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Pressure in fuel tank
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PostPosted: Sun Aug 26, 2001 6:14 am    Post subject: Reply with quote

Good point, my gas cap is red (prolly aftermarket) and with a hole in the plastic...
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PostPosted: Sun Aug 26, 2001 6:23 am    Post subject: Reply with quote

The gas cap only vents out it shouldn't vent in. I have the OEM cap on the car that I have a problem on but I've tried a new aftermarket which is on my other car and it makes no difference. The only thing I can think of is that the vent line to the charcoal canister is clogged up or else it is only a one way line meaning there is a check valve somewhere. I've tried a lot of things so if you got any ideas I'm listening.
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Blas Costagli  

PostPosted: Sun Aug 26, 2001 11:34 am    Post subject: Reply with quote

Porsche is looking in the cap venting idea.
Apparently, someone has made a replacement cap, original looking, which is not venting. Mine is a replacment from many years ago. I'll post what I find out Monday.
And jcope...I understand what your saying.
I note the emmisions testing stations around here can check the cap out...hmmm.
And AznDrgn
Shouldn't it vent in and not out....
Gasoline leaves the tank and is replaced by air...

[ This Message was edited by: Blas Costagli on 2001-08-26 11:36 ]
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kaffine krazed  

PostPosted: Sun Aug 26, 2001 12:09 pm    Post subject: Reply with quote

Some caps vent in and some vent out and some do both.

The gas cap is a convinet place to put a vent to let air in where it is away from road grime that might clog the vent if the located it on the tank. It will be a one way check valve to prevent fumes from escaping into the atmosphere.

Some vent out to prevent excessive pressure build up inside the tank from the gas evaporating I think this is now done with the charcaol canister but I'm sure some cars still have that as a spec for the gas cap to be on the safe side there will be a valve in the gas cap that opens at a specific pressure.
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PostPosted: Sun Aug 26, 2001 1:18 pm    Post subject: Reply with quote

You ever seen one of them weird gas cap critters with valves?
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PostPosted: Tue Aug 28, 2001 2:02 am    Post subject: Reply with quote

I'm glad to hear kaffine krazed say something more technical about how things are supposed to work.

I thought that negative pressure (vacuum) would be equalized with a vent from the atmosphere. And positive pressure (from vapor pressure and heat expansion) would be equalized by venting through the charcoal canister so that gasoline vapor could be removed, condensed and returned to the tank (for environmental reasons).

I'm pretty sure that the canister is responsible for removing ALL of the gasoline vapor and the tank has to send ALL of the vapor through it as long as the gas cap is closed. If not, this would completely defeat the purpose of the hydrocarbons emissions testing at the tail pipe.

I looked through Haynes and found only a single reference to "vent tubes" in the filler neck that must be disconnected if you want to remove the tank. No diagrams and no mention of the gas cap. I couldn't find anything whatsoever about the charcoal canister.

In my book about fuel injection systems, the canister is included in some of the simpler, less informative diagrams. It isn't mentioned in the discussion of the L-Jetronic or Motronic systems.

Also, the problem in my car re-appeared yesterday, so I paid close attention to what happened when I opened the gas cap (with the engine running this time). There was no change in the sound of the fuel pump. Also the pressure was higher (much higher) in the tank. It wasn't vacuum. It took 4 or 5 seconds for the pressure to equalize and the gas that came out was warm and full of vapor. Maybe we have different problems.
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PostPosted: Wed Aug 29, 2001 5:41 am    Post subject: Reply with quote

Your tank is designed to maintain a small vacuum when the engine is running. This keeps any vapors from escaping to the atmosphere. You should hear a rush of air anytime you open the cap to refuel. Excessive noise from your pump is a symtom of either a restricted fuel line or a pump that is worn. Since your car is running, it is unlikely that your fuel line is plugged or bent. Replace the fuel pump.
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Blas Costagli  

PostPosted: Sat Sep 01, 2001 11:21 pm    Post subject: Reply with quote

Ok all, we have the verdict from the dealer.
First of all, It's pressure not vacuum - could have fooled me... There is a electrical switch, located on the engine side of the fuel distributor, close to the valve cover, with two wires connected to it. They point down. Its function is to reduce the fuel pressure when starting the car up, cold I think he said. ( I believe it was a add-on item by Porsche to solve a fuel vaporization problem from the excessive heat under the hood.) Never-the less, the electrical connections were marginal at best. (1 loose, 1 corroded) So he replaced the wire end clips. And there appears to be a improvement in the starting. He also checked the fuel accumulator. There is a small screw on the bottom, if you remove it and any fuel comes out, replace it. If its dry it's fine. As always, clean fuel filters is a must...and the cost for all this at the dealer...$0.00..."Motor Werks of Barrington" - Barrington, Illinois...still one of the best Dealers...thanks Gene.
Hope this helps everyone out...

That was it...It Starts beautiful now..

[ This Message was edited by: Blas Costagli on 2001-09-08 05:24 ]
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