4 or 4x4, auto or manual tranny, stock fuel pump or electric, has fuel filter been changed, mileage, etc.

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Sorry, lol, 93 with a 94 engine. 4x4, auto trans, stock fuel pump, fuel filter has been changed less than a month ago, mileage is roughly 200k.

Are you having any issues, or is this just something you noticed and are concerned? The flow of fuel in the return lines is from the IP, through the injector caps, and back to the tank. It sounds like just a couple of small bubbles that get caught in the injector return cap that float back up towards the IP when you knock them loose. When you start the engine, the normal flow of fuel pushes them back to that cap, but seemingly no further. If it’s not causing an issue, I wouldn’t worry about it in this case.

TNBrett said:

Are you having any issues, or is this just something you noticed and are concerned? The flow of fuel in the return lines is from the IP, through the injector caps, and back to the tank. It sounds like just a couple of small bubbles that get caught in the injector return cap that float back up towards the IP when you knock them loose. When you start the engine, the normal flow of fuel pushes them back to that cap, but seemingly no further. If it’s not causing an issue, I wouldn’t worry about it in this case.

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It is causing issues. Occasional surge in power, which is rare but it has died on me while driving probably 6-7 times in the past two months. Would have to hop out, bleed the lines, then start it up again. The other day I almost got stranded at the grocery store because whenever I would try to start it with the front fuel tank, air would get in the line and I’d have to bleed it after every start attempt. The rear hasn’t given me any problems since then and it got me home.

What you describe sounds more like an issue on the supply side of things. A leak in the return lines can lead to hard restarts, but won’t cause any issues when it’s running. It sounds to me like your issue lies somewhere in between the fuel in the tanks and the lift pump. And likely between the front tank and the fuel tank selector valve. I say that because it sounds like you are sucking in air while running, and it sounds like the problem is primarily on the front tank. Do you know what kind of shape you shower heads are in? That could definitely do it, especially if it’s more prevalent around 1/4 tank.

TNBrett said:

What you describe sounds more like an issue on the supply side of things. A leak in the return lines can lead to hard restarts, but won’t cause any issues when it’s running. It sounds to me like your issue lies somewhere in between the fuel in the tanks and the lift pump. And likely between the front tank and the fuel tank selector valve. I say that because it sounds like you are sucking in air while running, and it sounds like the problem is primarily on the front tank. Do you know what kind of shape you shower heads are in? That could definitely do it, especially if it’s more prevalent around 1/4 tank.

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Not sure the condition of the shower heads, but I was considering deleting the front tank, as from what I’m reading online is the smaller of the two. Maybe if I just bypass the selector valve and run the rear tank straight to the engine, that would fix the issue? What’s your take on that?

If your FSV is letting air in, then it would help. I did that in my Blue Truck when the FSV stuck pulling fuel from the front tank and returning to the rear. That makes a mess and also makes for some pretty poor fuel mileage.

Well, everyone has their own way of approaching a problem. For me, I have to know the what and why of a problem before deciding how to go about fixing it. So, abandoning the front tank and FSV may hide the symptoms, but does it solve anything? What if the problem is simply the shower head on the front tank. So you start just using the rear tank, and two weeks later the shower head on the rear disintegrates and leaves you stranded again. Or it’s a spot on a steel line that’s rusted through and there are other spots that could fail at any time.
So for me, I would check whatever I could get to easily. Then maybe try testing with a temporary fuel supply i.e. feed and return line in a 5 gallon can. But I would honestly be pulling the bed pretty quickly to get easy access to everything and figure out exactly what’s going on. I could have the bed off in about an hour. I would also have new shower heads on hand for that, as their failure seems to be a “when” rather than an “if”.