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Author Topic: The mystery deepens -- fuel delivery question.  (Read 4144 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I have a Davco Fuel Pro 380 (superseded by the similar 382, if you want to have a look at one on Davco's web site at http://www.DavcoTec.com).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.
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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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« 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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« 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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« 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
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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
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« Reply #15 on: August 09, 2007, 01:21:18 PM »

Sean, I used a regular air nozzle with a rubber tip to blow back thru the line to the tank. It really takes very little pressure.

Needless to say I think, due to your knowledge, but be sure to have a fuel cap off when you do this so that someone can hear the air bubbles in the tank and to keep from over pressurizing the tank.
Richard
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« Reply #16 on: August 09, 2007, 01:43:39 PM »

Hi Sean,
The filter can be pushed around a little thru the fill cap.
I don't push it around much , the filter end cap has a depression in the top for the spring to set in so I never had a need to readjust it.
Run it at fast idle, if air is getting in you'll see it.
Remenber there is a drain valve at the bottom to be used when changing the filter, if it is not tight air will enter that way.
BUT still you should see the bubbles at fast idle.

The folks that say back pressure the fuel line with air may not know that MOST often there is a check valve in the fuel line put there by the manufacturer.  If that check valve is in good shape no air can go back to the tank.

Let us know when you run it at fast idle.
Frank
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« Reply #17 on: August 09, 2007, 01:57:01 PM »

Do you have rubber fuel lines anywhere in your coach?  If soosss, how old are they, and are they of the proper diameter to pass enough fuel when the fuel heats up?  Sometimes the insides of rubber lines swell up, thus restricting the fuel.  Kinda like my arteries. Smiley Smiley
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« Reply #18 on: August 09, 2007, 04:19:15 PM »


The folks that say back pressure the fuel line with air may not know that MOST often there is a check valve in the fuel line put there by the manufacturer.  If that check valve is in good shape no air can go back to the tank.

Not always in the supply line. MCI put them in the return line. Not sure what Neoplan did, but if there's a check valve in the supply line, that's the first place I'd look for a blockage.

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« Reply #19 on: August 15, 2007, 08:18:35 PM »

And the answer is (drum roll, please):

Kidney stones.

No, seriously -- debris in the fuel return system, at the restrictor orifice.  They showed me the debris -- it was smaller than a kidney stone.  You could hardly see it.

We took the coach in to Stewart & Stevenson in Albuquerque this morning, at the recommendation of Virgil Cooley over at PEDCO.

When they tested the fuel pressure, it was around 60PSI at idle, and it should be closer to 25 PSI.  So that told them right away where to look.  When they pulled the restrictor, they found the debris clogging it.  And they were confident that this was the sole reason for both the low power and overheating complaints.

And the good news for us -- they didn't even give us a bill.  They called PEDCO when they found the trouble, and PEDCO immediately agreed to take care of it.  There's no guarantee, of course, that this happened during the in-frame (although the debris was too large to make it through the fuel filters, so it is more likely it got in there while things were open).  And this, after all, is one reason we drove 400+ miles to PEDCO when we needed an in-frame -- we trust them, and know they stand behind their work.

I have not confirmed this yet, but I suspect the return restriction is also the reason our Kysor fuel pressure switch was cutting out the starter prematurely.  I still haven't found a replacement switch, so tomorrow I will put the original one back in and see if it's working now.

We had good power coming out of Albuquerque today, and we pulled a couple of good grades without overheating, at much higher speeds and in a higher gear than we were able to a few days ago.  So my fingers are crossed that our problems are behind us.

That said, the Davco Fuel Pro emptied out again on today's drive, and the coach, subjectively, feels a bit "sluggish" (although definitely not the level of problem we were having in Santa Fe).  So the jury is still out on the Fuel Pro issue.  I'll put more fuel in it tomorrow and we will monitor the performance moving forward.

-Sean
http://OurOdyssey.BlogSpot.com
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« Reply #20 on: August 15, 2007, 08:25:19 PM »


 Sean,

  I swear by the time you get everything that can go wrong, figured out, and fixed
 you will have a well earned    PHD in busology


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« Reply #21 on: August 16, 2007, 06:25:17 AM »

And the answer is (drum roll, please):

... at the restrictor orifice. 

So, what's a restrictor orifice, and where is it located?

Any photos of such?
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Craig Shepard
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« Reply #22 on: August 16, 2007, 06:53:54 AM »

And the answer is (drum roll, please):

... at the restrictor orifice. 

So, what's a restrictor orifice, and where is it located?

Any photos of such?

Craig,
The restrictor orifice sits on the return side of the fuel system and allows pressure to build up in the fuel rail. It's usually found on the head where the fuel return line is.
Basically it's a bushing that screws into the head with a small, (.080?) hole in it and the return fuel line connects to it.

There are a number of different sized orifices that may be used, depending on the application.
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« Reply #23 on: August 16, 2007, 07:20:26 AM »

Can anyone explain the logic behind this clogged return causing overheating?  I have been fighting overheating and have never heard of this before.   Is there a plugged port to check fuel pressure?
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« Reply #24 on: August 16, 2007, 07:45:46 AM »

Can anyone explain the logic behind this clogged return causing overheating?  ...   Is there a plugged port to check fuel pressure?


Chris,

Two ways:  First, and this is the lions share, any engine that is having trouble making power will have to "work" harder to achieve any result at the wheels.  More work means more heat, so engines with power trouble will generally heat up more.

Second, the injectors themselves are cooled by fuel.  The extra fuel flowing past the injectors and returning to the tank carries away this heat.  I find that my fuel return temperature is generally in the 120-130 range.  By further restricting the flow rate, less fuel flow is available to carry away this heat.

You can check your fuel pressure at the secondary.  There should be an "extra" port on the secondary mount.  Often, there is a tee on this port, with a pressure switch or sender on one side, and a plug on the other for priming.  Connect your gauge to this priming port.

-Sean
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« Reply #25 on: August 16, 2007, 07:49:12 AM »

Any idea on what is / was causing your fuel pro issues?
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« Reply #26 on: August 16, 2007, 09:31:28 AM »

Any idea on what is / was causing your fuel pro issues?


Jim,

The service manager at S&S felt that the restrictor blockage was 100% of my problem, and that the Fuel Pro is not implicated.  When I asked him about the level dropping, he felt that it was normal, and I should not worry about it without other symptoms such as low power.  I'm not sure I agree, so we are monitoring the situation moving forward.  I just filled the bowl back up to the top of the filter, and we'll see how far down it goes in today's driving.  The filter is brand new, so normal level right now should be at the very bottom of the media.

-Sean
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« Reply #27 on: August 16, 2007, 09:44:44 AM »

It seems to me that you were very fortunate that the trash all ended up at the restrictor and not in the injectors, unless it originated south of the injectors during assembly.

Len
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« Reply #28 on: August 17, 2007, 08:15:45 PM »

I sure learned something here.  Many thanks Sean for the detailed answer. 

I had no idea and would never think to look at a return line.  The strange part was you would think you would notice more 'pedal' resulting in higher fuel flow.  I'm sure that if it was obvious Sean would have immediately noted that, so it holds that it must mean a very slight increase in accelerator pressure to compensate for the lack of power resulted in the higher temp.  Glad its fixed Sean.

And a big thanks to Louise for the source books and travel guides.  Everyday thing for you guys but huge for the rest of us.


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