Air in fuel lines, stalls, happens again! - Diesel Forum - TheDieselStop.com
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99 & up 7.3L Power Stroke Engine and Drivetrain Discussion of the 99 & up 7.3L Power Stroke diesel engine and drivetrain in the 1999-Up Super Duty trucks and Excursions. No gas engine discussion allowed except on transmissions and drivetrain that pertain to all models. Please confine discussion of topics in this forum to those items that are specific to the 7.3L Power Stroke engine.

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Old 02-10-2012, 05:45 PM   #1 (permalink)
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Air in fuel lines, stalls, happens again!

Next chapter in my apparently unending saga...

I get air in the fuel lines when the truck is running. Yeah. I can let it idle for three to fifteen minutes, and it will eventually start straining and stall. I have a fuel pressure gage hooked up to the passenger side test port - pressure remains constant at 55 psi so I know it's not the pump (and it will drop when I purge the system, and come back up when I close it, so it's not the gage).

I have half a tank of fuel. so I know it's not the fuel pickup falling apart. Additionally, not running, I can open the test port, turn the key to turn the fuel pump on, and after it blows out the air it will send a solid shot of fuel. It's definitely got a good pickup in the tank.

She started doing this a couple of weeks ago. Messing around yesterday, I found that the sheet metal nut that holds the spacer on the IPR had fallen off and the IPR was loose on the solenoid. I replaced it, but it's possible the HPOP went over pressure and blew a seal somewhere. There's no obvious leaking on the exterior though.

I was able to diagnose and replace a couple of underperforming injectors, but that did not resolve the issue.

What happens is I start it up, it runs fine for several minutes with no indication of problems. Then she slowly sounds like she's straining more and more. ICP steadily rises from 450-550, up through 1100 or so, then she stalls. I purge the fuel lines, lots of air comes out, close and repressurize, and it happens over again.

Any ideas? I'm about at my wit's end here.
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Old 02-10-2012, 06:56 PM   #2 (permalink)
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I'm new to your "saga", so not sure what you've been told so far about this, but what I've been told is that the fitting on the suction end of the pump can let air in when the pump is running. I've been told to cut off the rubber part of the line from the tank to the pump, replace it with 3/8" (or 5/16") injector line and clamp with dual hose clamps on each end. (I'm gonna try it tonight...I broke the quick coupler on the suction side of the pump when replacing the pump.) Not sure if this will address your problem or not...
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Old 02-10-2012, 08:47 PM   #3 (permalink)
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you don't say what your milage on engine is but,does it run out faster if you rev it up? if it does ,could be injector tubes bleeding compression
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Old 02-12-2012, 04:49 PM   #4 (permalink)
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I don't think it's the tube leading to the fuel pump. If the truck isn't running, the fuel pump pushes non-aerated diesel to the heads. It only gets gas in the lines when it's running.

The truck just rolled over 200,000 miles. It doesn't take any certain amount of time to stall out, so I can't tell if revving it makes a difference. I've pulled most of the injectors and inspected them, and none have damage to the lower o-ring or the copper gasket.
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Old 02-12-2012, 07:05 PM   #5 (permalink)
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If it always stalls in a given amount of time sitting at idle, remove the fuel line from the back of the fuel pump and use some 3/8" hose on the back of the fuel pump dipped into a 5 gallon jug of diesel. Start her up and let it run pulling fuel from the jug instead of the fuel tank.

If that fixes the problem, you need to drop your tank. If not, start looking at your fuel pump to see if it's cutting out or weak.
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Old 02-14-2012, 12:47 PM   #6 (permalink)
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If it always stalls in a given amount of time sitting at idle, remove the fuel line from the back of the fuel pump and use some 3/8" hose on the back of the fuel pump dipped into a 5 gallon jug of diesel. Start her up and let it run pulling fuel from the jug instead of the fuel tank.

If that fixes the problem, you need to drop your tank. If not, start looking at your fuel pump to see if it's cutting out or weak.
The time varies. It's stopped raining so I'll do that today.

Thing is, fuel pressure doesn't drop. It remains steady at 55-65 right up till it stalls.
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Old 02-14-2012, 01:17 PM   #7 (permalink)
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We had a similar problem with a '99 F450 7.3. Run for a little while and die. Drove us nuts. Had about 300,000mi on it at the time. Finally traced it back to the pickup screens in the tank. They had plugged over time. We put a hose on the purge valve, let the pump run, and after a short time the flow dropped to nothing. If it is sucking air it would be between the lift pump and tank, since everything else is under positive pressure. Stick your purge line into a bucket and watch for bubbles with the lift pump running. No bubbles, no air.
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Old 02-14-2012, 01:19 PM   #8 (permalink)
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We had a similar problem with a '99 F450 7.3. Run for a little while and die. Drove us nuts. Had about 300,000mi on it at the time. Finally traced it back to the pickup screens in the tank. They had plugged over time. We put a hose on the purge valve, let the pump run, and after a short time the flow dropped to nothing. If it is sucking air it would be between the lift pump and tank, since everything else is under positive pressure. Stick your purge line into a bucket and watch for bubbles with the lift pump running. No bubbles, no air.
Yeah, but a clogged pickup screen would cause the fuel pressure to drop. I'm getting steady fuel pressure.
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Old 02-14-2012, 03:43 PM   #9 (permalink)
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***** but a clogged pickup screen would cause the fuel pressure to drop. I'm getting steady fuel pressure. *****

Maybe, maybe not! If it is getting no fuel to the pump due to air in the line, the pump cannot push the existing fuel forward. If there is not an actual leak in the pump, the pressure could be maintained for awhile without actual flow. Sounds weird maybe, but its a real life scenario.
I agree with the above posts that you need to pull fuel from an external source to test the possibility of air being sucked in prior to the pump. The stock pickup system in the tank is a known air sucking device as it ages. And the disconnect fittings on top of the tank are known to develop leaks. The entire problem described from all the above posts suggests air being sucked in the tank or fittings.
Tank is not too hard to drop and re-plumb the pickup per the "Hutch" mods described in many posts. Then cut the old fittings off and double band clamp a new fuel line to the pump.
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Old 02-14-2012, 06:26 PM   #10 (permalink)
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Nope. You're all wrong!

I replaced the fuel pump feed line (done it on a 2x4, never on a 4x4, and it's a pain). Before I lynched that down on the tank line, I ran her off a jug of diesel. Same result. Same result with the improved line.

Then I decided to get some vinyl tubing and replace the fuel filter feed line and head feed lines with sections of see-through tubing. Got that clamped down nice and hard (I expect the vinyl will happily melt at engine bay operating temps, so this is temporary) and purged the lines of air. Was able to see plenty of air come through, as expected. Then I turned her on. Everything looked fine for a few minutes, then I saw an air bubble rise from the head side of the drivers side head line.

I'm now absolutely certain that air (or some kind of gas) is being introduced at the head and starving the injectors.
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Old 02-14-2012, 08:32 PM   #11 (permalink)
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I replaced the fuel pump feed line (done it on a 2x4, never on a 4x4, and it's a pain). Before I lynched that down on the tank line, I ran her off a jug of diesel. Same result.
That's progress. If it still stalls and acts the same running from a jug of diesel, you've eliminated the tank as the problem.


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Everything looked fine for a few minutes, then I saw an air bubble rise from the head side of the drivers side head line.
Did the air come up the fuel supply line through the fuel filter bowl, or did the air appear out of the fuel filter and only go to the one head? How big was the bubble that showed up in the clear line? Just a bubble or a several inch long void with no fuel?
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Old 02-14-2012, 08:54 PM   #12 (permalink)
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Did the air come up the fuel supply line through the fuel filter bowl, or did the air appear out of the fuel filter and only go to the one head? How big was the bubble that showed up in the clear line? Just a bubble or a several inch long void with no fuel?
It came from the head side, definitely not the filter bowl side. In addition, neither the other head nor the filter supply line showed air bubbles. The bubble was several inches long (0.25" ID vinyl tubing) and it was growing. I didn't see any diesel behind it.

I guess my next step is to swap the injectors one by one to see if that resolves the issue.
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Old 02-15-2012, 05:45 AM   #13 (permalink)
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Good catch! Let's hope it's just an injector. A cracked head can also put air into the fuel system. Don't know how prone the 7.3s are to that, but I've seen it happen on other engines.
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Old 02-15-2012, 08:58 AM   #14 (permalink)
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Yeah, I've had injector issues before. Let's hope.
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Old 02-15-2012, 12:59 PM   #15 (permalink)
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When you pull injectors, check to be sure there isn't an extra copper seal in the injector hole. Also check the sealing point of the copper washers against the injector cups. Sounds like there may be a slight leak of combustion gasses into the fuel rail, displacing the fuel and starving the injectors. They won't inject combustion gasses very well, and it won't run on them either.

Does it start sounding ragged before it dies? That's a sign of fuel starvation. Possibly the ICP going up is trying to compensate, as well.

You may just want to replace the injector cups on that bank.

Hope you get it figured out.
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