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Author Topic: The mystery deepens -- fuel delivery question.  (Read 4286 times)
Sean
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« on: August 08, 2007, 03:19:23 PM »

OK, I have another question for the team:

After arriving in Santa Fe Monday, I noticed a loss of power.  Subjectively, it felt like a fuel delivery issue, and subsequent inspection confirmed this.

I have a Davco Fuel Pro 380 (superseded by the similar 382, if you want to have a look at one on Davco's web site at http://www.DavcoTec.com).  This combination water separator/fuel filter has a clear bowl with a visible cartridge filter media in it.  The way it is supposed to work (and has done flawlessly up to this point) is that the fuel level, visible in the bowl, starts out at about an inch up from the bottom.  Over time, as the filter media becomes plugged, the fuel level rises in the bowl, until it reaches the top of the media, which is when you are supposed to change the cartridge.

My media was less than half used when I left PEDCO, with the fuel level in the bowl being at about the 40% mark.  It remained there on several inspections over several days.

When I noticed the loss of power here in Santa Fe, the fuel level in the bowl had, mysteriously, dropped below the level of the retaining collar.  Davco technical bulletin 3097 advises that these symptoms indicate a missing grommet below the filter, or a broken spring above it.  So, today, I replaced the cartridge, grommet, and both O-rings, and inspected the spring.  All should now be normal, although the spring may be a bit weak from age.

Filling the bowl with fresh fuel relieved the power complaint.  But in just five miles of driving, the power again dropped, and, upon inspection, the fuel level is, once again, below the collar.

So here are my questions:

(1) Is it possible to have a fuel pump that's *too* strong?  For example, could the wrong fuel pump have been installed during the rebuild?  I ask this because a pump generating too much vacuum could be drawing the fuel out of the bowl faster than it can be replaced.  But, also, immediately after the rebuild, we had a problem with the 10psi Kysor fuel pressure switch sutting off the starter circuit before the engine was finished cranking.  That switch is now out of the circuit (I am still looking for a replacement), but, perhaps the switch is fine, and fuel pressure is simply building to 10psi sooner than expected -- another possible sign of a hyperactive fuel pump.  I've never heard of this, but I thought I'd ask.

(2) The more likely explanation for this is that the fuel supply line is restricted somewhere upstream of the Davco.  (Interestingly, this also happened the last time we had the engine rebuilt at PEDCO, and they replaced the main fuel line from the tank to the Davco.)  Since all was normal for ~800 miles after the in-frame, and this problem developed fairly suddenly, my immediate thought was collapsed hose, but these are double-steel-braid hoses.  So what diagnostic tips and tricks can you offer to help diagnose a main fuel line restriction?

(3) The only other explanation I can come up with for the symptoms is that the Davco unit is leaking someplace, allowing air into the bowl.  I've double-checked the assembly visually, but see nothing obvious.  So, can you think of a way to determine if it is possibility #2 or #3 without pressure-testing the Fuel Pro?

Any and all suggestions welcome.

-Sean
http://OurOdyssey.BlogSpot.com
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DrivingMissLazy
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« Reply #1 on: August 08, 2007, 03:36:40 PM »

Sean, is not there a check valve in the incoming line from the tank? Maybe it is full of trash restricting fuel delivery to the filter!
Richard

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I had a similar problem on my first bus and it would shut the fuel off completely. After sitting for several minutes it would start and run several miles. I blew air back thru the line and cleared it up. Never had any trouble again.

If you had an air leak in a hose or the filter you would lose prime I think.
« Last Edit: August 08, 2007, 03:49:14 PM by DrivingMissLazy » Logged

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« Reply #2 on: August 08, 2007, 03:42:33 PM »

Some things to check, as yo mentioned

An air leak somewhere in the line between the tank and the Davco unit.  How did Pedco damage the first line that needed rplaced, did they accidentally crush it?  Maybe they crushed it again in the same spot and now it leaks a small bit of air, or it damaged the inner liner.

Is there junk in your tank blocking the fuel line output, or even something in the line that is too large to get sucked through to the filter, that retricts flow.  How difficult is it on your coach to run a bypass fuel line?  even a temporary one would tell you if that's it.

I'd be suspicious of that spring, is there any way to replace it?  Potentially just by pass the filter with a hose, that would tell you if its the filter.



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« Reply #3 on: August 08, 2007, 03:46:48 PM »

My only suggestion would be that a vacuum guage on the suction side of the filter would tell you when you're approaching a blocked filter...and a pressure guage between your secondary filter and the engine would tell you if you fuel pump is operating according to specs.

Fuel is a solid. It cannot be compressed, like air.  It's doubtful that your fuel pump could overpower the suction side, unless there's an air leak there, then you could experience fuel starvation. But you'd have to be in two places at the same time to see air bubbles in the filter while you're placing your engine under load.

Not enough information for me to guess as to whether this is under load (moving) or no load RPM when sitting.  The other alternative is to bypass the questionable filter and see what happens.  You might wish to put a bypass line in the system with a shut off which would allow you to bypass that filter when on the road and the situation repeats itself.

If you're running a Detroit they still have primary and secondary filters which should do the job without benefit of the Davco.

Best I can offer guy..perhaps there are others betters versed in the shortcomings (if any) of a Davco filter.

Bob
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DrivingMissLazy
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« Reply #4 on: August 08, 2007, 03:53:27 PM »

EDIT;
I had a similar problem on my first bus and it would shut the fuel off completely. After sitting for several minutes it would start and run several miles. I blew air back thru the line and cleared it up. Never had any trouble again.

If you had an air leak in a hose or the filter you would lose prime I think.
Richard
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« Reply #5 on: August 08, 2007, 04:01:49 PM »

Sean- with the engine running look at the bowl of the Davco and see if you see air bubbles.  Also, make sure the top screw on plug has a good O ring in it.  Sounds like you're getting air into the system somewhere-that would explain the low level on the Davco.  I know the Davco 382 is rated up to 650hp, so it should have enough capacity.  Good Luck, TomC
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« Reply #6 on: August 08, 2007, 06:02:52 PM »

I agree with Tom. Sounds like it's sucking air, not enough fuel. Try to bypass the Fuel Pro, see what happens. Keep it simple. Let us know your results.  Cool
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« Reply #7 on: August 08, 2007, 09:02:01 PM »

Sean,  once I had a generator that got bugs in the tank. I would blow air into the line to and the problem was fixed until the screen filter in the tank would clog up again. hope you find the problem, Tomas
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DrivingMissLazy
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« Reply #8 on: August 09, 2007, 05:29:57 AM »

I'm willing to wager 20 bucks that it is not an air leak. Any air leak would very quickly cause the fuel pump to lose prime and that is not a symptom that Sean has identified. Anyone?
Richard
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« Reply #9 on: August 09, 2007, 08:10:02 AM »

We've had too many instances where the truck had an air leak that made for erratic engine performance, but not loosing its' prime.  This made for a difficult find.  Sometimes it was a collapsed fuel line, sometimes a fuel line that was porous and sucking air through its' walls, sometimes it was a cracked fuel/water separator, sometimes a blown fuel pump that was barely working-too many to list.  It usually has to be a farely large leak to loose prime.  Good Luck, TomC
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« Reply #10 on: August 09, 2007, 09:48:35 AM »

Just a few things to think about.
The space above the fuel in the FuelPro IS air.  The air has entered the dome some how !!  That air entered the dome from the fuel line or the O ring of the filter.
The FuelPro has a check valve built into the inlet fitting: fuel / air can come into the dome but not leave except to the engine.
Take the top fill cap off the dome , fill the dome to a known level with fuel, put the cap back on tight, start it up let it run on fast idle and watch the dome, if air is coming from the fuel line, bubbles will be seen in the dome and the fuel level will start to drop ( may drop very slow ).  If air is coming from the big O ring bubbles will be close to the inside of the dome.
 I have a 380 and I find it's important to clean both O rings and the O ring groove when I change filters.
 Good luck
Frank
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« Reply #11 on: August 09, 2007, 09:54:34 AM »

TOMC   Mentioned A worn engine fuel pump..........My bus at the time was not road worthy but in my shop the 8V71 would run Very eractally .some times ok some times not, drove me nuts for a few days. Installed a NEW pump..........problem solved  
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Sean
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« Reply #12 on: August 09, 2007, 10:15:12 AM »

... I have a 380 and I find it's important to clean both O rings and the O ring groove when I change filters.
...


Frank,

I also have a 380, and I don't have a "groove" for my bottom O-ring, that I can find.  The top O-ring compresses into an indentation in the dome, but, here again, no groove in the cap.  Am I missing something?  I did clean/lube the O-rings with fresh diesel before installing them.

BTW, I've already checked for bubbles, and I'm not seeing any other than right after putting the fuel in, as it clears the trapped air from the lower part of the housing.

Since you have a 380, I'll also ask:  when you have the whole thing assembled, can you wiggle the filter at all by sticking your finger through the fill hole in the dome?  Or does your spring hold the filter so tightly as to prevent that?

-Sean
http://OurOdyssey.BlogSpot.com
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« Reply #13 on: August 09, 2007, 12:18:16 PM »

I don't think it's your filter. I think you have a restriction in the line. Possibly at the pickup, possibly at a check valve, or other connection in the line. You're starving the pump for fuel as it can't pull enough through the restriction. I suggest you take an air nozzle and blow back through the line from the filter and see if the problem clears up. If it does, it will probably come back later on. You
might have some rust or other gunk in the tank that will need to be cleaned out.

Also, don't overtighten the filter bulb or cap. You might deform the o rings and create a leak
that way. Also, does your unit have a bottom unit on the head? Where the heater and
water separator are located? I believe there's an o-ring in that one, too.

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Sean
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« Reply #14 on: August 09, 2007, 01:12:32 PM »

I don't think it's your filter. I think you have a restriction in the line.

That was my initial guess, too.  However, it has been pointed out to me that, if all the fuel is draining from the bowl, then what is above the fuel must be air.  (It's not likely vacuum, as that would be a helluva vacuum based on how far the fuel has dropped.  And I can't imagine that much fuel could vaporize, so I don't think it's fuel vapor.)  That air has to be coming from somewhere.  Ergo, there must be an air ingress either in the supply line, or the Davco itself.  Does that change your opinion?

Quote
... I suggest you take an air nozzle and blow back through the line from the filter and see if the problem clears up. ...


If my latest fiddling with the Davco does not solve it, this will be the next thing I will try.  I need to run to Home Depot to build an adapter from an air fitting to the 3/4" NPT fuel line.

Quote
Also, don't overtighten the filter bulb or cap. You might deform the o rings and create a leak that way.

They are hand-tight, per the maintenance manual.

Quote
Also, does your unit have a bottom unit on the head? Where the heater and water separator are located? I believe there's an o-ring in that one, too.


No, the 380 has a solid aluminum bottom.  They didn't come out with the removable bottom until the 382.  (This caught me off guard the first time I changed the filter -- couldn't figure out what the extra gasket in the kit was for.  Same filter kit for both models...)

Thanks, Craig.

-Sean
http://OurOdyssey.BlogSpot.com
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