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'99 7.4MPI - Fuel Pressure Loss

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'99 7.4MPI - Fuel Pressure Loss

Old 07-22-2022, 07:57 AM
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Default '99 7.4MPI - Fuel Pressure Loss

This is an extension to my previous post but hopefully I've narrowed down whats going on. To start, I don't believe the issues is vapor lock like I previously thought.


Question: On a hot start, after sitting for a while, what could cause each of my engines to pull fuel from the fuel/water separator without refilling from the tank. This only happens on hot starts. Once started, engines run strong all day long. At the end of the day, engines are hot when we put the boat up on the trailer. Go back to the lake the next day & both engines fire right up. So why would it suck my fuel filters dry only on a hot start?


Both engines have an anti-syphon valve at the exit of the tank & tank vent is clear.


Background info: I have a pair of 1999 7.4MPI's in a 290 PowerQuest. Both motors run strong. Never have issues with a cold start. We'll typically run the engines for a while before we go sit at the local sand bar. Once we're ready to leave, the engines always fire up & run great for 30-60 seconds (no sputter, no signs that the fuel rail has vaporized). Then after those 30-60 seconds, both engines immediate shut down like they ran out of fuel. At that time, I cycle the key & can hear the fuel pump doesn't have any fuel in it. If I pull down the fuel/water separator, it's 1/2 empty. Only way to get the engines to re-start is to cycle the fuel pump until it's pulled all that air out of the fuel/water separator by bleeding the air out through the schrader valve.


Never had this issue last season. Replaced fuel/water separators over the winter. Now I have this issue every time we go out this year.

Any thoughts/suggestions would be greatly appreciated. Thanks!
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Old 07-22-2022, 08:01 AM
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To answer the ‘question’ That’s what vapor lock does.
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Old 07-22-2022, 08:14 AM
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SB Thanks for keeping up with this & commenting again.
I'm trying to wrap my head around this because if the fuel in the rail is vaporizing (which is my understanding of what vapor lock is), then why would it start & run so well for 30-60 seconds? In my head, this 30-60 seconds tell me that nothing between the fuel/water separator & the fuel rail had vaporized. Am I missing something here or thinking of this wrong?
Thanks in advance for your experience!
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Old 07-22-2022, 09:36 AM
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Its VL but not being caused by sitting with current problem so much because the fuel filters go empty

Its either the antisiphon valve, fuel line pickup, weak pump, collapsing fuel line, fuel tank vent
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Old 07-22-2022, 09:51 AM
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Not the pump it is sucking the filter dry
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Old 07-22-2022, 10:03 AM
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Thanks AllDodge . Do you think any of these are made worse when the engine is warm?
It's possible the fuel pumps are getting weak but they still push enough fuel at WOT. I can't imagine the antisiphon valve, fuel line pickup, a collapsing fuel line, or the fuel tank vent would be impacted by a hot start versus a cold start.

I did recently check the antisiphon valve (1 was dirty, I cleaned that out & it's still very functional). I recently checked the fuel tank vent thanks to your comment on my previous post & that's free & clear.
I did replace the fuel lines this past winter which is when the issue started but they didn't have any kinks. I'll look at re-routing these to ease some bends & see if that makes a difference.
I have not checked the fuel line pickup in the tank. I'll do this next time I run my tank down.
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Old 07-22-2022, 10:17 AM
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Did you verify when it won't start by opening the gas cap and listen for air being sucked in?

Are you using type USCG type A1 fuel line, and not just lines from local NAPA?
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Old 07-22-2022, 11:05 AM
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AllDodge - I have not verify by opening the gas cap while the system is vapor locked. I will do that if we go out this weekend. I have however seen the gas fumes coming out of the tank when pumping gas. It also doesn't burp or back up when filling at the gas pump on full speed.

As for the fuel line, yes, I used USCG Type 1 rated fuel line supplied by my local marina.
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Old 07-22-2022, 11:09 AM
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Originally Posted by cjanten View Post
Thanks AllDodge . Do you think any of these are made worse when the engine is warm?
It's possible the fuel pumps are getting weak but they still push enough fuel at WOT. I can't imagine the antisiphon valve, fuel line pickup, a collapsing fuel line, or the fuel tank vent would be impacted by a hot start versus a cold start.

I did recently check the antisiphon valve (1 was dirty, I cleaned that out & it's still very functional). I recently checked the fuel tank vent thanks to your comment on my previous post & that's free & clear.
I did replace the fuel lines this past winter which is when the issue started but they didn't have any kinks. I'll look at re-routing these to ease some bends & see if that makes a difference.
I have not checked the fuel line pickup in the tank. I'll do this next time I run my tank down.
I've seen brand new fuel line collapse on the suction side.
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Old 07-22-2022, 11:24 AM
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Originally Posted by cjanten View Post
Thanks AllDodge . Do you think any of these are made worse when the engine is warm?
It's possible the fuel pumps are getting weak but they still push enough fuel at WOT. I can't imagine the antisiphon valve, fuel line pickup, a collapsing fuel line, or the fuel tank vent would be impacted by a hot start versus a cold start.

I did recently check the antisiphon valve (1 was dirty, I cleaned that out & it's still very functional). I recently checked the fuel tank vent thanks to your comment on my previous post & that's free & clear.
I did replace the fuel lines this past winter which is when the issue started but they didn't have any kinks. I'll look at re-routing these to ease some bends & see if that makes a difference.
I have not checked the fuel line pickup in the tank. I'll do this next time I run my tank down.

So last summer it worked great, then over the winter you changed the fuel lines and since then it has been no good? I would check all connections between tank and fuel rail to see if any are loose and allowing air in the system.
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