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Fuel [message #302614]

Sat, 25 June 2016 17:27

jor

Messages: 43
Registered: June 2016
Location: Tucson

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Member

1. OCTANE: I'm reading the manual and it indicates that the 455 uses "87 average octane." I was advised by the PO to run 91 (rebuilt engine with 5,000 miles). What is the consensus here on appropriate octane for a stock 455 with stock ignition timing?

2. TANKS: I was under the impression that the GMC has two 25 gallon fuel tanks but the manual states, "...auxiliary tank normally contains seven to nine gallons." Not sure what that means. Also, I was told that fuel is drawn from tanks and that it is not necessary to switch from one to the other with the fuel tanks rocker switch on the dash. Is that the case?
Thanks.
jor

John O'Reilly 76 Eleganza II (quad bags, disc brakes w/ reaction arm. 3.70 gears, manny trans, headers, Patterson dist.) Tucson, AZ

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Re: Fuel [message #302616 is a reply to message #302614]

Sat, 25 June 2016 17:45

A Hamilto

Messages: 4508
Registered: April 2011

Karma: 39

Senior Member

jor wrote on Sat, 25 June 2016 17:27

1. OCTANE: I'm reading the manual and it indicates that the 455 uses "87 average octane." I was advised by the PO to run 91 (rebuilt engine with 5,000 miles). What is the consensus here on appropriate octane for a stock 455 with stock ignition timing?

2. TANKS: I was under the impression that the GMC has two 25 gallon fuel tanks but the manual states, "...auxiliary tank normally contains seven to nine gallons." Not sure what that means. Also, I was told that fuel is drawn from tanks and that it is not necessary to switch from one to the other with the fuel tanks rocker switch on the dash. Is that the case?
Thanks.
jor

1. STOCK 455 should run fine with no knocking or pinging on current 87 octane gas, even with ethanol in it. Some say they get about 10% better mileage on ethanol free... Over time the ethanol will dissolve the OEM fuel lines, many have replaced their lines with stuff that is not dissolved by ethanol. If a stock engine knocks or pings, it is too lean or it has too much spark advance.

2. When you run out of gas when the fuel selector was set on main, there would be 7 - 9 gallons of fuel left in the aux tank. The location of the filler plumbing keeps both tanks at the same level until they each get down to about 7 gallons. Then the gas only feeds from whichever tank the selector valve is set for.

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Re: [GMCnet] Fuel [message #302619 is a reply to message #302616]

Sat, 25 June 2016 18:05

Mr ERFisher

Messages: 7117
Registered: August 2005

Karma: 2

Senior Member

Read here
http://gmcmotorhome.info/tank.html

On Saturday, June 25, 2016, A. wrote:

> jor wrote on Sat, 25 June 2016 17:27
>> 1. OCTANE: I'm reading the manual and it indicates that the 455 uses
> "87 average octane." I was advised by the PO to run 91 (rebuilt engine with
>> 5,000 miles). What is the consensus here on appropriate octane for a
> stock 455 with stock ignition timing?
>>
>> 2. TANKS: I was under the impression that the GMC has two 25 gallon fuel
> tanks but the manual states, "...auxiliary tank normally contains seven
>> to nine gallons." Not sure what that means. Also, I was told that fuel
> is drawn from tanks and that it is not necessary to switch from one to the
>> other with the fuel tanks rocker switch on the dash. Is that the case?
>> Thanks.
>> jor
> 1. STOCK 455 should run fine with no knocking or pinging on current 87
> octane gas, even with ethanol in it. Some say they get about 10% better
> mileage on ethanol free... Over time the ethanol will dissolve the OEM
> fuel lines, many have replaced their lines with stuff that is not dissolved
> by
> ethanol. If a stock engine knocks or pings, it is too lean or it has too
> much spark advance.
>
> 2. When you run out of gas when the fuel selector was set on main, there
> would be 7 - 9 gallons of fuel left in the aux tank. The location of the
> filler plumbing keeps both tanks at the same level until they each get
> down to about 7 gallons. Then the gas only feeds from whichever tank the
> selector valve is set for.
> --
> 73 23' Sequoia 4 Sale
> 73 23' CanyonLands Parts Unit 4 Sale
> Upper Alabama
> "Posting on the GMCnet seems to always result in a cacophony of responses
> which only tend to cloud the solution rather than move you closer to it."
> Jim Miller
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
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>

--
Gene Fisher -- 74-23,77PB/ore/ca
“Give a man a fish; you have fed him for today --- give him a URL and
-------
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Alternator Protection Cable
http://gmcmotorhome.info/APC.html
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Re: [GMCnet] Fuel [message #302621 is a reply to message #302619]

Sat, 25 June 2016 18:34

jor

Messages: 43
Registered: June 2016
Location: Tucson

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Member

Thanks, guys. I now understand the great gas tank mystery! My knowledge base is growing (albeit slowly).
jor

John O'Reilly 76 Eleganza II (quad bags, disc brakes w/ reaction arm. 3.70 gears, manny trans, headers, Patterson dist.) Tucson, AZ

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Re: Fuel [message #302623 is a reply to message #302614]

Sat, 25 June 2016 20:49

JohnL455

Messages: 4447
Registered: October 2006
Location: Woodstock, IL

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Note that the generator dies with about 1/4 or so in the main so as not to strand you in the back woods. The important thing to realise is not to go down the road with the roof air on and run the Onan out of fuel under load. This is hard on the equipment and often burns out the bridge rectifier in the gen as it attemps to regulate voltage while engine is stoping, the current goes way up to the field windings in an attempt ti keep 120V

John Lebetski
Woodstock, IL
77 Eleganza II

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Re: Fuel [message #302625 is a reply to message #302623]

Sat, 25 June 2016 21:22

A Hamilto

Messages: 4508
Registered: April 2011

Karma: 39

Senior Member

JohnL455 wrote on Sat, 25 June 2016 20:49

...not to go down the road with the roof air on and run the Onan out of fuel under load. This...often burns out the bridge rectifier...

In the interest of thread drift, and to expand on that, any time the Onan is shut down under load (either accidentally or on purpose) it can take out the bridge rectifier.

Now that you got me thinking about it, I need to see if I can figure out a way to put a fuse in that line someplace that is easy to access. IIRC, Jim Miller said the field only sees about 4A under full load, maybe a 69 cent 5A fuse would save a $20 rectifier.

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Re: Fuel [message #302639 is a reply to message #302614]

Sun, 26 June 2016 08:15

jhbridges

Messages: 8412
Registered: May 2011
Location: Braselton ga

Karma: -74

Senior Member

Also note that in Norris and Coachmen upfits at least, the genset will pull the main tank completely empty. This is true of any installation where the installer was too lazy or too cheap to drop the rear tank and use the genset pickup. The culprits simply tee the genset feed into the main tank outlet line.

--johnny

Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons. Braselton, Ga. I forgive them all, save those who hurt the dogs. They must answer to me in hell

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Re: [GMCnet] Fuel [message #302644 is a reply to message #302625]

Sun, 26 June 2016 09:44

emerystora

Messages: 4442
Registered: January 2004

Karma: 13

Senior Member

Just put a 1000 volt rated bridge rectifier in and I don't think you will ever have that problem. You can google "1000 volt bridge rectifier" and find many priced fro $2 to $5.

If you have a local electronic supply shop you can likely buy it there for $5 as well

Emery Stora
77 Kingsley
Frederick, CO

> On Jun 25, 2016, at 8:22 PM, A. wrote:
>
> JohnL455 wrote on Sat, 25 June 2016 20:49
>> ...not to go down the road with the roof air on and run the Onan out of fuel under load. This...often burns out the bridge rectifier...
> In the interest of thread drift, and to expand on that, any time the Onan is shut down under load (either accidentally or on purpose) it can take out
> the bridge rectifier.
>
> Now that you got me thinking about it, I need to see if I can figure out a way to put a fuse in that line someplace that is easy to access. IIRC, Jim
> Miller said the field only sees about 4A under full load, maybe a 69 cent 5A fuse would save a $20 rectifier.
> --
> 73 23' Sequoia 4 Sale
> 73 23' CanyonLands Parts Unit 4 Sale
> Upper Alabama
> "Posting on the GMCnet seems to always result in a cacophony of responses which only tend to cloud the solution rather than move you closer to it."
> Jim Miller
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org

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Re: [GMCnet] Fuel [message #302773 is a reply to message #302644]

Tue, 28 June 2016 14:36

jor

Messages: 43
Registered: June 2016
Location: Tucson

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OP here. Got my first experience with the fuel supply situation described in this thread. The main tank gauge showed less than a quarter tank but the aux setting showed beyond full (pegged the needle). I selected the aux tank. Off I went to Discount Tire. On the way, I had to climb a steep hill. As soon as the rig started heading up, the engine quit. I switched back to the main tank and it fired right up. I went straight to the gas station and added 37 gallons. This rig has an electric fuel pump, btw.
jor

John O'Reilly 76 Eleganza II (quad bags, disc brakes w/ reaction arm. 3.70 gears, manny trans, headers, Patterson dist.) Tucson, AZ

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Re: [GMCnet] Fuel [message #302786 is a reply to message #302773]

Tue, 28 June 2016 16:34

A Hamilto

Messages: 4508
Registered: April 2011

Karma: 39

Senior Member

jor wrote on Tue, 28 June 2016 14:36

OP here. Got my first experience with the fuel supply situation described in this thread. The main tank gauge showed less than a quarter tank but the aux setting showed beyond full (pegged the needle). I selected the aux tank. Off I went to Discount Tire. On the way, I had to climb a steep hill. As soon as the rig started heading up, the engine quit. I switched back to the main tank and it fired right up. I went straight to the gas station and added 37 gallons. This rig has an electric fuel pump, btw.
jor

The gauge is built for 0 - 90 ohms. The float in the tank is attached to a variable resistor that is supposed to range 0 - 90 ohms.

Besides a stuck float, there are two other failure modes: A broken wire or bad connection will cause the gauge to peg one way. A short circuit in the wiring will make the gauge go to the other peg. I can never remember which is which.

You probably have a wiring problem, either an open circuit or a short circuit. A stuck float would probably not peg the gauge.

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Re: [GMCnet] Fuel [message #302807 is a reply to message #302773]

Wed, 29 June 2016 06:51

Matt Colie

Messages: 8547
Registered: March 2007
Location: S.E. Michigan

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jor wrote on Tue, 28 June 2016 15:36

OP here. Got my first experience with the fuel supply situation described in this thread. The main tank gauge showed less than a quarter tank but the aux setting showed beyond full (pegged the needle). I selected the aux tank. Off I went to Discount Tire. On the way, I had to climb a steep hill. As soon as the rig started heading up, the engine quit. I switched back to the main tank and it fired right up. I went straight to the gas station and added 37 gallons. This rig has an electric fuel pump, btw.
jor

John,

Anytime a GM guage of our period moves hard to full scale, suspect an open in the signal lead. This is just one of the things I think was a bad move on their part. Unfortunately, the sending units can easily go open when exposed to ethanol.
One of Jim Miller's favorite lines is that the FAA says that the only time you should trust a fuel level gauge is when it says "Empty".

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] Fuel [message #302839 is a reply to message #302807]

Wed, 29 June 2016 15:28

jor

Messages: 43
Registered: June 2016
Location: Tucson

Karma: 0

Member

Thanks for the tips on the gauge issue. This is one thing I'm going to fix before taking this baby anywhere, especially after reading the Gas Tank Mystery document. One thing I'll have to get used to is range of travel. Our last two rigs had big tanks (148 and 198 gallons) so I got used to a long time between fuel stops. On the other hand, having filled up my GMC once, I am elated on how easy it is to fuel. No worry about height and length. Just pull in like a pickup! I'm going to really enjoy the GMC.
jor

John O'Reilly 76 Eleganza II (quad bags, disc brakes w/ reaction arm. 3.70 gears, manny trans, headers, Patterson dist.) Tucson, AZ

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Re: [GMCnet] Fuel [message #302887 is a reply to message #302839]

Thu, 30 June 2016 15:46

Carl S.

Messages: 4186
Registered: January 2009
Location: Tucson, AZ.

Karma: 13

Senior Member

John,

I normally plan my gas stops, using gasbuddy.com for wherever it is most convenient, and least expensive, anywhere from 150 - 280 miles. I find that we need to stop, AT LEAST that often for other reasons anyway, so I don't find it inconvenient. Even if I didn't have working gas gauges, that system would work, although it is nice to be able to manage my fuel a little better than just by he odometer.

Carl Stouffer '75 ex Palm Beach Tucson, AZ. Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles, Solar battery charging. GMCSJ and GMCMI member

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