Gas tank/Cap Issues x2

[Amze]

Alright.. so in the last couple weeks this has been occuring. So if I stand behind my car at idle, you will her "Pshhhhh" literally sounds like air is just leaking out of a small hole of something of that such, right around the gas cap area.. almost exhaust/underneath area.

So.. I didn't think much of it.. was like, we'll its not affecting anything. Actually thought I possibly had a minor hole in my exhaust a couple weeks ago but that definitely isn't it. Ok so when I go to fill up my gas tank, especially if I haven't for a while and I tank the cap off "Psshhhhhhhhhhhshhhhhhh" and you can literally see the gas vapors coming out, it stinks to. Now first time this happened I waited 3-5 minutes till it stopps, filled it up.. worked fine, full tank. Went on driving, still hissed at idle.

Now next time I went to fill it up, took off the cap.. same thing "pshhh" so I just try to fill it up.. This is where I realize it is a real problem, try to put gas in and put about 3 dollars on the ground, 1 gallon of gas all shoots back out. So I am curious to whats causing this.

I was thinking its possibly.. my gas cap? has a crack in it? isn't working properly? anybody! help is very appreciated.

[zeven13]

Time to visit your local indy shop.

[Amze]

Haha, what exactly does that mean!?

[SeKKeY]

fuel filter clogged? right under right passenger door

[old guy]

In order for you to troubleshoot your problem you first have to know how the tank venting system works. When you go to fill the tank here is what happens:

first the pump nozzle swings open the cover in the fuel filler tube filling hole. The cover pushes a venting switch which allows the fumes(pressure) to flow down down the vent tube into the carbon canister. Use your finger to push the cover all the way open to feel the switch opening. If the venting switch is bad you are in trouble because the only way to replace it is to replace the tank.

So as you fill the tank the fumes flow through the venting tube to the charcoal canister mounted under the round plastic cover under the spare tire accessed from under the car. The carbon granules absorb the raw fuel fumes and clean air is vented out the canister through the LDP pump. The LDP(leak detection pump) turns into a vent when the car is turned off. That is the little filter that you see under the left rear wheel well liner. The fumes from the granules are pulled back into the engine by vacuum (regulated by the N80 valve) to be burned off when you start the engine. So the problem occurs when your tank doesn't vent properly as you fill the tank. The pressure build is what causes the gas to quit going in and the nozzle to shut off. The problem could be a clogged up charcoal canister or a non-functioning LDP.

The easiest way to determine if you have a blocked canister is to remove the large tank to canister hose on the passenger side of the canister after you remove the circular plate covering it. Then go fill your car. If the tank fills easily then most likely the canister is blocked. You can also have a defective LDP pump which will not vent. To determine if that is the problem you need to reconnect the large vent hose just removed, drive the car until fuel is low, and remove the small vent hose on the canister driver's side(this is the small hose along side the large hose on the passenger side). The large hose is an easy disconnect(push or pull ring, pull out hose). This small hose needs to have the clamp cut off and replaced with a normal hose clamp when done testing. If you can't fill the tank this time with the small hose off, then the canister is defective, if you can fill the tank with the small hose off then the LDP is bad. Change the canister first as this is the most likely problem.

If the problem is only occurring when you attempt to fill your tank immediately after a start up it could be a different problem. Right after start up (and at other intervals as controlled by the ECU) the ECU closes the evap purge valve (N80) and runs the leak detection pump (LDP) to slightly pressurizes the fuel tank. The pressure bleed down rate is monitored and if it drops too fast a CEL will result. For example a loose gas cap. If everything is OK the leak detection pump turns off and the N80 begins it's duty cycle to purge the tank fumes. So if you try to fill your tank in the first few minutes after start up you may find that the system is still pressurized from the leak detection diagnostic cycle and you will have difficulty getting the tank to fill.

If you made it this far I guess you are still awake!

[Amze]

Thanks for all that old guy, I read through it all.. now I'm more confused. And I guess I have some testing to do.

Anyone else have some opinions? Anyone with similar experiences?

[max09163]

I've had a CEL with a "small emissions leak" and my car takes 10 seconds to turn over right after I fill up. Not sure how to go about fixing this..

[mtbrider]

Try changing the gasket on the fuel filler cap, you can get the part at the dealer

[agy2k2]

this is great info. i am having the same problem with filling up at the pump. i literally can only put about 5 cents of gas in at a time before the pump stops. i am going to try these things tomorrow and hopefully i can figure it out.

[iamshayan]

i know this threads long dead but i wasn't able to fill up more than 3$ of gas at a time so i replaced the n80 valv now im having some other funky problems. it worked great for a bit i was able to fill up my gas full.... now when i get gas it fills up to like 30$ and then clicks full... i take the nozzle out and hear the "psssss" fromt the tank. i put it back in and fill it up again. filled it to $50.

[wbellinz]

Old Guy, You seem very knowledge able on the subject.

I read your post on how to tell if the charcoal canister or LDP is bad. I'm looking into this because my 2005.5 A4 2.0T has trouble filling up. The pump shuts off numerous times.

I took of the large hose into the charcoal canister and it fills up fine. I put the large hose back and then removed the small hose into the charcoal canister and it fills up fine as well

Based on your post it sounds like my LDP is bad. Would you mind explaining what the LDP pump does and why it would be causing this fill up issue?

Thanks,
Bill

In order for you to troubleshoot your problem you first have to know how the tank venting system works. When you go to fill the tank here is what happens:

first the pump nozzle swings open the cover in the fuel filler tube filling hole. The cover pushes a venting switch which allows the fumes(pressure) to flow down down the vent tube into the carbon canister. Use your finger to push the cover all the way open to feel the switch opening. If the venting switch is bad you are in trouble because the only way to replace it is to replace the tank.

So as you fill the tank the fumes flow through the venting tube to the charcoal canister mounted under the round plastic cover under the spare tire accessed from under the car. The carbon granules absorb the raw fuel fumes and clean air is vented out the canister through the LDP pump. The LDP(leak detection pump) turns into a vent when the car is turned off. That is the little filter that you see under the left rear wheel well liner. The fumes from the granules are pulled back into the engine by vacuum (regulated by the N80 valve) to be burned off when you start the engine. So the problem occurs when your tank doesn't vent properly as you fill the tank. The pressure build is what causes the gas to quit going in and the nozzle to shut off. The problem could be a clogged up charcoal canister or a non-functioning LDP.

The easiest way to determine if you have a blocked canister is to remove the large tank to canister hose on the passenger side of the canister after you remove the circular plate covering it. Then go fill your car. If the tank fills easily then most likely the canister is blocked. You can also have a defective LDP pump which will not vent. To determine if that is the problem you need to reconnect the large vent hose just removed, drive the car until fuel is low, and remove the small vent hose on the canister driver's side(this is the small hose along side the large hose on the passenger side). The large hose is an easy disconnect(push or pull ring, pull out hose). This small hose needs to have the clamp cut off and replaced with a normal hose clamp when done testing. If you can't fill the tank this time with the small hose off, then the canister is defective, if you can fill the tank with the small hose off then the LDP is bad. Change the canister first as this is the most likely problem.

If the problem is only occurring when you attempt to fill your tank immediately after a start up it could be a different problem. Right after start up (and at other intervals as controlled by the ECU) the ECU closes the evap purge valve (N80) and runs the leak detection pump (LDP) to slightly pressurizes the fuel tank. The pressure bleed down rate is monitored and if it drops too fast a CEL will result. For example a loose gas cap. If everything is OK the leak detection pump turns off and the N80 begins it's duty cycle to purge the tank fumes. So if you try to fill your tank in the first few minutes after start up you may find that the system is still pressurized from the leak detection diagnostic cycle and you will have difficulty getting the tank to fill.

If you made it this far I guess you are still awake!

[old guy]

The LDP also functions as a pressure relief flow valve. When the car is turned off the LDP is open to the atmosphere through the small paper filter underneath the drivers side rear wheel well cover. This prevents pressure from building up in your gas tank. As the pressure in the tank builds up the hydrocarbon fumes are pushed through the charcoal canister and get scrubbed before they eventually exhaust through the LDP vent. If the LDP vent doesn't open properly all the air that gets displaced when you fill your tank will have to exhaust through the filler neck instead of the LDP. Thus the hissing out of the filler neck when you attempt to fill the tank. Eventually this pressure is enough to trigger the shut off mechanism on the gas pump nozzle.

[wbellinz]

Thanks for the explanation!

The LDP also functions as a pressure relief flow valve. When the car is turned off the LDP is open to the atmosphere through the small paper filter underneath the drivers side rear wheel well cover. This prevents pressure from building up in your gas tank. As the pressure in the tank builds up the hydrocarbon fumes are pushed through the charcoal canister and get scrubbed before they eventually exhaust through the LDP vent. If the LDP vent doesn't open properly all the air that gets displaced when you fill your tank will have to exhaust through the filler neck instead of the LDP. Thus the hissing out of the filler neck when you attempt to fill the tank. Eventually this pressure is enough to trigger the shut off mechanism on the gas pump nozzle.

[old guy]

You are welcome;-)

[heywier427]

search!

remove or replace your charcoal canister.

[jayiszraw]

search!

remove or replace your charcoal canister.

lol this is one of the original threads for this.

[papadelogan]

I'm having this same issue and yes, I'm reviving an ancient thread, but I'm also wondering if the filter on the end of the LDP could be the culprit. It would have to be severely clogged, but look at the size of the charcoal canister. If that beast can clog up, why not that small filter? I'll be removing that filter before my next "test" fillup and report back on my results. It's unlikely, I know, but still possible, and probably could be replace with a $20 filter from a local auto parts store. Never hurts to try.

[ctoast]

I'm actually having a similar (though not the same) problem. Even after the low-fuel light comes on, I can't put in more than 37 l (9.7 gal) of fuel. The first 9-ish gallons go in easy, but the tank thinks it's full after that.

[old guy]

I'm actually having a similar (though not the same) problem. Even after the low-fuel light comes on, I can't put in more than 37 l (9.7 gal) of fuel. The first 9-ish gallons go in easy, but the tank thinks it's full after that.

Do you drive a quattro or FWD? The tanks are different between the two. The tank in the FWD holds more fuel since it doesn’t have to accommodate a drive shaft. The tank in the Quattro is basically a fat upside down “U” shape. Once the level drops below the center portion the gas is actually being stored in two separate containers. Gas is equalized between the two tanks by a venturi type pump called a suction jet pump (totally different than the SJP located on the back of the motor). There are no moving parts in the pump and it is powered by the gas flow from the fuel pump.

If the SJP doesn’t function properly you will not be able to access the fuel in one of the lower compartments. Consequently when the tank reads empty you will still have additional fuel in the tank and it will not require a full load to fill it back up.


Of course if you drive a FWD you can disregard this completely!

Edit: fixed my dumb ass mistake. Can't tell a FWD from an auto :-(

[Charles.waite]

Do you drive a standard or automatic? The tanks are different between the two. The tank in the automatic holds more fuel since it doesn’t have to accommodate a drive shaft. The tank in the standard shift car is basically a fat upside down “U” shape. Once the level drops below the center portion the gas is actually being stored in two separate containers. Gas is equalized between the two tanks by a venturi type pump called a suction jet pump (totally different than the SJP located on the back of the motor). There are no moving parts in the pump and it is powered by the gas flow from the fuel pump.

If the SJP doesn’t function properly you will not be able to access the fuel in one of the lower compartments. Consequently when the tank reads empty you will still have additional fuel in the tank and it will not require a full load to fill it back up.


Of course if you drive an automatic you can disregard this completely!

You mean FWD or Quattro right?

[ctoast]

Thanks Old guy! My car is a Quattro 5-speed, so I would have a U shape tank. If the SJP is the issue, would that be an expensive fix? Also, would that mean I've got a bunch of really, really old fuel at the bottom of one of the U legs?

[old guy]

You mean FWD or Quattro right?

Oops!! My bad. that's exactly what I meant!

I will correct the post for future reference.

[old guy]

Thanks Old guy! My car is a Quattro 5-speed, so I would have a U shape tank. If the SJP is the issue, would that be an expensive fix? Also, would that mean I've got a bunch of really, really old fuel at the bottom of one of the U legs?

The fuel should be fine because every time you fill the tank completely it will get mixed in with the fresh gas.

If that is indeed your issue it will require removing the back seat and accessing both gas tank panels. The one on the rider’s side houses the fuel pump and the one on the driver’s side has the transfer lines with the SJP. I have never messed with this feature so I can’t say first hand what it takes to replace it but it isn’t a sophisticated mechanism so it shouldn’t be too costly or too complicated.

I believe I recall reading a test procedure in the Bentley to determine if the transfer line is performing as it should. It required a flow meter and a few other specialized tools. If I was experiencing this problem I believe I would first check for any kinked fuel lines and then just replace the SJP.

[ctoast]

Thanks for the advice, I think going that deep into the gas tank is beyond my abilities. I'll have to take her into an indy shop

[ctoast]

Hey old guy, you were absolutely correct, the right-side SJP failed and had to be replaced. Just got my car back from the shop today and the problem appears to be fixed! Thanks!

[old guy]

Hey old guy, you were absolutely correct, the right-side SJP failed and had to be replaced. Just got my car back from the shop today and the problem appears to be fixed! Thanks!

Glad to hear that you got it fixed

[a4Tizz]

Sorry to bump an old thread but I need some advice here.

I have a 2007 a4 2.0T and I can't put gas in it without the pump shutting off instantly. I took it to the dealership and they said my carbon canister was bad and it would be around $700 parts and labor to replace it. I saw that I could get one online for about $180 so I told them I would do it myself to save money. So, before I ordered the part, I removed the carbon canister completely and just left the hoses open and tried to fill up and got the same problem.

It sounds like the problem is not my carbon canister, what do you guys think I should do at this point? LDP pump?

[old guy]

I removed the carbon canister completely and just left the hoses open and tried to fill up and got the same problem.

It sounds like the problem is not my carbon canister, what do you guys think I should do at this point? LDP pump?

Once you removed the canister the LDP became irrelevant. Take a look at the diagram. I suspect the vent line (28) from the filler neck to the canister is either crushed or clogged. If it is open on the end that connects to the canister your tank should be venting to atmosphere. Another possibility is a defective vent valve behind the metal flapper in the top of the fuel filler neck.

[a4Tizz]

Once you removed the canister the LDP became irrelevant. Take a look at the diagram. I suspect the vent line (28) from the filler neck to the canister is either crushed or clogged. If it is open on the end that connects to the canister your tank should be venting to atmosphere. Another possibility is a defective vent valve behind the metal flapper in the top of the fuel filler neck.

Amazing. Thanks for the diagram. I opened the gas cap and blew into the vent line (28) and air seemed to go through. I then closed the cap and blew in the vent line again and the air of course came back out. I performed a visual inspection and don't see any issue with the vent line being crushed. I suppose the only other thing it can be is a defective vent valve. Does this mean I'll need to replace my entire gas tank?

[a4Tizz]

Guys, can anyone point me in the right direction on how to change the vent valve on a 2007 audi a4? Thanks in advance

[Charles.waite]

Sawzall??? No?

[a4Tizz]

No Sawzall my man. Is it that involved?

[dennej1985]

Ok. My car can not be filled past 3/4. To my understanding that eliminates the suction jet pump from being the issue as that would only keep you from using fuel you have in that side of the tank. Since the tank is open at the top, filling would not be affected. I have disconnected each of the 3 hoses at the charcoal canister one at a time and am unable to fill the car no matter which hose is off. In fact I currently have them all off and can't fill the car so I don't see how pressure can possibly be building anywhere. I notice no hissing from the filler neck. No hiss when I take the cap off. I am able to easily blow air through the charcoal canister and each of the 3 hoses that connect to it (unhooked the n80 for the one) so in my mind that eliminates any kinked lines (or is there one involved I'm missing) what is my cars problem? And why does my car always seem to stump everyone with its specific quirky problems no one can solve? I have a friend that seems to think the sending unit(s) for the gauge might be bad? Would I have a sending unit in each side of the tank? How to test? I mean the dash gauge does read and seems consistent.

[old guy]

If I am reading your post correctly you don't have a problem with getting the gas to go into the tank but you do have a problem with being able to fill it up to where the gauge reads full. That sounds like a problem with your sending unit. Did you by any chance replace your fuel pump recently? Is so, read this: Clicky click. If not, then you could try cleaning the sender unit. Clicky click Be sure to read the next to last post in the thread before removing the sending unit. You don't have to pry off the white clips. Just push in on the large button.

[dennej1985]

Not quite right, sorry for the misunderstanding. When I can't fill anymore (pump stops right as gas begins flowing) my needle on the dash will be just below 3/4. I have never gotten it to fill beyond that point. The range estimator on the dash reads 330. I have not done a fuel pump on the car. Thank you for the sending unit thread. I will try that later today.


Sent while on my ridiculously long lunch using Tapatalk

[imnuts]

How much gas are you putting in when it stops?

[dennej1985]

It always takes 9-10 gallons. I have tried filling at the lowest speed possible and it still shuts the pump off

[old guy]

It always takes 9-10 gallons. I have tried filling at the lowest speed possible and it still shuts the pump off

I assume you are running it to where the gas gauge is somewhere near empty and then you are filling it. The tank holds a little over 16 gallons so my guess is that you are never going below a ¼ tank even though the gauge is reading empty. Either the sender unit is malfunctioning or the cluster gauge itself is inaccurate. I suspect it's the sending unit.

Here's my logic: You obviously aren't draining the tank completely if you can only add 9-10 gallons before the pump shuts off. I suspect the tank is full but the sender unit is sending incorrect information (hey, the tank isn't full!) to both the cluster gauge and the ECM since the gauge is reading ¾ full and the ECM is only recognizing 3/4 tank. Thus the 330 miles to empty. If the problem was only with the accuracy of the cluster gauge the gauge would read ¾ but the information from the sender to the ECM would still generate closer to a 400 miles to empty reading.

Use the linked DIY to check out the voltage of the sender unit and see what you get. I suspect you will find the problem there.

[dennej1985]

Got into the tank and it was still half full even though it read empty. my issue appears to have been that the float on my sending unit was hanging up on the center section of the tank only allowing it to travel part way up and down. I bent the rod just a bit inward and after a fill up my car reads full for the first time ever. Now I will just have to see how accurate it is with my bend.

[clemsongt]

The easiest way to determine if you have a blocked canister is to remove the large tank to canister hose on the passenger side of the canister after you remove the circular plate covering it. Then go fill your car. If the tank fills easily then most likely the canister is blocked.

So pull off the hose shown in the following pic which was taken from this DIY (props to plokm):



Attempt to fill up the tank. If you
Can fill up; then the canister is clogged - replace or clean (complicated) and enjoy.
Can't fill up; LDP is probably defective and canister might be too - proceed to step two.

You can also have a defective LDP pump which will not vent. To determine if that is the problem you need to reconnect the large vent hose just removed, drive the car until fuel is low, and remove the small vent hose on the canister driver's side. If you can't fill the tank this time with the small hose off, then the canister is defective, if you can fill the tank with the small hose off then the LDP is bad. Change the canister first as this is the most likely problem.

So to recap: reconnect the large hose shown in the first pic and now disconnect the hose shown in the pic below:



Attempt to fill up. If the first step resulted in unsuccessful filling up, and you
Can fill up - LDP is bad and canister is good.
Can't fill up - Canister is bad

Am I missing any other possible diagnoses? Do I have the hoses correctly labeled for steps 1 and 2?