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I have been having an issue with the fuel tank getting pressurized. I replaced the gas cap with a new one so that isn’t it. It seems to be a new issue I think after cleaning and messing with the sending unit. The only thing that I know I changed was when I took the sender out there was a fuel line fitting that I assume is the return (it wasn’t the pickup tube that goes to the pump) and there was no short tube on it. I put a short life of tube on it that looked to be the length of the ones in pictures I found.
Now even if sitting for a day or however long if I open the cap it hisses.
It isn’t sitting in the sun as it was in my garage.
Any ideas on where to start on this car or if this is a common issue. I’m told that all of the pumps and filters have been replaced on it before I got it. Any help would be appreciated as I’m new to this car and it is definitely a different bird than most that I have worked on with the multiple systems for the fuel with the kjettronic.
 

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Mine used to hiss. Since it was new.

I replaced the fuel cap on my 2004 VW because it didn't hiss when I removed it and I was getting a Check Engine Light. The new cap didn't fix either problem.

I just checked my '88 in the garage and it hissed when I removed the gas cap. I haven't even started it for months.
 

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Mine used to hiss. Since it was new.

I just checked my '88 in the garage and it hissed when I removed the gas cap. I haven't even started it for months.
In for discussion, I've quietly wondered about this for years. I hear it too and I've never noticed a pattern to it. On cars with a tank that vents into a valved charcoal canister, I can understand it on a hot day, but some of my Mk1s have a tank that vents directly into the intake. I run the lines myself during restoration so I know they're not clogged or kinked, and I still get the hiss sometimes. Doesn't make sense.
 

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It's part of the emissions system not to vent a tank to the atmosphere.

The first stage is to hold the pressure so it does not release. The second stage is if the pressure gets too much, it will vent into the carbon canister. That way, the vapors are captured. The last stage is vented to the atmosphere. Better to vent to the atmosphere vs rupturing the gas tank/lines/etc.

Since liquid fuel does not burn, the Reid Vapor Point [https://en.wikipedia.org/wiki/Reid_vapor_pressure] is changed in late spring and fall [depending on where you live]. This allows the engine to run better and to prevent vapor lock. If for some reason you have winter gas in your tank on a hot day, this could overload the system and cause more hissing than normal.

Either way, the system should always have some pressure since it cannot be vented to the atmosphere except in extreme instances. All manufactures have different ways of handling this but they are all very similar. If you do not have any pressure then it's possible to have small leaks that prevents any pressure build up.
 

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Ref: Reid Vapor Pressure, well there ya have it, I learn something every day here, thanks :thumbup:
 

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I learned this year ago when Mercedes had an issue with their fuel systems. The engineer recommended that we tell the clients to use mid grade or even regular. WTF, we're suppose to recommend super or their engines will blow up. Needless to say, almost every client was very happy with the running. Eventually, a software update was done. I was certain the engineer was blowing smoke [or vapors] but he was right. The clients were more than happy to pay less for gas too.

Another issue I learned is that Spring and Fall had the biggest issues with runnability complaints. Once I convinced them to change fuels or wait a month most customers said the problem went away.
 

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My tank builds way too much pressure. If I have more than a 1/2 tank, take it for a drive, and open the cap, it will literally shoot gas out! I haven't had time to go through my fuel system yet but it's so bad that I drive with the cap off and put it back on when I park it.

Sent from my SM-G970U using Tapatalk
 

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Could be the tank vent hose or tip over valve in the top of the wheel well. I had to temporarily cap mine off and it’s been building crazy pressure. Fixing it properly soon


Sent from my iPhone using Tapatalk
 

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My '77 Ford belches almost every time I fill it but not when I remove the cap. I filled it up today and it was cool out so I thought it might be OK.

It was running on fumes so I put the nozzle into the tank and stood back behind the car. After a few minutes the nozzle shut off. There was the tell-tale puddle of gasoline on the ground because it had benched and run out the drain tube. There was also fuel on the side because it was really windy today. (Utah had a big wind storm today). I had to fill the rest manually and the nozzle shut off a few more times and belched a bit each time until I got a full tankful.

I don't know if it's a late '70s Ford thing or if mine is messed up. The 1977 Ford Factory Repair Manual is not as good as a Bentley manual but I'll look there again.

I don't recall having trouble filling up my '78 Scirocco so I don't think it's a late '70s emissions thing.
 

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I learned this year ago when Mercedes had an issue with their fuel systems. The engineer recommended that we tell the clients to use mid grade or even regular. WTF, we're suppose to recommend super or their engines will blow up. Needless to say, almost every client was very happy with the running. Eventually, a software update was done. I was certain the engineer was blowing smoke [or vapors] but he was right. The clients were more than happy to pay less for gas too.

Another issue I learned is that Spring and Fall had the biggest issues with runnability complaints. Once I convinced them to change fuels or wait a month most customers said the problem went away.
I was told by the dealer mechanics to use regular in my '88 16V. I was in Texas and the knock sensor would dial back the timing until it was in the limp home mode. It ran like it was on two cylinders. Grinding gears was enough to upset the knock sensor. The A/C compressor coming on was another trigger. I don't think I ever took their advice but I wish I had so I would know if they were right or my younger self was right.

I also had a problem when it got hot where the starter would not crank the engine. It was like the engine was seized. It would push start and I got good at putting it in Neutral, pushing it in the parking lot, jumping in an putting it in gear. It would start every time.

It never had either problem in cooler climates like the PNW.

-OE
 

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I had problems on a B3 Passat which pulled a vacuum on the tank and warped it breaking the float assembly,

After the 2nd time, I drilled a hole in the cap, no problem, no more.

Friend at dealership said why dont you bring it in and let us fix it right,

I ask if they would replace the next gas tank? He had no answer.
 

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I had problems on a B3 Passat which pulled a vacuum on the tank and warped it breaking the float assembly,

... I drilled a hole in the cap, no problem, no more.
Mt boy has the same car, had the same problem, and we applied the same fix. We never found the root cause. The tank is serviceable but even though it's plastic, it didn't completely return to its normal shape. We checked what we could: return line, vent line, there's even a valve in the engine bay that controls the venting process and that still works.

Did you ever discover what caused yours to do this?
 

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I had one of my mechanics replace a crushed metal fuel tank. The next week, it came back crushed again. WTF! A crushed fuel tank is not the problem, it's the result a another problem. Yes, we took care of the customer since obviously it was our fault for not diagnosing the problem properly. The tech moved on to Arizona. I'm sorry for anyone that lives in Arizona and brought their Mercedes to the dealership.

Since you cannot vent fuel to the atmosphere, the purge control system takes care of this venting. The vapors go into the charcoal canister and the canister is 'purged' when the engine is running. This slight vacuum will draw the fuel vapors from the charcoal canister but air must be allowed to enter otherwise, yup you guessed it, the tank will collapse. I suspect every manufacture does the exact same thing with fuel vapor emissions. It's amazing how little vacuum it takes to completely collapse a fuel tank.

Usually you will find that the car was in an accident and the fuel evap lines are incorrectly installed/routed. The vent valve [the valve that allows air to go into the tank] may be faulty or clogged with debris.

Drilling a hole in the fuel cap will prevent the fuel tank from collapsing but that kind of butchery is not fix.
 
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