vented gas caps or not?

JCMAC · March 17, 2014, 06:48 PM

My 1972 413 Dodge has two gas tanks and no charcoal canister. A 27 Gal (aux) and a 40 gal (main) tank. I bought pressurized gas caps but last week the dyno man said get vented.
Do I really need to get vented or are my new non-vented caps OK to use?

John

Stripe · March 17, 2014, 06:59 PM

Vented. Your mech pump on engine will start straining to pull fuel when a vacuum is created after a while of driving.

moonlitcoyote · March 17, 2014, 07:15 PM

Quote from: Stripe on March 17, 2014, 06:59 PM
Vented. Your mech pump on engine will start straining to pull fuel when a vacuum is created after a while of driving.

Does this apply to all rigs or just some of them? I didnt know there was a difference.

DaveVA78Chieftain · March 17, 2014, 07:40 PM

Venting control has to do with emissions controls. California laws required that gasoline vapors be recirculated around 1970. That law went federal in 73. Since all chassis manufactures basically used non-vented gas caps. The air to the tank to prevent tank vacuum is controlled by the recirculation system. For a system like JCMAC's that does not have the recirculation system they use a vented cap. So yes MJ, you should have a non-vented cap.

Dave

JCMAC · March 17, 2014, 08:28 PM

OK, thanks for all of the input!
Both of the caps have a white button in the inside at the center. No air blows thru when you try to blow on the white button (sealing around it with your lips) , but if you use a tooth to depress the button while blowing, then one of the caps passes air while one does not. So are these vented caps, but one is not working properly?
I will check the part numbers from the store.

John

Stripe · March 17, 2014, 10:54 PM

Sounds like a vented cap. What's the part # or do you have a pic as well?

Froggy1936 · March 18, 2014, 12:16 AM

Note: If there is no way for air to get into the tank as fuel is sucked out The tank will collapse. Either a vented cap (home made if necc) or a ventilating syestem with charcoal canister One or the other must be used Most R/Vs are exempt from emmission testing except that forbidden chunk of land on the pacific coast Frank

Stripe · March 18, 2014, 04:13 AM

Quote from: Froggy1936 on March 18, 2014, 12:16 AM
except that forbidden chunk of land on the pacific coast Frank

:)rotflmao

Thundercloud · March 18, 2014, 06:34 AM

Interesting conversation. I would put vented caps on it after you smog it anyway if your not exempt. They want my 79 down at the station to smell my tailpipe. But I have a carbon canister and sealed gas caps. Also maybe consider a locking vented gas cap just to make double sure the tweakers don't syphon your 80 gallons out in the middle of the night. That would suck and be about 300 bucks down the tube.

Oz · March 18, 2014, 10:23 AM

Very good point, Thundercloud!

legomybago · March 18, 2014, 01:16 PM

Tweekers Suck...

JCMAC · March 18, 2014, 02:36 PM

OK, the caps I have are listed as "emmisions control" (non-vented). The white button must be an overpressure escape/safety device. I bought "vented, anti-surge" caps and they naturally look different.

Thanks for all the help,
John

moonlitcoyote · March 18, 2014, 09:54 PM

Quote from: DaveVA78Chieftain on March 17, 2014, 07:40 PM
So yes MJ, you should have a non-vented cap.

Dave

Thank you Dave, that was the only part I REALLY understood but it was the only part I really needed to know anyway

BrandonMc · August 22, 2018, 03:33 PM

Quote from: Froggy1936 on March 18, 2014, 12:16 AM
Note: If there is no way for air to get into the tank as fuel is sucked out The tank will collapse. Either a vented cap (home made if necc) or a ventilating syestem with charcoal canister One or the other must be used Most R/Vs are exempt from emmission testing except that forbidden chunk of land on the pacific coast Frank

Frank,

I have a vented gas cap, and have a long run to a brand new charcoal canister. Why do you think there is still much too much vacuum in my gas tank? Is the brand new vented gas cap not working correctly? I may have to ride without the gas cap on in the future - even though I have a vented locking cap.

I may also remove the auxiliary vapor canister hose so the only line into the brand new main charcoal canister is not the AUX but atmosphere. perhaps that is the restriction.

Rickf1985 · August 23, 2018, 09:45 AM

You have to make sure all of your vacuum lines are hooked up correctly and all are in good shape. If that vent line to the tank was open for any time it needs to be checked for obstructions, critters love to nest in open lines.

BrandonMc · August 23, 2018, 01:04 PM

Quote from: Rickf1985 on August 23, 2018, 09:45 AM
You have to make sure all of your vacuum lines are hooked up correctly and all are in good shape. If that vent line to the tank was open for any time it needs to be checked for obstructions, critters love to nest in open lines.

yeah, they aught to be alright, but I'll check em. I swear this vented gas cap isn't venting either

Rickf1985 · August 23, 2018, 04:46 PM

It is only supposed to vent in, not out. Are you sure the sound you here when you remove the cap is vacuum and not pressure? You have the same unit as me, you should have a large vent line going to the back of the RV from the tank. Why do you think you have a vent issue?

BrandonMc · August 23, 2018, 07:10 PM

Quote from: Rickf1985 on August 23, 2018, 04:46 PM
It is only supposed to vent in, not out. Are you sure the sound you here when you remove the cap is vacuum and not pressure? You have the same unit as me, you should have a large vent line going to the back of the RV from the tank. Why do you think you have a vent issue?

ok, let me explain.

I purchased a brand new vented gas cap, locking, and if it were venting inwards, like I expect, there wouldn't be a vacuum whooshing into the tank, and a pop when the tank equalizes to atmosphere when the cap is removed. Before I replaced the huge vapor canister, this pop was a HUGE pop.

Recently, I just replaced every soft fuel line, and the high pressure hose, and have A LOT of vacuum on the tank when I run the engine for ten miles. I'm really quite positive it's vacuum. A lot more vacuum than I used to have, after replacing the soft fuel lines.

I imagine this vented gas cap isn't venting in like its supposed to. Or the only other thing I can think of is the Aux Fuel Vapor Canister being plugged. I can hear the air system up front while running the engine.

Currently, my next steps are to:

First, test the fuel cap to see if it isn't dead on arrival for vented.

Second, disconnect the aux fuel vapor canister to alleviate an issue if there's a restriction there in that aux can. It could be a culprit because its old, And i can actually hear it hissing a bit. But wouldn't that vented cap let in enough air it wouldn't matter? Thanks, I appreciate all the sage advice.

Rickf1985 · August 23, 2018, 10:03 PM

You need to reopen this topic in the general motors forum. I keep getting screwed up because it is in a Dodge forum. I am super tired right now and the brain is not working but the canister should not have anyhting to do with the vacuum in the tank. I will look at my vacuum diagrams tomorrow.

RCND · August 24, 2018, 08:19 PM

Howdy All:

For those interested here is a typical Charcoal Canister System.

Take care.
Robert