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Fuel Smells

Fuel Smells

Alan Howard

Andy Morris wrote:

OK..this one is driving me nuts! I have fuel smells big time in my cat and I can't decide on the culprit. You can blow through the pressure valve in both directions so I know it is bad, but it doesn't seem that this alone could cause ALL of these symptoms. I would sure appreciate some help. Here are the observations:

  1. Smell builds up real bad in the cabin after the car sits a while (no matter what I do to stop it)
  2. Smell worsens after engine startup for first few miles, then tapers off.
  3. Disconnecting pressure valve from EGR side makes cabin smell even worse.
  4. Disconnecting pressure valve from tank side does not eliminate the smell.
  5. Leaving the filler caps unlatched is the ONLY thing that tends to kill the fumes.
  6. There are NO DETECTABLE fuel hose leaks anywhere. (I do know how to check them.)
  7. The car starts fine..does not seem to be any fuel pressure bleed off.
  8. Very small amount of fuel leak under driver side fuel tank after car sitting for a while. This happens even with that tank empty but is more severe when the tank is filled. This tank is fairly new BTW. Seems that the fuel is being "pushed" by one of the connections. Can't find anything wrong with the tank itself.
  9. You can blow through the pressure valve in both directions.
  10. Disconnecting the vapor separators from the tanks did not help at all.

Is all this JUST from the bad pressure valve, or are their other parts needed here?


Editor's note: Please also see Jim Isbell's book and Kirby Palm's book as there can be many sources of fuel smells.


The elusive fuel smell! This drives most S3 owners nuts.

You should not have any leaks at all from the driver-side fuel tank. This I would feel is most of the problem, especially if the smell is worse after it has been sitting. The way the fumes are getting inside the cabin is through the air exit plenum chamber under the rear shelf plus the 2 flaps in the vertical wall behind the rear seat.

The plenum camber has 4 flaps on it which are one-way valves (the best way to describe them). The flaps have a foam-backed material on them, the same material as the headlining. This material acts as the seal for the flaps. If the material has come off, any fumes in the boot area get into the cabin. The usual way of knowing that the material has come off the flaps is you hear an odd rattling sound from behind the rear seat (cow bells).

If you are getting fuel leakage from a supposedly empty tank I would check the fuel return valve to that tank. I assume you are in the USA and therefore driver side would be left hand side. The fuel return valves are located one each side in the rear wheel arch, behind a plastic cover, inboard of the wheel, very easy to find and remove. With power OFF the LH side valve is in the open position and the RH valve is in the closed position. The LH valve suffers from corrosion the most because it is mostly in the open position, i.e. because cars are parked and off most of the time. Therefore if there has been any water in the fuel it will corrode the valve. So if the LH valve is not seating properly it will get leakage through it when using the RH tank. I should have explained earlier there is one return fuel line from the engine to a T piece where it then returned to whichever tank is being used via the return valve.

One other point to look at, the final exit point for the ventilation system is through 2 rearward facing vents in the bottom of the spare wheel well, if these vents have been squashed closed by reversing over a rock etc, air cannot properly exit the car, when driven with the windows down or roof open air actually gets sucked out of the cabin and if there are any fuel vapours in the boot they will be drawn into the cabin.

Here endeth my epistle.


Date: Wed, 03 Dec 1997 07:36:10 -0600 From: "A. Gardner" Subject: [xj] Fuel odor in the cabin Matt Wehrle wrote >You can disconnect the vapor condensers in the luggage compartment in >minutes and just hook a bypass hose on to do a temporary test to see if >it clears the problem. I was conscientious of just bypassing the vapor >collection system and letting the vapor escape into the air. The >condensing coils make me sleep good at night and are a very low tech >solution to an aggravating problem. I hope that this will help someone >in the future. It seems to work well for me. If anyone sees a problem >with this I would sure appreciate feedback. Matt: you need to be aware that there are three connections to each vapour canister viz., two in the trunk and one to the tank filler cap. The latter is a rubber tube, just visible beneath the hinge of the cap, which connects to the tank via a one mm hole inside the filler neck. This is the expansion tank vent and needs to be diverted to your new condenser or you will have fuel vapour condensing inside the canister with nowhere to go. I believe the other connection, to the main tank via the trunk, is a vent + drain for the condensed fuel. The third connection removes excess vapour to the charcoal canister under the front wheel arch. I had a similar problem with fuel smells in my car which turned out to be a blockage in the tube from the vapour canister to the charcoal canister creating a lot of back pressure in the system. When my canisters inevitably rust through I may try your "bourbon" technology. Thanks for the tip. Tony Gardner XJ6 SIII 86 -------------------
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