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Old 06-12-2021, 06:36 PM   #1
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8.1L Starting Issue/ Vapor Lock?

I have '05' Fleetwood on an '04' P32 8.1L. Here's the story. Upon cold start up if I don't hesitate with the key and go right to "start" it fires up. If I turn to "on" and hesitate a few seconds then go to "start" it just cranks. The only way I've found to get it to fire is by removing the cap at the fuel rail test port and releasing fuel pressure at the schrader valve. Starts right up. If I drive any distance and shut it off it will not restart unless I release the fuel pressure. Also when it gets up to operating temp at highway speed often it just quits. Won't restart, you guessed it, until I release fuel pressure. I was thinking about relocating the fuel return line from the pump to one of the ports in the tank away from the pump (similar to Full.Monty's posting awhile back). I too, like SmilingSam, am baffled as to the fuel return line. The return line ties into the pressure feed line at a 'T' right in front of the fuel filter and then returns to the pump. How does this work? Rig has less then 15k, run perfect with exception of fuel delivery problem. New fuel pump, 68 psi at start up and 64psi at running temp. Thanks for taking the time to read this. Thoughts? Vroom.
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Old 06-12-2021, 09:32 PM   #2
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I thought they went to a returnless fuel system without a fuel pressure regulator in the 04 model year 8.1L, if not they at least moved the fuel pressure regulator off the drivers side fuel rail where it is on my 2002 8.1L P32. I would therefore start looking at the fuel pressure regulation system on the 04
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Old 06-13-2021, 07:27 AM   #3
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Why the new fuel pump with only 15k miles on the coach? Did you have the exact problem before the pump was replaced?
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Old 06-13-2021, 09:30 AM   #4
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starting issue

Good Morning,
The old fuel pump would not hold pressure. I was told that it was corrosion at the fuel pump check valve. At cold start up I would have to key the ignition 3 or 4 times, tripping the prime circuit, to get enough pressure to start. Pressure at best with the old pump was low 40s. It didn't happen over night. It was a slow decline with the old pump.

In regards to the fuel pressure regulator I'm told it is part of the pump. The only item that I can see on the drivers side fuel rail is the fuel pressure pulse damper.

This issue has started since the new pump was installed. I don't want to immediately point my finger at the new pump until I've exhausted all other possible avenues. I really don't look forward to pulling the tank again.

How much pressure is to much pressure at the fuel rail? This almost seems like problem.

Any other ideas? Thanks guys, Doug.
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Old 06-13-2021, 09:36 AM   #5
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Quote:
Originally Posted by Vroom View Post
Good Morning,
The old fuel pump would not hold pressure. I was told that it was corrosion at the fuel pump check valve. At cold start up I would have to key the ignition 3 or 4 times, tripping the prime circuit, to get enough pressure to start. Pressure at best with the old pump was low 40s. It didn't happen over night. It was a slow decline with the old pump.

In regards to the fuel pressure regulator I'm told it is part of the pump. The only item that I can see on the drivers side fuel rail is the fuel pressure pulse damper.

This issue has started since the new pump was installed. I don't want to immediately point my finger at the new pump until I've exhausted all other possible avenues. I really don't look forward to pulling the tank again.

How much pressure is to much pressure at the fuel rail? This almost seems like problem.

Any other ideas? Thanks guys, Doug.
You are correct the fuel pump assembly regulates the fuel pressure.

I believe the fuel pressure spec is approx 58-64 psi.

What is your pressure at the fuel rail?
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Old 06-13-2021, 09:53 AM   #6
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Your new pump is no good. Don't look for a good deal on a new pump. you get what you pay for.



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Old 06-13-2021, 10:29 AM   #7
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Fuel issue

Fuel pressure at cold start up is 68 psi then drops to 64psi at operating temp.
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Old 06-13-2021, 06:09 PM   #8
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Any possibility of blocked or pinched return line at top of tank.Or cross connected? Relieving pressure allows engine to start is confusing. My 03 8.1 fuel pressure is 58 psi, drops to around 50 a few minutes after engine shutdown.. 68 seems a little high. Most owners report pressures of 58-62 psi. Wish I had an answer for you.
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Old 06-13-2021, 09:45 PM   #9
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Any possibility of blocked or pinched return line at top of tank.Or cross connected? Relieving pressure allows engine to start is confusing. My 03 8.1 fuel pressure is 58 psi, drops to around 50 a few minutes after engine shutdown.. 68 seems a little high. Most owners report pressures of 58-62 psi. Wish I had an answer for you.
Your 03 uses a fuel pressure regulator mounted on the fuel rail. Fuel pressures will be in the 50s. In 04 they went to the fuel pump with built in regulator with fuel pressures is in the 60s.
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Old 06-14-2021, 02:02 AM   #10
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My 8.1 Vortec Vapor Lock Solution

So a few years back while driving from Phoenix AZ to Flagstaff I lost engine power engine was popping and cracking thru the intake pulled off the side of the road with my foot to the floor in park it would barely even run. After some trouble shooting I believe it was Vapor Lock after this fix it has never happen sense .

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Old 06-18-2021, 10:32 AM   #11
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Solved

It was a problem of my own making. When the original fuel pump would not hold pressure after shutting off the engine I took it upon myself to install an inline check valve. It worked well until the pump aged and was ready for replacement. At this point I installed the new pump and thought it would be okay to leave the check valve in place. Wrong, that's when the trouble started. With new pump making great pressure now and the fuel rail pulse damper doing its job of pulsing and the check valve blocking back pressure and not allowing the fuel pump to regulate the pressure, stick with me here, every time the damper pulsed allowing more pressure into the line, for each of those moments the injectors would not open from the over pressurization. Removed check valve, took out the guts, reinstalled the bare housing and the problem went away. A big thanks to everyone that offered their help. Very grateful, Vroom.
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Old 06-19-2021, 09:21 AM   #12
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Glad you have it solved. Sometimes we just have to take a step back and scratch our head a bit.
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Old 06-19-2021, 09:32 AM   #13
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