Excessive fuel tank pressure

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I had this pressure problem on a trip to Colorado a couple of years ago. When I opened the gas cap, tremendous pressure would be released and would keep releasing for a couple of minutes. I had to wait to fill up the tank. Simply changing the charcoal filter significantly reduced the pressure. That is all that was needed.

The interesting point was that I brought three 5 gallon cans in the Slee rear bumper can carrier and had no problems with pressure when I opened those can caps. What is the difference between those cans and my gas tank?
 
DARKNESS said:
Let's assume ethanol is the problem. New cars don't have this issue. Why? What is different about a modern evap system and how can we modify the 80's to mimic it?
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Great question. I will ask Google today and post my findings.
 
TexasJack said:
I had this pressure problem on a trip to Colorado a couple of years ago. When I opened the gas cap, tremendous pressure would be released and would keep releasing for a couple of minutes. I had to wait to fill up the tank. Simply changing the charcoal filter significantly reduced the pressure. That is all that was needed.

The interesting point was that I brought three 5 gallon cans in the Slee rear bumper can carrier and had no problems with pressure when I opened those can caps. What is the difference between those cans and my gas tank?
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What type of gas cans alot of older ones will vent from the cap into the atmosphere.
 
Excessive tank pressure in many makes and models is easy to find. Oddly most I found are 90's model years. I haven't yet read anyone suggesting the ethanol blend to be the issue. Some have blamed leftover winter blend that is more volatile than the summer. This forum may be the one who finds the smoking gun and develops a "fix". I've never been a member of a forum that was so full of technical wizards than this one. No pressure but this problem falls on us to solve. Here are links to a just a couple of examples where owners have experience excessive tank pressure.
Excessive Tank Pressure - CamaroZ28.Com Message Board
http://forums.corral.net/forums/general-mustang-tech/931082-excessive-pressure-gas-tank.html
 
My truck definitely runs better and I get less whoosh when I would run ethanol-free gas. And yes, I also get better gas mileage.

I used to be a huge PROponent of ethanol. Now, I wish it would go away.

All my cars run horrible on it, it's more expensive to run, the cars get worse mileage, and the ethanol eats stuff on old cars.
 
Since switching to a Rochester vapor canister, the VC120 from autozone, I have had no pressure in my tank.

The system check valve is in the inlet port of the oem canister. This check valve can become sticky and cause pressure to build in the tank. Some have reported a positive fix by cutting of be top of the factory canister, drilling out the check valve and JB welding the top of the canister back on.

What I noticed about the Rochester vapor canister is that it has the check valve in the port out to the intake manifold so if it did stick, that would not stop vapors from traveling from the tank to be stored in the charcoal canister until next start up.
 
So, I was in in Durango for the past two weeks; my house is at 6950'. I had issues with my 94 that had been sitting at the house since I last fueled it in early March. We drove it around town and once back in the garage I heard, "What's that smell?" Put ethanol free gas in it and it was better. I had never noticed the issue before and I had only driven the 94 in Durango during the colder months.

Drove my 97 from Dallas to Durango. Had horrible issues on the way up. Filled up in Childress, Tucumcari, and Bernilillo on the way up and it was worse at each stop. In fact, in Bernilillo I had to wait for the gassing to stop because it kept kicking off the pump. In Durango I purchased ethanol-free gas, and the issue was better the entire time I was there. We even drove up to Silverton (11000') and did some lite 4-wheeling. Of note, last week was the heat-wave and Durango was about 10 degF higher than normal.

Drove the 94 back to Dallas on Saturday. Filled up with ethanol-free in Durango and then that option was no longer available (at least easily accessible from the freeway). I had zero wooshing.

So, I think the ethanol is definitely a factor, but I think temperature and elevation played a roll too. I went from basically 7000' to 600' at my house in the DFW area. By the time I got to Amarillo the temp was 73 degrees. It had been in the mid 90's in Durango. 94 has been in my garage pretty much since I got back and zero gas smell. I tend to think the smell/venting I had on the 94 in Durango were related to the winter blend still being in the tank.

I have the VC120 in the box. I do think the charcoal in the 97 is spent.

To be continued...
 
TexasJack said:
I had this pressure problem on a trip to Colorado a couple of years ago. When I opened the gas cap, tremendous pressure would be released and would keep releasing for a couple of minutes. I had to wait to fill up the tank. Simply changing the charcoal filter significantly reduced the pressure. That is all that was needed.

The interesting point was that I brought three 5 gallon cans in the Slee rear bumper can carrier and had no problems with pressure when I opened those can caps. What is the difference between those cans and my gas tank?
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The fuel return line adds warmed gas to the tank?
 
I filled the 3 five gallon gas cans up before I left Texas and didn't use them until I was out all day running trails around Ouray, Silverton and Telluride. I had refilled my gas tank many times on the way up there. I never had any issues with the gas cans but continually had problems with the gas tank "whooshing" when I opened it. Again, the problem went away when I replaced the charcoal cannister. I have remained perplexed since this occured. I still don't have a problem with the tank and I replaced the cannister a couple of years ago. I used an OEM cannister.
 
So just as an update, I think between patching my pin hole leaks in the filler pipe and installing the VC120, I am getting a significant improvement in MPG. I haven't filled up yet, but I just got 152 miles out of the bottom half of a tank of gas and the light isn't on yet. If that is accurate, it would mean I was getting north of 300 miles out of a tank, which would be about 40 more miles than I have gotten before out of a tank. Not sure if that is related to the repairs, but it seems like it might make sense that getting the pressure in the tank regulated back to factory-ish specs makes a difference.
 
After running full tank of ethanol free I can report that while I did experience better mileage at 312 miles this tank, the excessive tank pressure was unchanged. I will say that I do not experience the 10 minutes of pressure venting that others have reported. I would conclude that issue stems from an obviously malfunctioning evap control system. Perhaps my tank pressure is normal. I don't know. I will continue to run ethanol free as I notice better cooling/more stable water temps(not sure why that would be), mileage, and a peppier performance.
 
I also had excessive tank pressure for the past few months. I am happy to report that after replacing the vapor canister with the VC120 last week there is no pressure upon removal of the gas cap.
 
Twinair said:
After running full tank of ethanol free I can report that while I did experience better mileage at 312 miles this tank, the excessive tank pressure was unchanged. I will say that I do not experience the 10 minutes of pressure venting that others have reported. I would conclude that issue stems from an obviously malfunctioning evap control system. Perhaps my tank pressure is normal. I don't know. I will continue to run ethanol free as I notice better cooling/more stable water temps(not sure why that would be), mileage, and a peppier performance.
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I'm with you on mileage and performance, but the water temp seems odd. The adiabatic flame temp (the temperature of the combustion of a correct mixture of fuel/air) is about the same for ethanol and gasoline: Adiabatic flame temperature - Wikipedia. I would think that ought to directly translate to the temperature of the cylinder and the amount of heat the water has to take away. In my short time with the ethanol free gas I felt the venting was reduced. I still have my VC120 in the box.
 
ThomasMore66 said:
I'm with you on mileage and performance, but the water temp seems odd. The adiabatic flame temp (the temperature of the combustion of a correct mixture of fuel/air) is about the same for ethanol and gasoline: Adiabatic flame temperature - Wikipedia. I would think that ought to directly translate to the temperature of the cylinder and the amount of heat the water has to take away. In my short time with the ethanol free gas I felt the venting was reduced. I still have my VC120 in the box.
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I'm with you there. I'm not ready to attribute stable water temps with the fuel change and I would think it to be a fallacy of false-cause to believe that without evidence. I do have my timing advanced to 6* and noticed detonation occurring for less than a second using ethanol-laced fuel during off-idle acceleration. Ethanol-free fuel=no detonation. I thought it interesting my observation of stable water temps after the switch. I filled up again yesterday with ethanol free and will continue to monitor and report my experience.
 
I'm sorry, but doesn't this just scream not enough coolant is going through the heater core or not enough air blowing though it?

I can't recall, but does the rear heater work normally?
 
I just read the first 6 pages of this thread and decided to skip ahead to 17 and see if there was a solution. Looks like there isn't!:rofl:

I have replaced my charcoal canister without permanent improvement. Twice I did this (both aftermarket). First time it worked but when the problem returned, the charcoal canister swap didn't fix it.

The only thing that seems to work consistently is keeping the tank more than 3/4 full. Highway driving is fine with any gasoline but as dozens of other posts have pointed out, changes in altitude and prolonged low range driving cause pressure build up.

Did anyone ever see improvement with this issue by means other than replacing the charcoal canister? I have a new radiator and fan clutch. I have a new fuel sock, new fuel tank hoses to the tank (filler and vent), new gaskets on the tank.

Anyone explore replacing the soft lines to address potential swelling?
 
I'll add this info for whatever it's worth.

I've had tank pressure for a while and at times it's so bad you can smell it as it evaps from the Charcoal canister.

About a month ago there was a thread where a member had a serious running problem with his truck and it turned out that the problem was a bad fuel pump relay. It wasn't passing the needed amperage even though it tested fine.

I subsequently changed mine out since my truck has over 300k and is SC'd. Also denso versions from rockauto was like 50.00, cheaper than what I would have paid for the dealer equivalent.

The following week I had a 12 hour trip down to Long Island. I filled up at the house and had the usual pressure release. I filled up upon arriving in Long Island and was surprised that there was no pressure in the tank. Then again when I returned home no pressure.

Any electrical motor that is starved amperage will run hot. Theoretically it's possible that the fuel pumps are running hot from a bad FPR and warming the fuel and creating the pressure.

I can't say for sure but it might be worth trying.
 
Noticed mine doing this today. I pulled the line from the tank to the charcoal cannister and there was a gurgling noise coming from it for about 30 seconds. Not sure if that hints at anything.

Can someone confirm for me: The fuel cap valve lets air in, the breather hose/valve lets air out? Is that right?
 
Well, I'm going to Moab tomorrow and I have a spare FPR in my trail spares. Looks like I'll be testing your theory soon, Rick. Will report back.
 
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