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Old 04-16-2011, 09:03 AM   #1
Join Date: Apr 2011
Location: Sebastian, FL
Posts: 5
Question My TJ sings

I am new on here and enjoy reading all the posts. It is very educational. Maybe someone can answer the problem I have.

I have a new to me 1999 TJ, 6cyl, auto Sahara. When it is running the fuel cap sings (buzz, whistle). When I shut it off and get out it is still making the noise. It lasts about ten seconds after shutdown the it slowly stops. It happens with a full or empty tank. It is always the same tone. The previous owner told me he replaced the fuel cap but just to be sure I replaced it again. If I open the cap while it is singing, I can hear a release of pressure but can't tell if it is a vacuum or pressure. Also when starting the engine, sometimes it will start immediately but sometimes it needs to be cranked for six to ten seconds before it fires. I don't know if the two are related.

Does anyone out there know how to stop the singing?? Thanks in advance for any ideas or solutions.


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Old 04-16-2011, 09:11 AM   #2
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Location: Escondido, Calif.
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Can't help you with a specific answer on the noise but the slow start after the Jeep sits a while has an easy solution. There is a check valve inside the fuel pressure regulator that sits on top of the fuel tank and is plugged into the top of the fuel pump. The check valve's job is to keep fuel pressure in the engine for fast starts, so the fuel doesn't drain back down into the fuel tank instead. When that valve goes bad and starts allowing the fuel to drain out of the engine's fuel rail that the fuel injectors are plugged into, a slow start is the result.

The fuel pressure regulator is only $80 or so and isn't difficult to replace but you do have to drop the fuel tank to get to it. The below pic shows what it looks like from when I swapped mine for the same reason earlier this year.

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Old 04-16-2011, 09:30 AM   #3

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Random thought-question for the better informed more knowledgeable jeepers--could the air sound be caused by the lack of regular pressure from the fuel pressure regulator?
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Old 04-16-2011, 03:27 PM   #4

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Old 04-16-2011, 03:50 PM   #5
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The pressure in the tank is caused by a piece of evaporative emissions gear called the leakdown detector. A small electric air pump runs a few seconds after engine start and pressurizes the tank and all the emissions hoses around the charcoal canister located inside the right rear corner of the tub. Then it measures pressures over a time period. A low pressure reading then sets an engine check light and posts one of the following codes:

P0456 - Very small leak detect
P0442 - Medium leak detect
P0455 - Large leak detect
P0457 - Gas cap loose or missing

The dealership uses a smoke generator to find leaks. You can also visually inspect for cracked hoses, there are quite a few, plus some inline check valves in the hoses considerably smaller than the one in Jerry's photos.
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Old 04-17-2011, 02:36 PM   #6
Join Date: Apr 2011
Location: Sebastian, FL
Posts: 5
Thanks for the information guys. I will order a new pressure regulator and when the tank gets empty again, I will drop the tank and change the part. I suspected that the loss of pressure was the starting problem but had no idea what to look for. I appreciate the help. Will report back with the results.


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