Hey guys,

My car smells of fuel around the cap area, hard to pinpoint it, ive been under the car too, had the plastic valve thing off and cleaned it which didnt help, theres no leaks.

After talking with a friend who also had an 87 GTA he said his filler neck had a rubber seal on it ? mine hasnt, just a round seal on the inside of the gas cap itself which was cracked, so i bought a new cap, but im not sure if its the right one or not.

Do i need a vented one or non vented cap ? you can also get one that has vacuum or doesnt have vacuum, im not even sure what that is !

The cap i got sent is vented, my old one appears to be non vented, so ive taken the seal off the new cap and put it on my old cap but hasnt made any difference, it doesnt feel like its sealing properly either, its a locking one too so i think i would prefer a non locking one as you have to wiggle the locking one around a bit to get it on properly

Im more interested if there should be a seal on the filler neck itself though, i didnt think there was such a thing ?

 

The Seal Is In The Cap.

 

If it smells to high heaven after you fill the tank, my vote is for failure of the vapor recovery lines. You fill up the tank, and fuel sloshes up into the vent lines and seeps out of the 26-year-old failed rubber hoses.

I only fill my 88 to about two-thirds.

A full tank is a guarantee that you'll be looking for a respirator until enough is burned off that it doesn't escape through wherever its getting out.

Fixing the vapor recovery lines is definitely on my to-do list.

 

Theres not much fuel in the car though, just over 1/4 of a tank. No harm in checking these lines out though but i dont notice any difference if its full or nearly empty to be honest.

I still need to know what kind of cap i should have on the car incase thats the problem

 

A stock replacement cap should be sufficient if you wish to replace the cap. Sometimes they go bad, but I can't see it putting out enough fumes to cause such a powerful odor.

 

Is a stock one for an 87 TPI vented or non vented though ?

i agree on that though, the smell is around that area but when the cars running you can smell it too so its coming out the exhaust aswell

 

It should probably be a vented cap.

Don't hold me to that. All the cars I've dealt with have had aftermarket caps.

 

hmmmm, pretty sure my current cap is non vented. I dont get why parts stores list two different types of cap for the same car, its really annoying

 

Could be a crack in the filler neck. Which if I'm not mistaken there is a recall on.


Posted from Thirdgen.org App for Android

 

Common problem. The solder where the fill neck meets the tank fatigues and starts to crack. Mine is currently so bad I can move the fill neck around a little. I noticed this the other day. Only fix is to remove the tank and replace with a new tank or have your fill neck resoldered to the tank like I'm doing today and then put yours back in.

 

Yes there was a recall for cracking around filler neck. The tank in my 91 Z28 was replaced under that recall way back when.

 

Missed this thread earlier so have just started one about where to get new tanks for the best quality.. Mine definitely was leaking but it only smelled of gas when full to about 3/4.. Below that level it doesn't smell and doesn't leak... So, if you smell it all the time, it might be something else...

 

I would suggest either a new cap or check and make sure the area where the cap screws on isn't bent anywhere. Mine was slightly banged up by the previous owner when he did the fuel pump so now it has a spot that doesn't seal all the way.

 

I got a non vented cap and its helped, however i think my car is running rich anyway so it smells when its running.

The other thing that troubles me is the fact i get no 'HISSSS' when i remove the cap, i used to before the problem started, so im guessing there is a leak in it somewhere as its not totally sealed up, therefor no hiss

 

It is my understanding that you should never hear any hiss when removing the gas cap , and here's why ;

Of the 4 lines coming from our fuel tank , 2 are the pressure feed and regulated return from the fuel rail . The other 2 are connected to the charcoal can and to the valve mounted by the driver's side rear axle . While the car is running and fuel is being removed from the tank by the engine burning it , there needs to be a way for air to replace the "missing" fuel in the tank so as to not build up a vacuum in the tank . (this is usually where a "vented" cap comes into play , letting IN air to replace the used fuel) The valve on the 4th line does this , lets air in so no vacuum is built up (if vacuum IS built up it can get to enough of a suction to compromise the fuel pump's ability to pump fuel) . Now , when the engine is not running , and the heat of the day causes the fuel in the tank to expand a tiny bit and give off vapors , those vapors are routed to the charcoal can as a way to relieve that pressure/vapor situation without venting it to the atmosphere . (the vapors get burned on the next start up) . Now , if the tank can take in air to replace the fuel being used by the engine , and it can vent air and fumes (into the charcoal can) to relieve pressure , then how could it ever build up ANY pressure or vacuum if all systems are functioning normally ?

I do believe my description of the fuel system's functioning is accurate , but I'm more than willing to entertain any other description which may further clarify the fuel delivery system's normal operation .

PS , my car has the factory supplied non vented cap , ALL emissions controls are in place and properly functioning , and I get no pressure or suction hiss when removing the gas cap , and no smells of fuel near the car at any time (it lives in a garage and would be smelled for certain if a leak were present)

 

Your described the system correctly. But the system is designed to hold pressure. When I used to do emission inspection when our cars where new I had to do a pressure test on the fuel tank and it had to hold pressure and the fuel cap had to hold pressure.
The things that commonly go bad are the valve on the tank that only let air in.
Gas cap. Non vented on our cars.
Rubber lines
The connection between the filler neck and tank, You can use JB weld to seal it.
The O ring for the top of the tank and fuel line , gas gauge, fuel pump.
The purge valve on the charcoal canister and then the charcoal canister it self, And vacuum lines.
I would start with a pressure test and make sure it is holding pressure.

 

Now this is exactly the info I was looking for , in a roundabout way i guess , and why I wanted the fuel system's proper operation to be further discussed . It would make great sense to maintain the tank at a few ? pounds above atmospheric pressure as that would aid the fuel pump in pushing fuel through the system if it was in effect seeing it's input supply of fuel at a slight pressurization .

So , do you happen to know the actual designed operating pressure of the system ? I will be doing a pressure test now somewhat soon as per your revelation of the system's need for a few pounds in the tank while running .

Also , Exactly HOW much of a hiss should one be listening for ? I have never had the hiss , don't smell fuel , and have no outward symptoms of any trouble other than the forementioned missing hiss .

Thank You again for expanding on this conversation , I have done searches here several times wondering about this , and never found a satisfactory definitive answer as to the pressure question .

 

PS ,

If you now do a search with "fuel tank pressure" , OUR thread comes up on top , with a whole bunch of not quite as helpful ones to follow .

 

So , to recap my question , Does anyone know the normal operating pressure for the fuel tank as per the discussion above ?

 

I am unsure what the pressure should be, just that the system should hold pressure.
I never listened for a his, If the system did not hold pressure I would use a soap and water solution in a spray bottle to find the leak,

 

New question, same problem...

'91 Z28 5.7 auto stock with 70,000 miles.

Stock gas cap, no smell, "check engine light" comes on. No difference in performance.

Bought vented cap, gas smell, no " check engine light". No difference in performance.

Check charcoal canister and ventilation line? How do I check?

By the way, I only get the smell with drivers side window cracked open. Pulling from the back of the car, I think. Maybe through the hatch seals.

 

First you need a non vented gas cap.
Next before you check anything else find out what code is setting off your check engine light, then we can figure things out from there.
For the charcoal canister not much can go wrong as long a there is no cracks or anything like that in it, Make sure the line from the tank to the charcoal canister has no blockages and flows free. Now the purge valve on the charcoal canister can go bad. Depending on the year of the car it is operated by vacuum only or a combo of engine vacuum and the ECM.
But like I said find out first what code your car is sending out.

 

Maybe this is a good time to say my car has no charcoal canister, or any of the emissions stuff for that matter, its all be removed.

Im wondering where a line from the fuel tank is going if the charcoal canister has been removed ? theres no loose hoses or anything in the engine bay

 

I'm not sure where on your car that would be, On mine it is on the right front frame rail in the engine compartment, Just a solid metal line. If your steering wheel is on the left side then that's where it should be.
Now the problem with the charcoal canister removed is that it is a part of the fuel system. The gas tank is supposed to have a small bit of pressure in it. On top of the charcoal canister set a purge valve that allows some extra pressure in the form of fuel vapors to be released into the engine. The pressure that is needed in the tank helps the in tank fuel pump. There is a vent valve on the tank. Some think and say it is a two way valve , But it is a one way valve to allow air into the tank so a vacuum is not created, If there is a vacuum in the tank then the fuel pump ends up working harder, And gas boil sooner in a vacuum, and holds of in a pressurized system.
But you can not block of the hose that is coming off the tank going to the charcoal canister, To much pressure is not good either for gasoline as the vapors can become dangerous. the secrete to keeping a good balance i the purge valve. you can run one without the charcoal canister.
But you still need to find out what codes your car is throwing.

 

I'm pretty sure that the tank hissing when you remove the cap on an empty tank is normal, and good.

When ECM commands the EVAP purge from the cannister, the EVAP purge solenoid OPENS the solenoid valve, and allow engine vac to pull the vapors from the cannister, AND the tank. Usually this command is at highway speed. Tanks vacuum develops.

If vacuum does NOT hold for 'X' seconds at the end of the vac draw, the ECM sets an EVAP code. A bad gas cap, or an un-tightened gas gas, won't allow vacuum to hold. DTC EVAP gets set.

Filling the tank FURTHER after the pump shuts off, allows liquid fuel to get into the vapor recovery line. This can be the smell of gas...

Vented caps are for venting OUT, not in. Pretty sure about that...

 

The only code i get is the the MAF burnoff circuit failure, which is because i need to leave my burnoff relay unplugged, if its plugged in the car starts and immediatley stalls ! so theres some dodgy wiring going on which needs looked at.

Inspight of this, the car runs fine, ive fitted quite a few new parts to it in the 8 n a half years ive had it and it idles nice, has plenty of power, just a slight drop in RPM when i lift of the accelerator, that only started earlier this year, ive been told it might be the lockup solenoid in the transmission which i will change when i do the fluid and filter.

My fuel pump seems fine, im assuming its the stock one unless some previous owner has removed the tank and fitted a new one instead of cutting an access panel behind the rear seats. When it primes i can hear what sounds like a vacuum being made along with the electronic whir ? sounds like something sucking in air alongside the whirring noise, then they both stop and thats when i start the car

 

Do you have K & N?

Too much oil will foul the MAF element. MAF fluid cleaner, CAREFULLY, with a tiny paintbrush...

 

I have yeah, but ive never sprayed it with the oil stuff, i dont use the car much so its not really getting clogged up.

 

Not on these cars.
ECM operates the CCP, assumes it is operating correctly and adjusts the fueling accordingly.
Period
There is no feedback ( or DTC ) for the CCP not operating

 

When the SES light comes on it ,is a indication you have a problem so you are supposed to pull the error codes
from the computer 1st to see what the problem is ;
BEFORE throwing parts and $$$ at it.

 

About four months too LATE.
With no CCP connected then the fumes from the tank are just going to leak out where ever they can when there is pressure in the tank;
with a NON vented cap obviously from around the cap as you first described back in described in April

 

Weird, well my car has never had to charcoal canister since ive had it, nearly 9 years, and it never used to be like that.

I havnt really noticed any smells when the car is just standing in the garage now so i think its alright to be honest, just the smell when its running which is to be expected

 

There should never be a vacuum in the tank, There is a one way vent valve on our tank that lets air in to prevent a vacuum. Gasoline in a vacuum boils sooner than in atmospheric pressure or under operating pressure.

 

Fun tellin' what's WRONG, ain't it? Especially when it's 99% RIGHT yUp! (and 100% right on other cars)

OP probably wants to know what's RIGHT. Or what to do, to MAKE it right.

 

At this point I don't believe the OP is having a problem with his fuel system, As stated it is running fine, maybe just a bit rich , For the other problems mentioned it's more than I can get into over the net and not actually looking at the car, And I'm not going to go to England to do that.

 

And the purge system ( if in place ) is open to the canister with the key off.
As previously noted;
if there is visible pressure when the cap is removed ,then most likely the purge system is blocked and not allowing the pressure to be vented.

The bypassed fuel is heated as it passes through the engine bay so obviously more prone to evaporation once it is under reduced pressure in the tank.

 

The purge valve on the charcoal canister is closed when the engine is off, It is normal for there to be pressure in the tank, The fuel system and vapor recovery system is supposed to hold pressure. I used to be a state emission inspector and one of the test I had to perform was a pressure test on the fuel tank, It was to hold pressure and not drop off at all for ten minutes this was with the engine off, if it didn't hold pressure the car failed, we even had to test the gas cap to see if it would hold pressure.

 

The only time fuel vapor ( pressure ) is released is when the engine is running, but there is always some pressure in the fuel tank, The whole system was designed that way.

 

Can i get to any hoses from under the car that might be perished ? ive never investigated around the fuel tank before, probably because its hidden by the exhaust and ive never had too !

Ive got a new NON VENTED cap on but its still the same, smells after running the car, doesnt smell if its been standing a day or so.

 

Yes you can get to some of the hoses from under the car, There is a one way vent under your car going to the fuel tank. Its on the driver side This vent only lets air into the tank it should never vent out. This could be bad or the rubber hose that it connects to. You can check this vent by removing it and see if you can blow air threw it, You can also see if it will hold a vacuum or not, It should not hold a vacuum, and you should not be able to blow threw it.
Are you having any other issues with your car ?

 

I wouldn't use anything to touch that dinky MAF wire, the spray cleaners work well enuff with out having to touch it.

 

Not to pick an argument with TAPerformance but the later models (mine is a 92) use a two way valve (fuel tank vent near the rear axle, not the fuel canister vent near the engine). I took mine apart and it is as I described early in this thread.. It is definitely a two way valve. I have pulled them from cars as old as 87 and they were all a two way valve.. If it popped open to relieve excessive pressure, it might have gotten some dirt in it and stuck slightly open... However, it sounds like your problem is a leaking gas tank if you are smelling gas after it has been filled up.. To check the recall on that, remove your gas cap door and then remove the inner shroud around the fuel filler neck.. Use a flashlight and shine it onto the connection of the fuel filler neck and the gas tank.. You will see stains on the tank there if it is leaking as the recall mentioned. Yes, if you get the documentation off the internet for the fuel filler leak and take it to a Chevy dealer, they will fix the fuel tank leak (they have too if you read the documentation at no cost to the owner).. I know for sure because I just had mine done and the Chevy district manager approved the fix.. The documentation gives the vin numbers and it says specifically independent of the condition, age or ownership of the vehicle... They even have to rent you a car while they fix it.. Your really have to tear into it to see if it is a leak, again, first hand experience... By the way, I have pictures that I can send that shows my leak if you want it but it won't tell you much unless you pull some covers off your tank.. With a new tank and a cleaned two way fuel tank vent, my car is now free of fumes for the first time in about 10 years... Good luck...

 

Well LarryD. If that is a two way valve as you say then the part of the emission test that I was doing back in the 80's to late 90's would of showed something else and of bean a waist of time. Part of the emission test was a pressure test on the fuel tank. where I would put a pump with a gauge and pressurize the tank up to 15psi It had to hold that pressure and have no drop in it what so ever or the car would fail. This was the first test I had to do for the emission test both in GA and in MO,. It had to hold pressure for 20 minutes, while this part of the test was going on we had to check for all other emission crap was on the car, egr, charcoal canister air pump and so on. The total emission test is supposed to take 1 hour no less,
Also according to the epa No fuel vapor should be allowed to escape from the fuel system. At the same time our cars were built is the same time they started putting vapor recovery system on fuel pumps at the gas stations. And those new pumps hated when a car had to much pressure built up and made it difficult to fuel up those cars.
If this vent valve was a two way as you believe It would allow fuel vapors into the air, And the fuel tank would not be able to hold pressure as it was designed to. Even the fuel pump needs pressure to work properly, If this valve ( a one way valve ) is bad the electric in tank fuel pump will go bad. If the seal on the sending unit is bad or any way like the filler neck being loose or anything that allows loss of pressure will cause the fuel pump to go bad. The valve was designed to allow air into the fuel tank so a vacuum would not be created. And so pressure ( fuel vapors ) could not escape.. If gasoline or any liquid is in a vacuum it will lower the temp at which it can start to boil or turn to vapor. So instead of boiling starts at 175 degrees now it will start at 100 degrees in a vacuum.
Now you do not have to believe anything I have said here. But everything can be verified if you want to do the research.

 

Sometimes its hard to get a straight answer here :/ But anyway this valve you guys are speaking of is the small white thing that clips on near the rear axle yeah ? ive removed this and cleaned it all up as it was totally black but it didnt make a difference.

So i guess i should check the actual hose instead, and then maybe look into taking the shroud off inside where the filler cap is ?

As far as taking my car to a dealer for the recall, i live in England and Chevrolet dealerships dont sell proper Chevy's here, they are re-badged euro boxes, so i think they would just tell me to **** off if i brought in a 26 year old Trans Am and said fix that !

 

At 15 psi the gas tank would look like a balloon. It has been a while since you did these tests, it may be that the memory is a bit foggy on the actual pressure that was used.

The pressure information listed is from the '92 FSM.

The vapor pressure control valve (CCP system) prevents the tank from pushing vapors into the canister until the tank pressure exceeds 5 KPa (0.72 psi).

The tank vent (honk valve) is there as a safety for excess pressure, and to allow air ingress as the fuel is consumed (2-way valve).

With tank pressure it will vent at 5.5 - 7.6 KPa (0.8 - 1.1 psi).

Under vacuum it will allow air ingress between 0.88" - 1.9" Hg.

There are two types of canisters in use. A solid bottom and an open (filtered) bottom. The solid bottom types have a vent on top marked AIR. This is to be open to the atmosphere.

Note that while in a purge cycle the ECM checks the BLM to be sure that it doesn't drop too low. If it does the ECM will back off the purge volume. The purge is controlled via PWM of the solenoid.

Note that the charcoal canister is open to the atmosphere. Either through the bottom filter on the older ones, or the air vent on the newer ones. The canister will never have any pressure in it.

If the vapor pressure control valve (CCP system) is installed backwards then the tank can not vent to the canister. The tank vent valve will then open to the atmosphere once the pressure builds past it's limit.

RBob.

 

Thank You for this clarification RBob , I really appreciate this info !

Now , if we are to believe GM's Field Service Manual , the tank should never attain more than One PSI , correct ? ... So where does this leave the guys who report the large "wooshing" sound of escaping air as they remove their gascap ? I have inspected , LITERALLY , every inch of my fuel system , have no leaks or gas smells with the proper non vented cap , and have never encountered either pressure nor vacuum while removing the cap . My car lives in a garage , so I'd smell any leakage for sure . My car has , in fact , been garaged it's whole life , has only 74K miles , and not one spot of rust anywhere which includes all of the fuel and brake likes (sunny day driver ONLY since new , and never seen rain) .

So which is it , SHOULD there be a woosh sound when removing the cap or not ???

 

Just spoke to a friend with the exact same car as mine, 87 GTA with a 5.0 TPI and he said his sucked in air when removing the cap, but since replacing his fuel filter last year it doesnt do it anymore.

Maybe its partly down to the cleanliness of the fuel system too

 

GM has changed the tank venting and CCP system over the years. If anything, it may be that the vapor pressure control valve is bad. This is the one that is close to the charcoal canister and is the first to open once the tank pressure increases a little.

If that valve doesn't seal then no pressure will build, and, no fumes will escape either. They will be held in the CCP canister.

With my '92, on a hot Summer day there is a _lot_ of hissing when removing the gas cap (from pressure). This is after driving for a bit and stopping to refuel. So much so that I loosen the cap and let the hissing stop before removing it.

RBob.

 

TA, thanks for your insight on the emissions testing process. I am not arguing with you about holding pressure with the cap and the emissions test. That is correct and I have had that test done many times on my car. However, I believe the vent valve is an emergency over-pressure valve set to open a relatively high pressures. It does require pretty good pressure for it to pop open (I didn't have a pressure gauge to measure the opening pressure) and probably higher than the emissions test pressure for the cap. Blowing on it hard, it will pop open and then close again very quickly. Again, this is based on personal experience on 5 valves with broken mounts from pull apart yards and tearing them apart to see how they work so I could fix one for my car. My car is a stock RS and the valve has never been changed so I know it came from the factory with that valve.

Just trying to provide the information from my experience that might help troubleshoot his fume problem (my problem though was the fuel filler neck). That valve could be stuck open with dirt as all the ones I checked had dirt and some leaked... I might see if I can get a pic of the inside of a valve from my shop (located 150 miles away) this weekend when I am down there.. I threw away the research parts though I believe..

So, as with all the help here, the user can decide whether or not the information provided is helpful and gives them a direction to look, especially since there is so little information on theory of operation available. Oh, and thanks RBob, your pressures sound in line with what I experienced.

 

I can't emphasize enough to check your fuel filler neck for leaks.. I would have sworn I didn't have a leak until I took it apart and it was a bad leak... Fuel was running down from around the fuel filler neck and being captured on top of the the muffler heat shield... Can you say a boom waiting to happen... One tell-tail indicator was that the fumes were really bad after filling it up but would get less and less as I drove the car and fuel level dropped below the filler neck...

gta knight.. Get the recall info from the internet.. Make sure it is the full recall document.. It worked really good for me with GM.. You might have to go to the district manager level or higher to get it fixed but when I showed them the actual recall documents, they decided they had to do it...

init4fun, I now have a new fuel tank, new fuel pump, new fuel tank gasket, new gas cap, cleaned and working gas tank vent (one near rear axle and working as described above), rebuilt fuel injectors, and new fuel regulator.. Lastly no leaks.. I get a pressure whoosh when I open the tank..