When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Two years ago I had the original fuel tank vent line clogged as many have had. I bypassed the metal vent tube with a 1/4" rubber fuel line and drove along my happy way until this week. When refueling I got a large amount of fuel vapor out of the tank. My first thought was it's time for a charcoal cannister or the short metal lines in the wheel well were bad (they were not). I looped the tank vent line directly back into the throttle body line in the wheel well. I still had too much tank pressure. So I disassembled the system and checked each part, nothing obvious. I blew out both metal lines in the wheel well. Removed and cleaned the tanks vent drain T-fitting. Tested all hoses at the sender as well as the vent valve on the sender itself and the 2 year old rubber line I used to bypass the original. I also checked everything in the wheel well, the charcoal canister, the purge valve, the hoses, the tank control valve in the wheel well and the metal lines again. The new lilland tank sender is probably a year old but I tested it anyway. Nothing was bad and no fuel in the lines. I blew out all the lines with air and checked the entire route of the rubber hose and saw nothing pinched.
The problem is I do not know how much vapor comes from hot fuel. The 1/4" lines do not carry a lot of air flow so neither my air hose nor my lungs could get a sense of how much or how little it is capable of passing vs how much vapor flow is created. I can tell you everything had about the same feeling of air passing.
I broke out the vacuum tester and tried to pull vacuum on the sender's vent tube, it passed with flying colors.
I wonder if I am the victim of poor engineering and or the perfect storm of modern ethenol winter fuels, hot air temperatures and a lot of heated return fuel from the injection system. Winter fuels contain additional butane for better vaporising during the winter months. Is it possible the vent valve ball ( i do not know if it is actually a ball) in the sender is too light and vapor flow itself can lift the float and seal the tank or once a splash of fuel closes the valve and the air pressure holds it shut? If it was stuck my use of air pressure to test simply pushed it back open. Is it possible todays winter fuels can overwhelm a 40 year old 1/4" evap system in hot weather? With my old sender it did not.
So that is where I am at. I do not see anything obvious but the amount of air the system can pass is not great. As far as the tank control valve just know that even if the vacuum line to it fails the tank control valve itself is made to be opened by tank pressure at 15psi.
Your thoughts are welcomed. I am trying to get my car ready to sell and I want this solved before I do so.
Donny
Last edited by Vets-Vet; Feb 23, 2026 at 02:21 PM.
That is a possibility but I would like to, if possible, contribute a little to the environment.
I respect that. A vented gas cap is not the same as a bucket of gas evaporating into the atmosphere. Unless there is pressure nothing is released. The fumes are contained unless pressure builds and a small amount is released. Dan
It's not the fuel. You have a restriction somewhere. Is your rubber line fuel resistant or just regular vacuum hose? Make sure the vent line from the tank to the tankpressurecontrolvalve is not plugged. It is the only route for air to exit the fuel tank. With the engine off, The tankpressurecontrolvalve will hold only a small (I think 1/2 to 5/8 psi) amount of pressure before it allows it to enter the canister. Remove the gas cap and blow back through the line after disconnecting it from the tankpressurecontrolvalve. It should have no restriction. Another thought, the sound of air rushing out sounds just like air rushing in. I learned this the hard way. I turned a 20 gallon tank into a 15 gallon tank because the wrong gas cap caused a vacuum in the tank. Many parts houses list both vented and non vented caps for our cars. The correct fuel cap is VENTED with a built in check valve so air can go in but not out. It is the only way air can get into the tank as the fuel level drops.
Thanks for the help and responses.
Most of the things in your post I described as tested in my first post. I have the system disassembled and have cleaned and tested everything.
The rubber line was fuel hose which I am replacing because there seems to be a difference rear to front. In my original post I underlined and highlighted air out just to make sure that was it noted, I could definitely feel air out, I had this once before for I am fairly familiar. A 1/4" fitting is what comes off the tank and 1/4" line does not have a massive air flow especially at appx 17ft of hose. A fresh brand new 20 ft of 1/4" hose has plenty of resisteance.
At this point I have cut open the canister and am replacing the 40 Y/O bituminous charcoal and the hose (again) from tank to wheel well. Of the entire system, those are the two most restrictive but not by much.
When a vehicle is first parked, the fuel is hot and plenty of fumes are still being released, my closed up garage would probably smell of gas with an at-the-cap vent.
The correct fuel cap is VENTED with a built in check valve so air can go in but not out. It is the only way air can get into the tank as the fuel level drops.
I've never found a straight, difinitive answer if this is true or not.
Mainly just bunch of beating around the bush without actually answering the qestion or knowing the answer to that question.
I now have an OEM gas cap from GM parts direct, part number 22525300, it arrived in an ac Delco box, says it fits Corvette 1986-1996 and many other GM vehicles. You can see the check valve. You can suck air in through it but not blow out through it. Look at the diagram. There is no other path for air to enter the tank. The tank pressure control valve has a check valve that holds a slight amount of pressure when the engine is off but doesn't let air back into the tank. I installed a non vented cap thinking it was correct. After a long drive my car was sputtering and running rough. My gas gauge was low so I thought I should fill up. I couldn't get the fuel cap off. I had to use slip joint pliers to open it. Really big whoosh when I cracked the seal. The sides of the tank were sucked in because of the vacuum. It was a lesson learned the hard way. The bladder was still holding but the metal had holes rusted in it so I replaced the tank. My advice, use an OEM gas cap with the correct part number and you can't go wrong.
I now have an OEM gas cap from GM parts direct, part number 22525300, it arrived in an ac Delco box, says it fits Corvette 1986-1996 and many other GM vehicles. You can see the check valve. You can suck air in through it but not blow out through it. Look at the diagram. There is no other path for air to enter the tank. The tank pressure control valve has a check valve that holds a slight amount of pressure when the engine is off but doesn't let air back into the tank. I installed a non vented cap thinking it was correct. After a long drive my car was sputtering and running rough. My gas gauge was low so I thought I should fill up. I couldn't get the fuel cap off. I had to use slip joint pliers to open it. Really big whoosh when I cracked the seal. The sides of the tank were sucked in because of the vacuum. It was a lesson learned the hard way. The bladder was still holding but the metal had holes rusted in it so I replaced the tank. My advice, use an OEM gas cap with the correct part number and you can't go wrong.
I replaced my cap when I first bought and had it transported up here from Florida.
One of the first things i did was replace the fuel cap with a locking one but I did keep the OEM ne. I'll have to dig it out and take a look at it and compare it to this one you've shown here, just out of curiosity.
I'd actually ditch the aftermarket locking cap in favor of gong back to th OEM one if I could find a new gasket for it. The one on it now is as hard as a rock.
I went for a ride with only the sender and rubber hoses in the system. Everything up front was open. When I returned the tank had pressure.
The 2 year old LILAND GLOBALSUCO10 vent (rollover) valve had failed barring any trash type problem. I removed the very first rubber line off of the fuel senders metal vent fitting and nothing came out. Removing the gas cap however produced quite a bit of pressure, (yes it was pressure). I tapped on the area where the metal tube is welded to the flat part of the sender and some pressure began to release from the vent tube.
I had purchased the more expensive sender in hopes it was built better.
The problem is I do not know how much vapor comes from hot fuel. The 1/4" lines do not carry a lot of air flow so neither my air hose nor my lungs could get a sense of how much or how little it is capable of passing vs how much vapor flow is created. I can tell you everything had about the same feeling of air passing.
Prior to last week, I wouldn't have guessed very much. Until now that I discovered the CCP System duty cycle ramp rate and duration seems to have a very significant effect on throttle response.
From a thread a while ago, there's a point right after a hot startup where the throttle response is razor sharp... I mean sharp-sharp-sharp...(some might call it "hair-trigger" sharp). It lasts for about 30-40 seconds and then you lose that. After that time period, the response is still fantastic, but I can tell the difference. I.e., if someone got in my car and drove it around, they probably wouldn't notice the difference, but I do.
Its also not something i can detect in the [Tunerpro] data, since i captured the operation in that time period a few times trying to see what the difference was between that 40 seconds and everything after that.
... provides the context for what I"m saying in this link...
I know this is all qualitative information and doesn't really provide you with a flow number... but if I'm seeing the engine respond like this just by feeding gas fumes into the engine at a faster rate, the fuel must be producing quite a bit of vapor. I guess you figure that there's quite a bit of liquid surface area from which vapors can boil off.
But with all this in mind, I went through and set the CCP parameters such that the CCP duty cycle goes to 100% as soon as I get on the throttle at all times. Also not allowing the ECM to modify the duty cycle based on INT or BLM. The fact that GM was adjusting the CCP duty cycle based on INT and BLM seems to indicate the fuel vapors coming through the CCP are not trivial to the A/F mixture.
Thanks for the reply,
That was my reasoning that if the rollover valve was light, could vapor flow lift / push the valve shut? Even if that is not the case, if a full fuel tank sloshed around and closed the valve, high pressure could hold the valve closed, just like using compressed air to hold the valves in position when replacing valve stem seals. I rarely fill the tank full, I am beginning to wonder if the rollover valve didn't get washed enough with fresh fuel and now has varnish on it.
Think about the possibility of hot winter fuel with some blend error where the butane was higher than it was supposed to be. In the end I am going back and replacing the new line with 5/16th's, it will require clamps everywhere. I did cut out the metal restrictor where the rubber line from the rear meets the metal pipe in the wheel well. Will I need to modify the rollover valve (or can I), I guess I am going to find out.
Winter fuel vs old EVAP system.
