Vapor Lock... or poor gas tank vent?

[John Burns]

Hi all,

For years (decades) I have seen many posts and interesting remedies regarding "vapor lock", gas smells when the car is parked in the garage, hard to fill the tank, that big whoosh that happens sometimes when you take the gas cap off... etc.  My classic car experience has been mainly with 68 and 70 Camaros and a 70 Monte.  These cars all have terrible factory fuel tank venting- if any at all!  I drive these cars a lot in all sorts of temperatures.  Certainly these issues are more prevalent in the warmer months, but who wants a car they can only drive when it is cold?

Here is what I discovered: a gas tank needs a vent- and the bigger the better!  A "vented cap" isn't going to do the job.  It will spew fuel if you have a car that "leaves hard" (ask me how I know 😉) and it is often difficult to fill the tank without it spitting up the fill pipe.  I haven't seen anyone really address this- they talk about venting but I've seen some questionable attempts.  "Tanks" has maybe the closest diagram but it is important to get the vent well above the fill neck.  This is hard to do on these cars- unless you run the vent pipe through the trunk.  (NOT into the trunk!)   Some makes (Pontiac and Mopar) made various attempts at this in the 60s but they used really small pipe.  The photo is the vent pipe in my 68 Camaro.  (It has an Aeromotive tank with a vent in the top of the tank.)  This is a 3/8" hard line that runs from the tank, through the trunk floor, and then back to open atmosphere under the car (and I wish the line was 1/2" actually).

The important thing to remember is that a big vent will NOT let fuel vapors out of the tank!   I have seen people say their hot rod makes their garage smell like gas.  It isn't vapor leaking through the lines...  What is happening is that the tank gets pressurized and pushes fumes out.  If a tank is properly vented- and this is important- it will not push fumes out!  The vapors will stay right where they are in the tank if the tank pressure is the same as atmospheric pressure.

And my theory on "vapor lock": when the tank becomes pressurized, it will push fuel up the line, through the needle and seats, and flood out the engine.  OR if the tank goes into vacuum, it won't allow fuel to flow up the line.  Yes, electric fuel pumps can overcome some of this vacuum / pressure, but it is really hard on the pump.  A nice mechanical pump and a well vented tank gets the job done.

Next time your tank "whooshes" when you open the cap, or you don't want to drive it because it might "vapor lock", or the wife is complaining she smells gas in the garage- take a hard look at how well your tank is vented!

 

Edited August 23 by John Burns
Grammar

[S.Maug]

love it John !my 70 sin city same problem since I bought her in 79. always has driven me nuts, and is a pain.

[Cooltech]

I also been having this problem, I have been looking at the roll over valve and trying to see if that will work. I will let everyone know when I get to that.

Cheers!

[jft69z]

Here's what I've done on my Monte and also the Camaro to provide a vent point, higher than the tank. Seems to work fine.

https://www.firstgenmc.com/forums/index.php?/topic/23913-72’-fuel-tank-vent/#comment-221856

https://www.firstgenmc.com/forums/index.php?/topic/28631-converting-oem-tank-to-fuel-injection-question/#comment-258419

 

[1970mcss]

Joe, where is that vent? behind the tank? Where does that line that you put the filter on go on a factory setup without a canister? I have the same problem the gas smell in my 70.

 

 

[jft69z]

47 minutes ago, 1970mcss said:

Joe, where is that vent? behind the tank? Where does that line that you put the filter on go on a factory setup without a canister? I have the same problem the gas smell in my 70.

It's in front of the gas tank Paul, up high, above the rear differential. It's part of that collection canister behind the back seat area.

My car is a 72, not sure if a 70 has that hardware.

[Dtret]

15 minutes ago, jft69z said:

It's in front of the gas tank Paul, up high, above the rear differential. It's part of that collection canister behind the back seat area.

My car is a 72, not sure if a 70 has that hardware.

70 does not. 

[1970mcss]

Thanks, is there another way to vent the 70??

[jft69z]

5 minutes ago, 1970mcss said:

Thanks, is there another way to vent the 70??

Depends on your tank. Do you have any vent ports on it, or just one up top?

[1970mcss]

I will have to take a look. I've never had the tank down

[John Burns]

5 minutes ago, 1970mcss said:

I will have to take a look. I've never had the tank down

A stock style tank with vents is relatively inexpensive.

 

https://www.opgi.com/air-fuel-delivery/fuel-tanks/1970-76-fuel-tank-kits/fuel-tank-kit-1970-chevellemonte-carlo-20-gal-w-2-vents-ch31518.html?size=&color=GV&gad_source=1&gclid=EAIaIQobChMIwOPx0vzlhwMV9jEIBR3P4BjYEAQYASABEgIVA_D_BwE

[John Burns]

On 8/7/2024 at 9:40 PM, jft69z said:

Here's what I've done on my Monte and also the Camaro to provide a vent point, higher than the tank. Seems to work fine.

https://www.firstgenmc.com/forums/index.php?/topic/23913-72’-fuel-tank-vent/#comment-221856

https://www.firstgenmc.com/forums/index.php?/topic/28631-converting-oem-tank-to-fuel-injection-question/#comment-258419

 

Hi Joe- I see in your link Bruce Stang posted this from his 71 Monte assembly manual.  I'd imagine all of those vents are connected inside that "Separator ASM"- pretty cool!  And the 71 tank had 3 vents- they figured it out and yes, you can't have too much venting!  I didn't know these existed and they are only available used, but I'm going to try to buy one.

[Scott S.]

I don't believe it was factory available in '70. 

[Dtret]

John. The separator sits behind the back seat. It sits on a completely different type of panel than a 70. The panel behaves the back seat is completely different too. I have one of the separators here somewhere if you’re interested. 

[S.Maug]

cool

[72 Monte Carlo]

I left my tank vent intact on my 72. I figured it was there with a purpose.

[Rob Peters]

I think it was part of the federally mandated emissions things back then.

rob

[stangeba]

I believe I also posted about the very small blue plastic restrictor, that I almost threw out when I replaced all my rubber hoses shown in that picture above in the top hose that has two arrows twisting in the hose.

 

[S.Maug]

Interesting. I've never had sin's fuel tank out. But have had a crazy amount of sometimes pressure or sometimes vacuum at fuel cap when removed for refueling. I'm sure this translates to acceleration issues and generally when hot lack of engine performance. Got to try and diagnose with everyone's help when I get back To her in Illinois. 

[Dtret]

34 minutes ago, S.Maug said:

Interesting. I've never had sin's fuel tank out. But have had a crazy amount of sometimes pressure or sometimes vacuum at fuel cap when removed for refueling. I'm sure this translates to acceleration issues and generally when hot lack of engine performance. Got to try and diagnose with everyone's help when I get back To her in Illinois. 

A 70 does not have this separator. Check your cap to make sure it’s a vented cap. 

[S.Maug]

I believe I have replaced it with a vented cap in 1981. Issue has continued. Frustrating that issue has been for me

[John Burns]

1 hour ago, S.Maug said:

I believe I have replaced it with a vented cap in 1981. Issue has continued. Frustrating that issue has been for me

A vented cap won’t vent enough to solve the issue!

[fountain4ever]

I replied to another post about this as we do allot of these at our shop in Phoenix. We install a Vapor Trapper on just about all the cars we work on due to this exact same issue that you are discussing here. We just finished a 1972 Monte Carlo. I have always felt the fuel tank venting was probably never properly sized on these and many other cars from this era. When you drive them, then stop for fuel you get this rush of pressure. Clearly, if the tank were venting properly, you would not have pressure nor vacuum in the fuel tank. Gas tanks need to breathe - both in and out. The weather (heat and cool) causes the tanks to breathe and the tiny hole in the gas cap is (in my opinion) way too small. However - if you open it up you now have a fuel smell issue in your garage. So, by installing this charcoal vapor canister to the vent on the fuel tank, you now have the tank venting properly and you eliminate the fuel smell. It is really a basic concept that all new cars have and that is why they do not smell like gas! Hope this helps! Oh, and the nice thing with these is that they are super easy to install and work on every single application. They are rechargeable so every so many years you can refresh them. Very nice parts in my opinion. 

 

The next issue is when you install EFI on your Monte Carlo - you now go from 4-6 psi of fuel pressure to 40-60 psi and a return line that constantly circulates with this higher-pressure fuel pump. This magnifies this whole issue of venting the tank and the gas smell gets almost unbearable. That is why we started doing this. It is really a fairly simple issue but not one that people really spend time on. Vent the tank, go through a charcoal canister and them back to atmosphere. Tank will vent, garage will not smell like a major fuel spill! 🙂 

Edited September 16, 2024 by fountain4ever

[fountain4ever]

I read a response above from John Barnes and I agree with everything he is saying about the tank vent and pressure, but I feel people need to consider some additional things that also cause the fuel tank to "breathe". On most of these cars the fuel tank is positioned right between the exhaust pipes. If you laser thermometer the fuel tank after diving, it could be in the 150-degree range from the testing that we have done. Heating and cooling of the fuel causes this exchange of air from inside the fuel tank. The weather and the temps inside and outside the tank all play a part in the tank venting as well. A properly vented tank will have far less "vapor" not being pressurized however, I always prefer to route the vent line up as high as possible first to allow that fuel surge with a full tank of fuel to not be able to get into the charcoal canister. Remember when you are driving with a full tank of gas - the acceleration, braking and turning causes the fuel to slosh around and can sometimes surge out the vent lines. Just be mindful when routing the vent lines. If you do this and install a charcoal canister like the one we use, you will eliminate that unwanted fuel smell and eliminate most of the issues discussed here! 🙂

[1970mcss]

Thomas, when you add the Vapor Trapper do you have to tap the line into the filler neck? I have a 70 so I don't have extra vent lines. Any advie would be much appreciated. Let me know, thanks