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Excessive fuel tank pressure

Old 07-23-2016, 11:01 AM
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Boeing 717
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Default Excessive fuel tank pressure

When I remove gas cap I get a blast of air coming out of tank. If I leave cap on after driving I can hear air escaping out of cap (a whining hissing noise). I have looked at parts diagram and do not see a ventilation valve, only shut off valves. Does anyone know where I should start looking for this problem? Or is it a problem at all?
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Old 07-23-2016, 11:44 AM
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You sure it's air coming out, and not going in?
The tank has a check valve to let fresh air in when the fuel level goes down, so there isn't a vacuum in the sealed tank.

The valve is buried under the passenger side trunk carpet in the mess of hoses back there, perhaps the valve is faulty or the hose is clogged with dirt?

Excess pressure (warm fuel vapors in the tank) should be dealt with via the evap system/charcoal canister under the DS front fender.
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Old 07-23-2016, 03:36 PM
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Yea it's definitely pressurized and releasing air when I open the gas cap.
Could it be the bleeder valve, vacuum valve (both after the charcoal canister) or the charcoal canister itself?

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Old 07-24-2016, 01:35 PM
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Pin 33 on the dme plug controls the tank vent solenoid. I can check mine later today but most of my other cars purge after the engine is warm on the overrun. There is a small plastic line from the throttle to the dash pot (purging will only occur with the engine running)

Also there is a larger line near the engine support off the intake that runs through the dme controlled solenoid, the vacuum controlled dash pot and then onto the carbon canister under the drivers side fender liner.

I would put a finger on the valve to see if you can feel it pulse. I would see if the larger vacuum line is plugged with oil residue. (My intake has some fig newton type lining...maybe guides, stem seals or long term faulty AOS seals)

If you have difficulty filling up, I would look at the check valve at the tank...92s are different...soft tops are different than coupes.

If you are taking short trips or the dme temp sensor isn't showing the engine being warm, it may not have enough time to suck out the over pressure from jostling the fuel while driving.
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Old 07-24-2016, 03:13 PM
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Great info thank you.
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Old 07-24-2016, 04:42 PM
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Originally Posted by thomasmryan View Post
Pin 33 on the dme plug controls the tank vent solenoid. I can check mine later today but most of my other cars purge after the engine is warm on the overrun. There is a small plastic line from the throttle to the dash pot (purging will only occur with the engine running)

Also there is a larger line near the engine support off the intake that runs through the dme controlled solenoid, the vacuum controlled dash pot and then onto the carbon canister under the drivers side fender liner.

I would put a finger on the valve to see if you can feel it pulse. I would see if the larger vacuum line is plugged with oil residue. (My intake has some fig newton type lining...maybe guides, stem seals or long term faulty AOS seals)

If you have difficulty filling up, I would look at the check valve at the tank...92s are different...soft tops are different than coupes.

If you are taking short trips or the dme temp sensor isn't showing the engine being warm, it may not have enough time to suck out the over pressure from jostling the fuel while driving.
Hmm, any pictures of this solenoid that vents? Always wondered if something like this was what triggered the tank thinking it is full thing.
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Old 07-24-2016, 06:17 PM
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le book (24/28 DME diagnostics) says to check for a pulse within 7 minutes of starting a warm engine.

the rear/cowl vanity cover needs to be removed. the electric valve is right at the tip of the clutch master/ 3 o'clock of the brake booster.
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Old 08-04-2016, 12:34 AM
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Hi - did the OP resolve this issue? I occasionally smell fumes in the cabin if the car sits over night, but no fume odors in the engine bay. And filling the gas tank causes the fill nozzle to repeated "pop" and I have to release and squeeze the handle multiple times.

Is this the check valve near the full tube?
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Old 08-04-2016, 05:31 PM
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I have not figured it out but based on everything I've read so far it's most likely #23.
Could be #35 or #21 also or in my case since I have a rough idle possibly the fuel pressure regulator isn't working properly.
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Old 08-09-2016, 12:14 PM
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Guys can anyone tell me how to test the fuel pressure regulator and the "bleeder valve" part number 23 in pic above (aka ventilation valve) before I pop $350 bucks on 2 new ones?
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Old 08-09-2016, 02:37 PM
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Lindsey sells a drilled and taped fuel rail cap for attaching an aftermarket fuel pressure gauge.

Did you ever feel the purge valve after a warm start? If no, test the resistence of the DME temp sensor. The wire spades are exposed so they can get green and fuzzy. And be sure the small black vacuum line is intact. It should be easy to test these two items that purge the tank.
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Old 08-09-2016, 08:12 PM
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Thank you Thomas. I believe the solenoid purge valve you are talking about is the #23 in the parts diagram that I posted above, correct? Same as the ventilation valve?
So I wait seven minutes and then what? Do I have to remove the hose and then se if it's moving air? Which side of the valve do I test?

Where is the dme temp sensor, is that the one right at the front of the engine above the belts?
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Old 12-19-2017, 10:15 PM
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Originally Posted by Boeing 717 View Post
Thank you Thomas. I believe the solenoid purge valve you are talking about is the #23 in the parts diagram that I posted above, correct? Same as the ventilation valve?
So I wait seven minutes and then what? Do I have to remove the hose and then se if it's moving air? Which side of the valve do I test?

Where is the dme temp sensor, is that the one right at the front of the engine above the belts?
Did you ever solve this? I see in my thread (https://rennlist.com/forums/968-foru...l#post14680337) you replaced vacuum valve?
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Old 12-20-2017, 07:24 AM
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Don't think so. I did replace that valve but then moved on to other things and haven't looked into it any further yet.
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