Home » Public Forums » GMCnet » Saturday fuel line Fun and mystery
| Saturday fuel line Fun and mystery [message #243730] |
Sun, 16 March 2014 08:48  |
Joe Weir
Messages: 769
Registered: February 2013
Location: Columbia, SC
Karma: 7
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Senior Member |
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After the rebuild, it was very hard to prime the gas lines, and after shutting down it became apparent by the sizable pool of fuel, that the rubber lines were shot.
Finally got a sunny Saturday off to get the GMC properly jacked, blocked, and leveled to allow access to the tanks. Compounding the issue is the fact that the Coach is not fully on the concrete driveway and the driveway is higher on one side, so some shade tree engineering was necessary to get everything stable.
The fuel was pouring from the combo switch area but prior to getting the coach back in the air, I could not see what else was bad and had assumed all the lines would need to go.
When I was able to slide under I found a bit of perplexing PO disease. The aux tank 3/8" rubber supply line comes off to the side of the tank to a galvanized "T". The front of the T is 5/15" and runs to the combo valve, but there is a facet style fuel pump (with 5/16" size fittings) spliced between the T and the Valve. How none of that leaked with 3/8 rubber line is a testament to the clamps.
OK, perhaps a homemade anti-vapor lock addition, I think. But what is really curious is that the back of the T off the aux tank runs to a 1/4" hard line that disappears somewhere back at the main tank. I will have to drop the second tank to find out where it terminates, as the rear corner where the onan is is the closest to the ground thanks to the slope of the driveway - not enough room for me to slide under and poke around right now.
It has to be a supply line, and I only have two tanks. The main line for the primary tank is hooked to the combo valve (and looks to be an unmolested run from the tank as far as I can tell now) and the GMC was running fine before the rebuild on that tank. Perhaps it is taking fuel from the onan pickup as some sort of incline assist fuel solution? Interesting idea, even if the execution was a bit dodgy.
Anybody heard of this before?
76 Birchaven - "Wicked Mistress" - New engine, trans, alum radiator, brakes, Sully airbags, fuel lines, seats, adult beverage center... those Coachmen guys were really thinking about us second hand owners by including that beverage center...
Columbia, SC.
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| Re: [GMCnet] Saturday fuel line Fun and mystery [message #243732 is a reply to message #243730] |
Sun, 16 March 2014 08:53  |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Senior Member |
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Joe,
Here's a link to a search of the Photo Site for "Fuel Tank."
http://www.gmcmhphotos.com/photos/search.php?searchid=60361&cpage=1
I'll bet you'll find a picture that will answer your questions.
Regards,
Rob M.
Sydney, Australia
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426
-----Original Message-----
From: gmclist-bounces@temp.gmcnet.org [mailto:gmclist-bounces@temp.gmcnet.org] On Behalf Of Joe Weir
Sent: Monday, March 17, 2014 12:49 AM
To: gmclist@temp.gmcnet.org
Subject: [GMCnet] Saturday fuel line Fun and mystery
After the rebuild, it was very hard to prime the gas lines, and after shutting down it became apparent by the sizable pool of fuel,
that the rubber lines were shot.
Finally got a sunny Saturday off to get the GMC properly jacked, blocked, and leveled to allow access to the tanks. Compounding the
issue is the fact that the Coach is not fully on the concrete driveway and the driveway is higher on one side, so some shade tree
engineering was necessary to get everything stable.
The fuel was pouring from the combo switch area but prior to getting the coach back in the air, I could not see what else was bad
and had assumed all the lines would need to go.
When I was able to slide under I found a bit of perplexing PO disease. The aux tank 3/8" rubber supply line comes off to the side
of the tank to a galvanized "T". The front of the T is 5/15" and runs to the combo valve, but there is a facet style fuel pump
(with 5/16" size fittings) spliced between the T and the Valve. How none of that leaked with 3/8 rubber line is a testament to the
clamps.
OK, perhaps a homemade anti-vapor lock addition, I think. But what is really curious is that the back of the T off the aux tank
runs to a 1/4" hard line that disappears somewhere back at the main tank. I will have to drop the second tank to find out where it
terminates, as the rear corner where the onan is is the closest to the ground thanks to the slope of the driveway - not enough room
for me to slide under and poke around right now.
It has to be a supply line, and I only have two tanks. The main line for the primary tank is hooked to the combo valve (and looks
to be an unmolested run from the tank as far as I can tell now) and the GMC was running fine before the rebuild on that tank.
Perhaps it is taking fuel from the onan pickup as some sort of incline assist fuel solution? Interesting idea, even if the
execution was a bit dodgy.
Anybody heard of this before?
--
76 Birchaven - New engine, trans, brakes, airbags, fuel lines, seats, upholstery, beverage center...
Columbia, SC.
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Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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| Re: [GMCnet] Saturday fuel line Fun and mystery [message #243734 is a reply to message #243732] |
Sun, 16 March 2014 09:14 |
Joe Weir
Messages: 769
Registered: February 2013
Location: Columbia, SC
Karma: 7
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Senior Member |
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Thanks, Rob.
I'll have it down soon enough, but the more I think of it " low fuel aux tank fuel starvation when climbing hills" is making more and more sense. Would a 1/4" hard line be enough to feed the 455?
When people talk about documentation that comes with a vehicle it often means manuals and literature. With an old vehicle, a journal explaining repairs and modifications would be so much more helpful... 
| USAussie wrote on Sun, 16 March 2014 08:53 |
Joe,
Here's a link to a search of the Photo Site for "Fuel Tank."
http://www.gmcmhphotos.com/photos/search.php?searchid=60361&cpage=1
I'll bet you'll find a picture that will answer your questions.
Regards,
Rob M.
Sydney, Australia
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426
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76 Birchaven - "Wicked Mistress" - New engine, trans, alum radiator, brakes, Sully airbags, fuel lines, seats, adult beverage center... those Coachmen guys were really thinking about us second hand owners by including that beverage center...
Columbia, SC.
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| Re: [GMCnet] Saturday fuel line Fun and mystery [message #243737 is a reply to message #243734] |
Sun, 16 March 2014 09:21 |
Joe Weir
Messages: 769
Registered: February 2013
Location: Columbia, SC
Karma: 7
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Senior Member |
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I should add that on the to-do list I have been chasing an odd intermittent Onan start problem.
Never noticed if was when the vehicle was running on the aux tank...
The joys of multi owner vehicles...
76 Birchaven - "Wicked Mistress" - New engine, trans, alum radiator, brakes, Sully airbags, fuel lines, seats, adult beverage center... those Coachmen guys were really thinking about us second hand owners by including that beverage center...
Columbia, SC.
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| Re: [GMCnet] Saturday fuel line Fun and mystery [message #243751 is a reply to message #243749] |
Sun, 16 March 2014 11:03 |
Joe Weir
Messages: 769
Registered: February 2013
Location: Columbia, SC
Karma: 7
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Senior Member |
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Matt,
The fuel was leaking from the connections to the combo valve mostly, and tightening the standard hose clamps did not help. I replaced the line from the combo valve forward, but that was not the main culprit. If I was on the side of the road somewhere, I could just splice the ends with fresh line as the leaking appears to be from hardened and cracked hose ends. But, the hoses themselves show age cracking, so it would only be to get home.
I was not aware the transmode did not have a fuel tap for the onan in the main tank - that makes sense though.. The 1/4 inch line has a collar loose on it like it was a section of pre-made brake line that was pressed into service, so maybe the PO "hardened up" the onan line when it went bad.
My concern with this setup would be the aux facet pump competing with the onan fuel pump for supply. If you have two pumps teed off the same supply, can one overpower the other? Or am I not thinking of it correctly?
[quote title=Matt Colie wrote on Sun, 16 March 2014 10:38]| Joe Weir wrote on Sun, 16 March 2014 10:21 |
Joe,
Yes, there are discoveries to be made with PO (mis)rigged vehicles.
I suspect that the small line is for the Onan. Remember, your Birch was shipped as a Transmode and so it did not have the fuel pickup was not installed. Coachman put a T into a fuel line, and that is the limit of what I know about that....
Did you identify where the fuel was leaking?
Matt - twixt Louiville and Nashville.
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76 Birchaven - "Wicked Mistress" - New engine, trans, alum radiator, brakes, Sully airbags, fuel lines, seats, adult beverage center... those Coachmen guys were really thinking about us second hand owners by including that beverage center...
Columbia, SC.
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| Re: [GMCnet] Saturday fuel line Fun and mystery [message #243753 is a reply to message #243751] |
Sun, 16 March 2014 11:11 |
emerystora
Messages: 4442
Registered: January 2004
Karma: 13
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Senior Member |
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Is the "combo valve" you are referring to the Pollack fuel tank electric selector valve or is this something else that the prior owner added?
Emery Stora
> On Mar 16, 2014, at 11:03 AM, Joe Weir <joeweir@gmail.com> wrote:
>
>
>
> Matt,
>
> The fuel was leaking from the connections to the combo valve mostly, and tightening the standard hose clamps did not help. I replaced the line from the combo valve forward, but that was not the main culprit. If I was on the side of the road somewhere, I could just splice the ends with fresh line as the leaking appears to be from hardened and cracked hose ends. But, the hoses themselves show age cracking, so it would only be to get home.
>
> I was not aware the transmode did not have a fuel tap for the onan in the main tank - that makes sense though.. The 1/4 inch line has a collar loose on it like it was a section of pre-made brake line that was pressed into service, so maybe the PO "hardened up" the onan line when it went bad.
>
> My concern with this setup would be the aux facet pump competing with the onan fuel pump for supply. If you have two pumps teed off the same supply, can one overpower the other? Or am I not thinking of it correctly?
>
> [quote title=Matt Colie wrote on Sun, 16 March 2014 10:38]Joe Weir wrote on Sun, 16 March 2014 10:21
>> Joe,
>>
>> Yes, there are discoveries to be made with PO (mis)rigged vehicles.
>>
>> I suspect that the small line is for the Onan. Remember, your Birch was shipped as a Transmode and so it did not have the fuel pickup was not installed. Coachman put a T into a fuel line, and that is the limit of what I know about that....
>>
>> Did you identify where the fuel was leaking?
>>
>> Matt - twixt Louiville and Nashville.
>
>
> --
> 76 Birchaven - New engine, trans, brakes, airbags, fuel lines, seats, upholstery, beverage center...
> Columbia, SC.
> _______________________________________________
> GMCnet mailing list
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| Re: [GMCnet] Saturday fuel line Fun and mystery [message #243760 is a reply to message #243753] |
Sun, 16 March 2014 12:24 |
Joe Weir
Messages: 769
Registered: February 2013
Location: Columbia, SC
Karma: 7
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Senior Member |
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My apologies to Mr. Pollack, it is the fuel tank electronic selector valve.
| emerystora wrote on Sun, 16 March 2014 11:11 |
Is the "combo valve" you are referring to the Pollack fuel tank electric selector valve or is this something else that the prior owner added?
Emery Stora
|
76 Birchaven - "Wicked Mistress" - New engine, trans, alum radiator, brakes, Sully airbags, fuel lines, seats, adult beverage center... those Coachmen guys were really thinking about us second hand owners by including that beverage center...
Columbia, SC.
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|
|
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| Re: [GMCnet] Saturday fuel line Fun and mystery [message #243823 is a reply to message #243751] |
Sun, 16 March 2014 18:19 |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
Karma: 7
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Senior Member |
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| Joe Weir wrote on Sun, 16 March 2014 12:03 |
Matt,
The fuel was leaking from the connections to the combo valve mostly, and tightening the standard hose clamps did not help. I replaced the line from the combo valve forward, but that was not the main culprit. If I was on the side of the road somewhere, I could just splice the ends with fresh line as the leaking appears to be from hardened and cracked hose ends. But, the hoses themselves show age cracking, so it would only be to get home.
I was not aware the transmode did not have a fuel tap for the onan in the main tank - that makes sense though.. The 1/4 inch line has a collar loose on it like it was a section of pre-made brake line that was pressed into service, so maybe the PO "hardened up" the onan line when it went bad.
My concern with this setup would be the aux facet pump competing with the onan fuel pump for supply. If you have two pumps teed off the same supply, can one overpower the other? Or am I not thinking of it correctly?
|
Joe,
The conditions where one of the two pumps could over power the other from the tank pickup could only occur if one of the "socks" the subject pickup was in real big trouble. If this was the case, the main engine/APU fight would be the least of your issues at that moment.
That is my experience based opinion.
Matt
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
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| Re: [GMCnet] Saturday fuel line Fun and mystery [message #243835 is a reply to message #243823] |
Sun, 16 March 2014 19:12 |
Joe Weir
Messages: 769
Registered: February 2013
Location: Columbia, SC
Karma: 7
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Senior Member |
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Matt, thanks. I won't worry about it then. I know squat about engineering a fuel supply and I didn't stay at a Holiday Inn Express last night...:^)
The GMC engine was running fine, fuel-wise, before the leaks.
76 Birchaven - "Wicked Mistress" - New engine, trans, alum radiator, brakes, Sully airbags, fuel lines, seats, adult beverage center... those Coachmen guys were really thinking about us second hand owners by including that beverage center...
Columbia, SC.
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| Re: [GMCnet] Saturday fuel line Fun and mystery [message #243863 is a reply to message #243835] |
Sun, 16 March 2014 21:15 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Senior Member |
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Joe,
What you need to take away from that though, is that when you do drop the
rear (Main) tank, you should plumb the Onan to the left rear fitting on the
tank, provided for that purpose. Since that pickup is positioned above the
bottom of the tank, it will prevent the Onan from stranding you with no
fuel in the Main.
While I won't dispute Matt's conclusion about one pump not starving another
when they compete for the source, I have had the Onan die several times on
a Coachmen fitted with the main-line tee and the only obvious cause seemed
to be that the 455 was starving the Onan. Could certainly have been
something else, but why take the chance?
Former Columbia, SC resident ('56-58 @ USC).
Ken H.
On Sun, Mar 16, 2014 at 8:12 PM, Joe Weir wrote:
>
> Matt, thanks. I won't worry about it then. I know squat about
> engineering a fuel supply and I didn't stay at a Holiday Inn Express last
> night...:^)
>
> The GMC engine was running fine, fuel-wise, before the leaks.
>
>
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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| Re: [GMCnet] Saturday fuel line Fun and mystery [message #243866 is a reply to message #243863] |
Sun, 16 March 2014 21:47 |
Joe Weir
Messages: 769
Registered: February 2013
Location: Columbia, SC
Karma: 7
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Senior Member |
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Thanks, Ken. The current setup is kind of ragged, so I would be re-plumbing it anyway.
Would there be a capped off fitting, or will I need to fabricate that?
You need to come back and visit sometime, it has changed a bit since you left. I'll buy the beverages.
| Ken Henderson wrote on Sun, 16 March 2014 21:15 |
Joe,
What you need to take away from that though, is that when you do drop the
rear (Main) tank, you should plumb the Onan to the left rear fitting on the
tank, provided for that purpose. Since that pickup is positioned above the
bottom of the tank, it will prevent the Onan from stranding you with no
fuel in the Main.
While I won't dispute Matt's conclusion about one pump not starving another
when they compete for the source, I have had the Onan die several times on
a Coachmen fitted with the main-line tee and the only obvious cause seemed
to be that the 455 was starving the Onan. Could certainly have been
something else, but why take the chance?
Former Columbia, SC resident ('56-58 @ USC).
Ken H.
|
76 Birchaven - "Wicked Mistress" - New engine, trans, alum radiator, brakes, Sully airbags, fuel lines, seats, adult beverage center... those Coachmen guys were really thinking about us second hand owners by including that beverage center...
Columbia, SC.
|
|
|
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| Re: [GMCnet] Saturday fuel line Fun and mystery [message #244000 is a reply to message #243734] |
Mon, 17 March 2014 20:04 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Senior Member |
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Joe,
Dunno, however, since GMC used 3/8" lines it would be logical to assume that's what is needed.
Regards,
Rob M.
Sydney, Australia
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426
-----Original Message-----
From: Joe Weir
Thanks, Rob.
I'll have it down soon enough, but the more I think of it " low fuel aux tank fuel starvation when climbing hills" is making more
and more sense. Would a 1/4" hard line be enough to feed the 455?
When people talk about documentation that comes with a vehicle it often means manuals and literature. With an old vehicle, a
journal explaining repairs and modifications would be so much more helpful... :d
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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| Re: Saturday fuel line Fun and mystery [message #244002 is a reply to message #243730] |
Mon, 17 March 2014 20:14 |
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WildBill
Messages: 232
Registered: January 2014
Karma: 1
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Senior Member |
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If your talking about getting rid of the t and when you drop the tank plumbing the onan to the factory fuel feed strictly to the onan my recollection is that the tank fitting is 3/8" npt. I think the factory ran 1/4" fuel hose to the onan but might have been 5/16 s just a 90 degree npt to hose barb needed.
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