Onan electric gas shut off valve just before the fuel pump.

B

Bob Dunahugh

New member
Sep 17, 2012
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Pulled the Onan out of the burned GMC. To put it in the Mouse house GMC. As it had 345 hrs. on it. Verses the 1045 hrs. The 345 hour one has the electric gas shut off. There are 122 VIN's between the 78 GMC's. The burned GMC was built in Dec of 77. The Mouse house in Jan of 78. I don't see the valves purpose.

Bob Dunahugh

78 Royale
 
> Pulled the Onan out of the burned GMC. To put it in the Mouse house GMC. As it had 345 hrs. on it. Verses the 1045 hrs. The 345 hour one has the
> electric gas shut off. There are 122 VIN's between the 78 GMC's. The burned GMC was built in Dec of 77. The Mouse house in Jan of 78. I don't see
> the valves purpose.
>
> Bob Dunahugh
> 78 Royale
The valve does not have a purpose on the GMC. Onan made them standard in case the generator is installed with the gas tank above the carburetor.

Remove it and eliminate a potential point of failure.
--
73 23' Sequoia 4 Sale
73 23' CanyonLands Parts Unit 4 Sale
Upper Alabama
"Highest price does not guarantee highest quality."
 
A lot of people have removed them. I do see a reason why you might want to keep it. Especially on a Royale.

On a Royale the gas feed to the Onan is "Teed" into the same main tank feed line that feeds the 403/455 engine. If the float in the Onan carb is down
allowing the needle and seat valve to remain open, it is possible that the main engine fuel pump could draw fuel and eventually air back through the
Onan fuel line.

I'm not positive that the Onan inline fuel pump would allow this to happen but it is a something to think about.

I have never tried to move fuel in a reverse direction through one of those pumps. We do move fuel in a forward direction through one of those pumps
all the time on low wing airplanes. On a low wing airplane, fuel is pumped by a mechanical pump on the engine and the electric pump is normally
turned off. The electric is only turned on in the case of a mechanical pump failure.

On a GM built coach (not transmode) the fuel to the Onan is pulled from a different port on the main tank so this issue should not exist with the
valve removed..

It is something to think about.

--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Thanks for the info. I'll pull it out. As then it's just one less thing to go wrong. Then to the Royale fuel supply set up. When I put all new fuel lines in. I had move the Onan fuel supply line to the tank port. That set up is better on several points. Some may not know it. But that gen fuel pickup. Won't let the gen run you out of gas.
Bob Dunahugh
78 Royale

________________________________
From: Bob Dunahugh
Sent: Saturday, November 26, 2016 9:46 AM
To: gmclist
Subject: Onan electric gas shut off valve just before the fuel pump.

Pulled the Onan out of the burned GMC. To put it in the Mouse house GMC. As it had 345 hrs. on it. Verses the 1045 hrs. The 345 hour one has the electric gas shut off. There are 122 VIN's between the 78 GMC's. The burned GMC was built in Dec of 77. The Mouse house in Jan of 78. I don't see the valves purpose.

Bob Dunahugh

78 Royale
 
> The valve does not have a purpose on the GMC. Onan made them standard in case the generator is installed with the gas tank above the carburetor. Remove it and eliminate a potential point of failure.

With all due respect, the valve DOES have a purpose. It prevents the fuel inventory in the pump’s inlet chamber from being drained back to the tank via siphon action - thus permitting the unit to achieve a successful start with less cranking.

Remember that the GMC as it came from the factory was not provided with a primer button - so with an empty pump chamber you would be cranking the engine with no chance of it starting while the pump was bringing a new inventory of fuel up from the tank. The design was such that it would be easy for a non-mechanically-oriented GMC owner to get it to start without fiddling around.

I installed the primer button on my coaches AND I kept the solenoid valve. Even if the valve were to go bad it would only take about 2 minutes to remove it and stick the hose right on to the pump inlet.

One could always try to find a check valve to replace the solenoid…

—Jim
1977 Eleganza
1977 Royale
Hamilton, OH
 
>
> > The valve does not have a purpose on the GMC. Onan made them standard in case the generator is installed with the gas tank above the
> > carburetor. Remove it and eliminate a potential point of failure.
>
> With all due respect, the valve DOES have a purpose. It prevents the fuel inventory in the pump's inlet chamber from being drained back to the
> tank via siphon action - thus permitting the unit to achieve a successful start with less cranking.
>
> Remember that the GMC as it came from the factory was not provided with a primer button - so with an empty pump chamber you would be cranking the
> engine with no chance of it starting while the pump was bringing a new inventory of fuel up from the tank. The design was such that it would be easy
> for a non-mechanically-oriented GMC owner to get it to start without fiddling around.
>
> I installed the primer button on my coaches AND I kept the solenoid valve. Even if the valve were to go bad it would only take about 2 minutes to
> remove it and stick the hose right on to the pump inlet.
>
> One could always try to find a check valve to replace the solenoid...
>
> --Jim
> 1977 Eleganza
> 1977 Royale
> Hamilton, OH

Jim,

Will the gas travel backwards through the pump? I think the answer is yes because on several occasions when I have had the gas line disconnected at
the carb all of the gas flowed out of te line and back to the tank. Then I had to prime and prime to get it pumping again.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Jim,

I don't disagree with you, however, I direct your attention to Parts Book 78Z / Section 28 - Furnishings / Page 28-51 / Figure
28.044E - Pump - Fuel, Onan Generator / Key 8 - Spring - cup and valve

Isn't this part a check valve that only allows the fuel to flow in one direction when the piston shuttles back and forth? If it
didn't I don't understand how the pump would pump.

The reason I don't disagree with you is because the check valve may allow fuel to slowly seep past it.

Regards,
Rob M.
The Pedantic Mechanic
USAussie - Downunder
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426
USA '77 Kingsley - TZE 267V100808

-----Original Message-----
From: Gmclist [mailto:gmclist-bounces] On Behalf Of Jim Miller
Sent: Sunday, November 27, 2016 5:26 AM
To: gmclist
Subject: Re: [GMCnet] Onan electric gas shut off valve just before the fuel pump.

With all due respect, the valve DOES have a purpose. It prevents the fuel inventory in the pump's inlet chamber from being drained
back to the tank via siphon action - thus permitting the unit to achieve a successful start with less cranking.

Remember that the GMC as it came from the factory was not provided with a primer button - so with an empty pump chamber you would be
cranking the engine with no chance of it starting while the pump was bringing a new inventory of fuel up from the tank. The design
was such that it would be easy for a non-mechanically-oriented GMC owner to get it to start without fiddling around.

I installed the primer button on my coaches AND I kept the solenoid valve. Even if the valve were to go bad it would only take about
2 minutes to remove it and stick the hose right on to the pump inlet.

One could always try to find a check valve to replace the solenoid.

Jim
 
> I don't disagree with you, however, I direct your attention to Parts Book 78Z / Section 28 - Furnishings / Page 28-51 / Figure 28.044E - Pump - Fuel, Onan Generator / Key 8 - Spring - cup and valve
>
> Isn't this part a check valve that only allows the fuel to flow in one direction when the piston shuttles back and forth? If it didn't I don't understand how the pump would pump.

Hi Rob,
Yes, the pump does have a check valve that is fundamental to its operation and there may or may not be some fuel captive about it and in the hose going up to the carb. Considering that the compartment temperature can go north of 210F after shutdown (it is much less during operation and I have experimentally measured it under both operational conditions) then some or all of that fuel might have boiled off after the last run.

The inlet fitting in the side of the pump body leads into a plenum area where the mesh filter basket is located and this location can hold an additional bit of fuel not to mention the fuel that is captive between the solenoid valve and the pump inlet.

In addition to the faster-starting issue that I raised, KenB also brings up a point why it would be beneficial on certain model coaches to have the line closed hard by the valve.

I’ve worked on vehicles for 40 years and I have rarely if ever seen a superfluous component (especially a relatively expensive one) on any of them. If GM or Onan put the valve there then they did so for a reason which may or may not be obvious to observers decades later.

—Jim
1977 Eleganza
1977 Royale
Hamilton, OH
 
Jim,

You're right!

I had not considered the location of the pump check valve in the fuel flow stream. With the solenoid removed there is nothing to
stop the flow of fuel from the pump back into the gas tank.

Regards,
Rob M.
The Pedantic Mechanic
USAussie - Downunder
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426
USA '77 Kingsley - TZE 267V100808


-----Original Message-----
From: Gmclist [mailto:gmclist-bounces] On Behalf Of Jim Miller
Sent: Sunday, November 27, 2016 1:53 PM
To: gmclist
Subject: Re: [GMCnet] Onan electric gas shut off valve just before the fuel pump.

Hi Rob,
Yes, the pump does have a check valve that is fundamental to its operation and there may or may not be some fuel captive about it
and in the hose going up to the carb. Considering that the compartment temperature can go north of 210F after shutdown (it is much
less during operation and I have experimentally measured it under both operational conditions) then some or all of that fuel might
have boiled off after the last run.

The inlet fitting in the side of the pump body leads into a plenum area where the mesh filter basket is located and this location
can hold an additional bit of fuel not to mention the fuel that is captive between the solenoid valve and the pump inlet.

In addition to the faster-starting issue that I raised, KenB also brings up a point why it would be beneficial on certain model
coaches to have the line closed hard by the valve.

I've worked on vehicles for 40 years and I have rarely if ever seen a superfluous component (especially a relatively expensive one)
on any of them. If GM or Onan put the valve there then they did so for a reason which may or may not be obvious to observers decades
later.

-Jim
1977 Eleganza
1977 Royale
Hamilton, OH

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If it is not giving you a problem, I would leave it in there.

BTW, Mine does not have one. If you are going to throw it away, I'll take it.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
> With all due respect, the valve DOES have a purpose. It prevents the fuel inventory in the pump's inlet chamber from being drained back to the
> tank via siphon action - thus permitting the unit to achieve a successful start with less cranking.
>
> Remember that the GMC as it came from the factory was not provided with a primer button - so with an empty pump chamber you would be cranking the
> engine with no chance of it starting while the pump was bringing a new inventory of fuel up from the tank. The design was such that it would be easy
> for a non-mechanically-oriented GMC owner to get it to start without fiddling around.
>
> I installed the primer button on my coaches AND I kept the solenoid valve. Even if the valve were to go bad it would only take about 2 minutes to
> remove it and stick the hose right on to the pump inlet.
>
> One could always try to find a check valve to replace the solenoid...
>
> --Jim
> 1977 Eleganza
> 1977 Royale
> Hamilton, OH

To those thinking that the check valves in the Bendix clicker pump should do the job, remember that when the pump is dead headed, it keeps
clicking.....
That means it has to have internal leakage. Indeed, in my experience, they all do.

Matt
--
Matt & Mary Colie - Members GMCMI, GMCGL, GMCES
'73 Glacier 23 - Still Loving OE Rear Drum Brake with Applied Control Arms
Now with both true Keyless and remote entry
SE Michigan - Twixt A2 and Detroit
 
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