| vapor pressure in fuel tank [message #138611] |
Thu, 11 August 2011 22:47  |
armandminnie
Messages: 864
Registered: May 2009
Location: Marana, AZ
Karma: 2
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Driving through southern Utah today the temperatures were in the low 90s and we had altitudes that ranged from 5000 to more than 7000 feet. At a couple of points along the way we had minor symptoms of fuel starvation consistent with vapor lock but I did not believe it because since I installed the full time electric fuel pump I have not had a problem while driving (restarting a hot engine is the only symptom these days) no matter the outside temps or altitude.
My wife suggested that we loosen the gas cap - her brother's idea - and since I had just read about someone else having a gas cap issue I decided to give it a try. After about 100 miles since the last fillup we stopped at a rest area and I loosened the gas cap. I immediately heard the rush of air I expected but it was a rush of air - and hot gasoline - out of the tank and all over my hand. I quickly put the cap back on and then slowly released all the air. If I tried to let it out too fast I got gasoline again. It took at least a full minute and maybe more to slowly release all of the pressure.
The confusing thing is that my vent line to the fuel/vapor separator in the rear driver's side wheel well is open to the air. I know this because fuel coming from it during a fillup is my signal that the tanks are pretty full.
Any ideas or suggestions as to why this happened?
Armand Minnie
Marana, AZ
'76 Eleganza II TZE166V103202
visit my gmc blog
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| Re: vapor pressure in fuel tank [message #138626 is a reply to message #138611] |
Thu, 11 August 2011 23:44  |
George Beckman
Messages: 1085
Registered: October 2008
Location: Colfax, CA
Karma: 11
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| armandminnie wrote on Thu, 11 August 2011 20:47 |
Driving through southern Utah today the temperatures were in the low 90s and we had altitudes that ranged from 5000 to more than 7000 feet. At a couple of points along the way we had minor symptoms of fuel starvation consistent with vapor lock but I did not believe it because since I installed the full time electric fuel pump I have not had a problem while driving (restarting a hot engine is the only symptom these days) no matter the outside temps or altitude.
The confusing thing is that my vent line to the fuel/vapor separator in the rear driver's side wheel well is open to the air. I know this because fuel coming from it during a fillup is my signal that the tanks are pretty full.
Any ideas or suggestions as to why this happened?
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I think you are on the right track with the separator. This is one of the early smog goodies that actually can work and makes sense. It vents the tank, capturing the fumes in the charcoal canister and lets you burn it later as fuel.
I would find out why the separator is leaking gas. It should not. They can warp, or have a bad hose. If it is not warped, I would make sure you can blow through it from bottom to top. The little ball can get stuck at the top and not let the tank vent to the canister. Of course check hoses to the tank and to the canister and canister to carburetor/TBI for leaks.
If you need another separator Sirum invented a replacement that is available at most GMC shops. Some Internet Corvette stores have the original. It is a good unit to have working; no sense paying for gas and letting it boil/evaporate off when warm. Plus it stinks.
'74 Eleganza, SE, Howell + EBL
Best Wishes,
George
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| Re: vapor pressure in fuel tank [message #138635 is a reply to message #138611] |
Fri, 12 August 2011 01:48 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
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What I am hearing is two different problems.
The first is vaporizing or boiling of gas in the tank. This is you prime problem. Three things affect the boiling or vaporization of the fuel:
1. Fuel temperature
2. Outside air pressure which changes by altitude.
3. Reid Vapor Pressure of the fuel in the tank.
Item 1. I would start by looking at ways to reduce the fuel temperature. Insulation, reflective paints (like painting the tanks white on the bottom), and heat shields on the various components all will help. GM knew that they had a problem with California gas in the 1970's so they added a couple of exhaust heat shields on California coaches only. That is only one example of one possible thing you might do. The gas has only gotten worse in California so I believe you need to look at doing more to keep it cooler.
Item 2. Not much you can do about this one. I had some auto fuel vaporize one time at 4000 feet in an airplane on a 90 degree day. Not much you can do about this other than be aware of it and drive at higher elevations during the cooler parts of the day.
3. Winter mix fuel and alcohol mixed fuels vaporize at lower temps so avoid using those kinds of fuel when ever possible. I keep track of where I can buy non-alcohol fuel and try to use it exclusively. I also try to buy fuel in the same locale where I intend to drive.
The second problem is one of venting which you would not normally have to worry about if you weren't boiling the fuel in the first place.
I do not understand how you could be building up pressure in the tank(s) when you say you have an open vent hose running direct to the tank. The separater can't be the problem if the hose to it is disconnected.
With the tank not full, I would remove the gas cap and then blow LOW PRESSURE shop air back through that vent hose and into the tank to verify that the hose is really open. I'm wondering if the body isolation pads in your coach have squished down and the body is now squeezing off the vent hose where it passes between the body and the frame. I really do not understand this problem as you described it.
Good luck,
Ken B.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: vapor pressure in fuel tank [message #138664 is a reply to message #138635] |
Fri, 12 August 2011 08:33 |
armandminnie
Messages: 864
Registered: May 2009
Location: Marana, AZ
Karma: 2
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Ken,
I also don't understand how this could happen with the fuel vapor separator broken and open to the outside air. 100 miles earlier I had filled up and fuel came out of it so I know that, even if it is crushed somewhat, the amount of pressure that was on the gas cap would have surely vented through it.
Right in the middle of writing this I looked at a very nice fuel line schematic from 'RJW' at palmbeachgmc.com - I can't find the diagram on the site now but I did save a copy of it some time ago. It shows a vent line from each tank teed into a single line to the fuel vapor separator. Maybe one of those lines is crushed before the tee. I guess I will be dropping the tanks again this winter.
Armand Minnie
Marana, AZ
'76 Eleganza II TZE166V103202
visit my gmc blog
click here to visit gmcws.org
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| Re: vapor pressure in fuel tank [message #138667 is a reply to message #138664] |
Fri, 12 August 2011 08:52 |
Keith V
Messages: 2337
Registered: March 2008
Location: Mounds View,MN
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| armandminnie wrote on Fri, 12 August 2011 08:33 |
ago. It shows a vent line from each tank teed into a single line to the fuel vapor separator. Maybe one of those lines is crushed before the tee. I guess I will be dropping the tanks again this winter.
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That would be my guess...one tank isn't venting properly.
Also your gas cap should be relieving pressure, so maybe that needs to be replaced, Stant # 11807 for my 1975
Keith Vasilakes
Mounds View. MN
75 ex Royale GMC
ask me about MicroLevel
Cell, 763-732-3419
My427v8@hotmail.com
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| Re: [GMCnet] vapor pressure in fuel tank [message #138678 is a reply to message #138669] |
Fri, 12 August 2011 09:25 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
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Clean air or not, I'd keep a venting gas cap except for the actual
inspection. IMHO, that could be a safety issue. It's conceivable to me
that the pressure could build enough to rupture a tank or, at the least,
blow a tube fitting apart -- such as the elbows from the fill tube into the
tanks -- you could be dumping 40+ gallons of fuel.
Ken H.
Americus, GA
'76 X-Birchaven w/Cad500/Howell EFI+ & EBL
www.gmcwipersetc.com
On Fri, Aug 12, 2011 at 10:00 AM, Armand Minnie <armand@minniebiz.com>wrote:
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> I am not sure but I think that a venting fuel cap violates some clean air
> rules. They check it every two years and a couple of years ago made me
> replace it. Arizona is almost as bad as California for this stuff.
> --
>
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Large Wiring Diagrams
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76 X-Palm Beach
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| Re: [GMCnet] vapor pressure in fuel tank [message #138686 is a reply to message #138682] |
Fri, 12 August 2011 09:45 |
Keith V
Messages: 2337
Registered: March 2008
Location: Mounds View,MN
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| armandminnie wrote on Fri, 12 August 2011 09:37 |
Ken, good suggestion about the venting gas cap. I will run with a loose gas cap until I can find a venting one to use in between smog checks.
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I'm pretty sure the cap is part of the emissions system and it must be the correct one. Putting the wrong cap on would be considered tampering with the emissions system.
The emissions check should be that the cap vents at the correct pressures
( you gotta use their own rules against them sometimes)
Keith Vasilakes
Mounds View. MN
75 ex Royale GMC
ask me about MicroLevel
Cell, 763-732-3419
My427v8@hotmail.com
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| Re: vapor pressure in fuel tank [message #138730 is a reply to message #138664] |
Fri, 12 August 2011 15:14 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
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If one of the vent lines was squished or plugged before the tee, the pressure would simply flow to the other tank through the big fuel filler line.
I still do not understand what is causing the failure to vent problem. That line to the separater has to be plugged or plugging up on the fly some how.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: [GMCnet] vapor pressure in fuel tank [message #138736 is a reply to message #138730] |
Fri, 12 August 2011 16:38 |
Chuck Garton
Messages: 54
Registered: June 2006
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Armand,
I have owned a GMC since November 1977 and live in the Mojave Desert
in California. I have cured vapor lock problems only to have
California change the gasoline.
I wrote an article that is still posted on the Pacific Cruisers web
site under "shadetree" (www.GMCPC.org) titled third fuel tank
installation. It also covers everything I have done to combat vapor
lock.
One time, I encountered pressurised fuel tanks. The problem turned out
to be the vacuum line from the carbon canister to the carburetor had
broken.
This is a ported vacuum (0" hg at idle) and does not show up as a
"normal" vacuum leak. After replacing the vacuum line, no more
pressurized fuel tanks.
Chuck Garton
77 Kngsley 455
Ridgecrest, CA
On Fri, Aug 12, 2011 at 1:14 PM, Ken Burton <n9cv@comcast.net> wrote:
>
>
> If one of the vent lines was squished or plugged before the tee, the pressure would simply flow to the other tank through the big fuel filler line.
>
> I still do not understand what is causing the failure to vent problem. That line to the separater has to be plugged or plugging up on the fly some how.
> --
> Ken Burton - N9KB
> 76 Palm Beach
> Hebron, Indiana
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| Re: [GMCnet] vapor pressure in fuel tank [message #138740 is a reply to message #138736] |
Fri, 12 August 2011 17:20 |
armandminnie
Messages: 864
Registered: May 2009
Location: Marana, AZ
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Chuck, thanks for the input on the pressurized fuel tank. I broke my carbon canister some time ago and just plugged the vacuum line with a bolt. The line from the fuel/vapor separator to the cannister is just hanging there under the wheel well liner. Fuel drips from the fuel/vapor separator when the tanks are near full.
Today, I drove all day with the gas cap loose. Of course the temperatures are getting a little lower - I don't think we ever got over about 88 degrees today in northern Utah and Idaho.
Armand Minnie
Marana, AZ
'76 Eleganza II TZE166V103202
visit my gmc blog
click here to visit gmcws.org
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| Re: [GMCnet] vapor pressure in fuel tank [message #138752 is a reply to message #138693] |
Fri, 12 August 2011 18:51 |
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Matt Colie
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Registered: March 2007
Location: S.E. Michigan
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| Gary Bovee wrote on Fri, 12 August 2011 11:55 |
Armand,
I had the same thing happen to me the other day. I got stopped for 20 minutes for a road repaving project. When traffic starting moving again I stopped at a gas station a couple a minutes down the road from where I had been stopped. The outside temp was above 90 degrees. The tanks were about a 1/3 full. When I start to loosen the gas cap air started rushing out and then gas. I quickly tighten the cap. Then I very slowly started to open it again and let much more air escape. I heard one of the tanks go pop when the air was releasing. I have installed a new fuel vapor separator within the last year. I have replaced all the fuel lines/vent lines in the last three years. I have installed new body pad so none of the lines are pinched.
Back to the drawing board to check out what is going on.
Gary Bovee
Red Bluff, CA
1978 Royale by Coachmen
GMC Idiot's Guide
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Gary,
Don't doing work for no reason....
You just hit the mix that does not work.
The tank was venting to the carbon canister(s) as it should, but all that time you were waiting, the canisters could not purge.
That only happens at road load (this is even more critical with and Open-Loop fuel control (what most of us have)) because if it had purged to an idling or stopped engine, this would be are real big problem.
The canisters were full, the tanks were hot and there was nowhere for the vapor to go, so it was just waiting for you.
Matt (30 years a Detroit lab rat)
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] vapor pressure in fuel tank [message #138755 is a reply to message #138752] |
Fri, 12 August 2011 19:35 |
armandminnie
Messages: 864
Registered: May 2009
Location: Marana, AZ
Karma: 2
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Matt, you said that the carbon canister(s) could not purge so the vapor had no where to go.
I no longer have a carbon canister installed (temporary - because I broke it) but I thought that I remembered it leaking liquid gasoline after fuel fill-ups a couple of years ago. When I was trying to repair it I seem to remember some openings at the bottom that allowed a view of the carbon fabric inside of it. Both of those things caused me to assume that the carbon canister could also vent gasoline vapors if needed.
Armand Minnie
Marana, AZ
'76 Eleganza II TZE166V103202
visit my gmc blog
click here to visit gmcws.org
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| Re: [GMCnet] vapor pressure in fuel tank [message #138782 is a reply to message #138780] |
Fri, 12 August 2011 22:06 |
armandminnie
Messages: 864
Registered: May 2009
Location: Marana, AZ
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I also would think that excessive in-tank pressure would not cause any kind of hesitation or fuel starvation symptoms. I wasn't concerned about the fuel starvation any more, I was more concerned with why, after using about 15 gallons of fuel, it would still come spewing out of the filler when I opened it - especially with an opening to the atmosphere in the vent line.
Armand Minnie
Marana, AZ
'76 Eleganza II TZE166V103202
visit my gmc blog
click here to visit gmcws.org
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