What motor has the best mileage?

[Matt Colie]

Hal,
Thank you for the complete and clear explanation.
(Now he tells me!)
Did you think to look up the adjustable pork chops that someone sells?
Matt_C

 

[RoadWarrior]

Here is a image of what someone posted of the high tee setup from the photosite
http://www.gmcmhphotos.com/photos/in-tank-fuel-pumps/p53352-20130719-141801.html

Which motor gets the best mileage: MINE-- just like everyone else's, and now that gas has skyrocketed it "never gets enough gas mileage."

I suppose relocating the "tee" may have some advantage.

In that picture it shows the fill line, like many going in and uphill, slightly so when you try to fill the tanks the fuel rushing down the tube wont allow the air to flow out of the tank (the fill line is actually the main source of venting). There is no possible way the tank could vent during filling through what is commonly called the vent line. Basic Physics: large volume flow of fuel into the tanks cant be vented through a small line. The purpose of that line is for topping off the tank once it has clicked off.

My coach when I got it had a terrible time burping up gas and was very difficult to get the tanks really full. So much so that my PO had a mat that unfureled when the gas door was opened to protect the paint.

Referencing the Assembly Manual I discovered that there were supposed to be two clamps on the fill line (I used a large electrical conduit clamp) supporting the horizontal part of the line and of course it was supposed to have a slight downward angle. Just like all plumbing systems. Take out that slight drop in any plumbing line and you develop what I see most complaining about.

My coach now fills as fast as the pump can deliver it and never burps or foams.

 

[gerryg]

Which motor gets the best mileage: MINE-- just like everyone else's, and now that gas has skyrocketed it "never gets enough gas mileage."

I suppose relocating the "tee" may have some advantage.

In that picture it shows the fill line, like many going in and uphill, slightly so when you try to fill the tanks the fuel rushing down the tube wont allow the air to flow out of the tank (the fill line is actually the main source of venting). There is no possible way the tank could vent during filling through what is commonly called the vent line. Basic Physics: large volume flow of fuel into the tanks cant be vented through a small line. The purpose of that line is for topping off the tank once it has clicked off.

My coach when I got it had a terrible time burping up gas and was very difficult to get the tanks really full. So much so that my PO had a mat that unfureled when the gas door was opened to protect the paint.

Referencing the Assembly Manual I discovered that there were supposed to be two clamps on the fill line (I used a large electrical conduit clamp) supporting the horizontal part of the line and of course it was supposed to have a slight downward angle. Just like all plumbing systems. Take out that slight drop in any plumbing line and you develop what I see most complaining about.

My coach now fills as fast as the pump can deliver it and never burps or foams.

The fill tubes enter the sides of the tanks so once the tanks are half full the fill tube would be beneath the surface of the fuel. At this point I think that no air can escape from the tanks back up the fill line so it all needs to come out the vent lines.

 

[Hal]

Hal,
Thank you for the complete and clear explanation.
(Now he tells me!)
Did you think to look up the adjustable pork chops that someone sells?
Matt_C

Yes Matt, I'm using those also. The sockets in the LCA's were iffy also so I used some(sockets) from some lower Revcon arms I had which are WAY more robust. I moved the sockets a bit for better clearance also.

 

[RoadWarrior]

The fill tubes enter the sides of the tanks so once the tanks are half full the fill tube would be beneath the surface of the fuel. At this point I think that no air can escape from the tanks back up the fill line so it all needs to come out the vent lines.

They do enter in from the sides, as they do on every vehicle I have seen except tractors and motorcycles, but at least on mine the port is located at the top of the tank. If all works well, air rushes out at that point until the tank is about 90-95% full; at which point the gas nozzle should click off. Some would consider that a full tank, but the tank can be "topped-off" by slowly filling the tank, at which point the "fill vent" will allow a slow fill for the remaining 5%. GM has published a number of technical memos over the years describing just this, although I didn't do a search on our coaches specificly.

If the fuel line is tilted up, venting is limited primiarly to the small vent fill line, which can't handle a high flow fill. Air still trys to gush up the main fill tube causing turblince in the gas fill, and since gas foams up easialy, you get burping of gas, and a restricted flow.

Here is a image from gmcmhphotos.com: http://www.gmcmhphotos.com/photos/gas-tank-rework/p35292-gas-tank-rework.html

 

[RF_Burns]

Here are a couple close-up photos showing the fill tube position on my tanks:

Front: http://www.gmcmhphotos.com/photos/fuel-tank-lines/p61890-fuel-tank-lines.html

Rear: http://www.gmcmhphotos.com/photos/fuel-tank-lines/p61888-fuel-tank-lines.html

I'd say the top of the fill tube is at about the 2/3 mark, but then the low rear ride height and front position of the filler port might give you 3/4 tank before the filler is submerged. So at least 1/4 of the tank must fully vent through the vent tubes.

Just my farmboy shade-tree mechanic way of seeing things.

 

[Keith V]

Lower you right rear as much as possible with some lowerig on the left then slow fill. no burbs !!!455 for me the mileage is an insignificant factor.. Larry 78/455 XPB Pearl

Does this really help? I mean the vents are in the center of the tanks so if the right is lower the vents will be covered sooner. it seems leveling the coach, like for camping, would be a better solution
I have 3/8" High T vents so I don't have that problem

 

[Matt Colie]

Well, we have a 23 and by all accounts (and my first mathematical model) should be worse than a 26. (because there is less length in the fill pipe. ) Normal gas pumps are limited to 10 GPM by EPA rules and so most are set up for 9+. I can fill from near dry to full in so little time I can't clean the windshield or do anything else. On a number of occasions, I have put more than 50 gallons in the tanks.
When I had a bad suspension compressor, I wouldn't dare lower the coach just to put fuel on. Now, dump the black tank, that may happen if I was at a fair ground I was not leaving soon.
Matt_C

 

[RoadWarrior]

Here are a couple close-up photos showing the fill tube position on my tanks:

Front: http://www.gmcmhphotos.com/photos/fuel-tank-lines/p61890-fuel-tank-lines.html

Rear: http://www.gmcmhphotos.com/photos/fuel-tank-lines/p61888-fuel-tank-lines.html

I'd say the top of the fill tube is at about the 2/3 mark, but then the low rear ride height and front position of the filler port might give you 3/4 tank before the filler is submerged. So at least 1/4 of the tank must fully vent through the vent tubes.

Just my farmboy shade-tree mechanic way of seeing things.

The first question that comes to mind; are those GM tanks, or aftermarket? Although GM dealt with many vendors, often for the same part. Mine are much closer to the top of the tank and have less curve at the corners. Nothing wrong with "farm boy shade Tree" common sense; but it is rare. You probably can only get a fast fill for about two thirds when the coach is level, but as you mentioned, the back being lower would help.

Like I indicated before, the "fill vent" was added to vehicles to allow topping off the tanks without them burping and spilling gas on the ground (environmental). Following the "farm boy shade Tree" common sense rule; expecting the tank to fast vent through that small hose would be like a plumber installing a 1/2" vent line on a toilet. It wouldnt flush very well because of the small diameter of the vent.

 

[RF_Burns]

I can see where it would lower the fuel level at the fill ports and allow more fuel in the tanks. But I just fill till it gurgles and then a bit more. I'm a 200 mile or half tank left per fill guy.