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elevatorbernie
Expert
Date registered: Aug 2006
Location: Vancouver B.C. Canada
Vehicle(s): 1989 280GE
Posts: 1347
| gas tank pressure problem
I have what seems to be a gas tank pressure or venting problem, was wondering if anyone has had it before and what is the cause. Intermittently I have very strong smell of fuel inside the cabin and occasionally I have a pressure build up inside the gas tank itself. Sometimes when refueling I am barely able to hold on to the gas cap when it's removed and two hands are required to turn it also due to the amount of pressure behind it...alot of pressure is built up inside that tank. I would guess around 100psi or more. When I have fumes inside the cabin there seems to be little to no pressure inside the tank. Today the tank made a large pop noise as it contracted when I released the pressure while removing the gas cap. I have visually inspected the fuel venting valves and hoses inside the passenger side rear panel...all looks normal. I suspect I have 2 problems...1st is somehow I'm getting full line pressure returning to the tank my guess is the accumulator bladder is ripped. 2nd, this is being compounded by the vent valves not venting, either due to excess pressure jamming them or damage due to the pressure or just old age. Any thoughts or theories welcomed. Thanks, regards Bernie.
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DUTCH
Administrator Doppelgänger
Date registered: Apr 2006
Location: US, GA, Atlanta
Vehicle(s): 2015 Audi Q7 3.0 TDI,2018 Sprinter
Posts: 9963
| RE: gas tank pressure problem
My experience with the vent valve in the gas cap of my 1984 280GE was that it worked very poorly. Sometimes it would hold pressure. Other times it would not and would leak fuel if I parked up hill after a fill up. |
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J.R.
Elite Veteran
Date registered: Apr 2006
Location: Southern Maryland
Vehicle(s): 00 G500, 09 911 4S, 11 Cayenne S, 86 280GE (sold)
Posts: 828
| RE: gas tank pressure problem
I would first check out the tank vent system. Gas tanks need a way to safely vent.
I recall on my 86 280GE that there was a charcoal canister that had a line back to the fuel tank area. The canister's other line was connected to the engine intake manifold somewhere. When I dropped the gas tank for a clean out and hot lining application, every hose I was touching was either a vent line running to the saddle tanks, the fueling neck vent, feed line to the pump or a return line. What I didn't recall was messing with the line feeding the charcoal canister. So my theory is that is a line running from near the top of the fueling neck has a "T" that connects to the charcoal canister. There is probably a valve in that line somewhere for rollover protection against fuel loss and fire.
When I look up my old 280GE VIN, there are displays of vents lines as described above. Some were not applicable, some were.
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DUTCH
Administrator Doppelgänger
Date registered: Apr 2006
Location: US, GA, Atlanta
Vehicle(s): 2015 Audi Q7 3.0 TDI,2018 Sprinter
Posts: 9963
| RE: gas tank pressure problem
J.R. - 9/7/2014 10:38 AM
I would first check out the tank vent system. Gas tanks need a way to safely vent.
I recall on my 86 280GE that there was a charcoal canister that had a line back to the fuel tank area. The canister's other line was connected to the engine intake manifold somewhere. When I dropped the gas tank for a clean out and hot lining application, every hose I was touching was either a vent line running to the saddle tanks, the fueling neck vent, feed line to the pump or a return line. What I didn't recall was messing with the line feeding the charcoal canister. So my theory is that is a line running from near the top of the fueling neck has a "T" that connects to the charcoal canister. There is probably a valve in that line somewhere for rollover protection against fuel loss and fire.
When I look up my old 280GE VIN, there are displays of vents lines as described above. Some were not applicable, some were.
A lot of that, including the charcoal canister, was probably added by the Federalizer. It certainly was on my 1984 280GE. |
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MudMonkey
Veteran
Date registered: Jun 2013
Location: CO
Vehicle(s): '03 G500
Posts: 274
| Re: gas tank pressure problem
Charcoal canister was at fault for this on one of my late '90's 4x4 trucks. Simple replacement, and back to good.
Not sure if it's your issue... but if it has never been replaced, it's certainly a possible cause. |
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elevatorbernie
Expert
Date registered: Aug 2006
Location: Vancouver B.C. Canada
Vehicle(s): 1989 280GE
Posts: 1347
| RE: gas tank pressure problem
thanks for the replies...I haven't got a charcoal canister my G is as it left the factory...never federalized. However, there are 3 factory installed valves visible inside the passenger rear corner panel. They are also on the epc. If one was plugged it would explain the non venting issue, but why do I have pressure and not vacuum? As fuel is consumed I would think there would be vacuum created in the tank....not pressure. I mean it's really a lot of pressure not just a pssst of normal gas vapor pressure, I'm talking big pressure like you cracked open a propane bbq cylinder...the last time it happened it almost blew the cap from my hands...yes 2 hands on cap. Somehow it's building pressure in the tank and the longer I drive the more pressure created, also seems the lower the fuel level the more pressure is built up..which might be normal if air was going in but fumes not venting out. When I removed the cap the tank contracted with a very loud pop...it was really under alot of pressure...not normal gas vapor.. But, I'm surprised the saddle tank hose hasn't blown off. None of the tanks vent hoses are pressure rated either. Only the fuel lines are rated for that kind of pressure. Also I've had that tank off before and it's not some cheap thin tin can, I got to think to expand it would take more than vapors. I'm convinced return fuel line pressure is somehow pressuring the tank. What happens if the accumulator leaks?
Edited by elevatorbernie 9/7/2014 9:08 PM
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AlanMcR
Expert
Date registered: Apr 2006
Location: US, CA, Los Altos
Vehicle(s): G300DT E300DT 230SL
Posts: 3500
| RE: gas tank pressure problem
Fuel goes in cold. Then when you are driving the excess fuel returning from the engine is quite warm. The the warm fuel and surrounding air expands, causing the pressure.
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w.james
Veteran
Date registered: Jun 2006
Location: New Jersey
Vehicle(s): 84 280 GE 617A 5sp getrag
Posts: 239
| Re: gas tank pressure problem
Do you have the aux set up the the wheel wells? they have a pressure equilzer that goes ove the door and is connected by rubber pipe. That could be your smell |
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J.R.
Elite Veteran
Date registered: Apr 2006
Location: Southern Maryland
Vehicle(s): 00 G500, 09 911 4S, 11 Cayenne S, 86 280GE (sold)
Posts: 828
| RE: gas tank pressure problem
AlanMcR - 9/8/2014 12:16 AM
Fuel goes in cold. Then when you are driving the excess fuel returning from the engine is quite warm. The the warm fuel and surrounding air expands, causing the pressure.
I have had situations where I had excessive pressure when opening a gas cap, never a G, and never at the pressures you are seeing. Yes you should be concerned. I would be too.
To expand on Alan's comment. The warming of the fuel in the tank, whether from the return fuel heated by the engine compartment, road heat, exhaust (mine ran over the top right hand side of the tank and there was an insulating blank placed on the tank to protect the tank), exhaust leak in that same area, heat from the radiator-engine-exhaust as it exits underneath the truck, etc, it all raises the temperature of the contents of the fuel tank and causes fuel to convert from liquid to gaseous state at the surface. A failing fuel pump may generate excessive heat that is added to down stream in the engine compartment. The pressure building up has no where to go because the normal vent line to the engine intake manifold is not accepting an input because there is not pathway...it's blocked. A stuck valve (or crimped line) can prevent the pressure/fumes from leaving the tank and at the same time, the same problem, or a different valve is not allowing the engine vacuum to pull from the tank. If there are electrical connections to the valves, check your fuses, check for corrosion at the connections, etc. If possible, with the area well ventilated, depressurize the tank by opening the cap. Leave the cap off. disconnect the vacuum hose at two ends that takes all of the valves out of the loop. Blow air through the line going to the intake manifold and to the tank. They should both be open. The one going forward should be open as I do not think there are any additional valves in the engine compartment. If it is not open, research the problem. If its okay, take a piece of fuel line and two male splices and mate the two open ends to yield a no valve circuit. replace the fuel cap. Fire it up and test for vacuum at the port. Put the line back on and take it for a drive. Test the cap when you are finished with the drive. I would also double check that you do not have an exhaust leak at the fuel tank. I have never had to perform a test like this, but this is how I would approach it.
As to the high pressure pump and fuel return mechanics. Let's assume that there is no off gassing of the fuel and temperature is constant. It is a closed loop system. You can take fuel from the tank, pressurize it and return it to the tank and there will be no change in tank pressure. To pressurize the tank is this scenario, you need external inputs. The truck's fuel system, generates a pressure loss due to fuel consumed, all other variables held constant.
When I had my tank out of the truck, I recall that none of the lines related to tank pressurization and venting where friction fit. They all seemed to be robust lines with clamps. Some where factory, others were Federalizers as Dutch mentioned.
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elevatorbernie
Expert
Date registered: Aug 2006
Location: Vancouver B.C. Canada
Vehicle(s): 1989 280GE
Posts: 1347
| RE: gas tank pressure problem
I only have one aux saddle tank its on the passenger side. Thanks all for the ideas, im going to try and troubleshoot the problem.
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