Vented Gas Tank Cap - How to ID ?

[Drbrown]

Question: How to identify a vented gas cap. Attached photos show my made-in-mexico stainless steel gas cap; has been on car forever. Never suspected a problem with it. Don't see any vent holes inside or outside the cap, and of course there's no separate vent pipe on the tank.

I ask because I think yesterday my '47 experienced vapor lock or a gas shortage of some type. After driving in city abt an hour, engine "stumbled" a little starting up from a traffic light after shifting into second gear, then ran okay for several blocks then quit completely. Coil felt hot (likely engine heat). Let sit over night; started up and ran fine in garage. Will get on road tomorrow to load test. (we are in winter season here; yesterday air was in 30's)

Gas line rising up and onto fire wall to mechanical fuel pump has thin shiny foil type insulation covered by snap-on corrugated-type plastic electrical wire covering .... maybe not thick enough or line passes near exhaust pipe too close ?) Gas line from mechanical pump to carb has no insulation.

Before starting this AM, tested new 12 volt Bosch Blue coil; read 3.6 ohms cold. Started engine; read 14.0 volts at coil (+) which fell to about 13.8 after a few minutes. At same time had /14.2 volts at alternator output. Turned heater motor and 16 amp electric radiator fan on high and reading at coil fell to abt 12.8 volts; heater motor only = reading of 13.5 volts at coil. So seems no electric system concerns unless the dizzy condenser malfunctions when hot .

STATS: Original to vehicle 59AB; 35,000 m on rebuild; new batt; 12 volt neg grd system; new 12 volt 63 amp one-wire alternator; new crab w/points and Bosch Blue coil from Charlie NY; newly rebuilt model 59 carb and mechanical fuel pump; Airtex back-up pump at tank; fuel = 3 psi at carb; new gas tank of recent vintage; Champ H-11 plugs; vacuum at idle = 19.5 Hg (very slight indicator flicker could be a sticky valve, otherwise steady).

[waterboychuck]

I can't say for sure if your cap is vented or not but back in 69/70 time frame I was driving a Ford Van and someone stole the gas cap. I took it to the dealer to have service done and they replaced the cap as well. Shortly after that I was sitting at a traffic light and had about 1/2 tank of fuel. When the light turned green I stepped on the gas and the van acted as if it ran out of gas. I called a tow truck. Hope this helps.

[petehoovie]

Seems to me that the WARNING label on the cap is telling you that the cap is not vented. A vented cap would not cause 'FUEL SPRAY'....

[40cpe]

My cap is exactly like that, down to the decal. Came from Carquest. I've run them on two early Fords for years.

[TJ]

I've also used those caps on my '40s and never had a problem. Sounds like your problem just developed. If you've been running that cap for awhile with no problems I would not suspect the cap as the culprit.

[Tony, NY]

Sponsored Links (Register now to hide all advertisements)

Maybe you need some dry gas. Remember that from years ago? Probably not needed with today's gas. The cap looks to be non-vented type.

[40cpe]

I would think you need to isolate the problem when/if it stops again. Check for spark at a plug and for a stream of gasoline from the squirt nozzles in the carburetor.

[paul2748]

Easy way to find out if it's the cap. If you can get the engine to run until it stalls, immediately remove the cap. If you get a swoosh sound, then it's the cap.

If you have run this cap for along time with no problems, then in all probabilty its not the cap.

[Drbrown]

Thank you for these responses. After 4 months of ignition problems, when engine quit this time I again became a 78-year-old guy panicking in the roadway. I was pretty sure Charlie NY's new points crab had finally fixed my old ignition issues. So I first briefly tried the electric pump but in hind-sight not long enough to matter anyway. Then, although gauge wasn't quite on empty, thought maybe ran out of gas. When I opened the gas cap I didn't notice any suction sound. I added 5 gal and left cap off. Still thinking gas supply problem I was able to hear gas squirting inside carb using linkage.

Based on comments, will give the cap a chance. Possible future fix: Drill a very small hole thru outer shell, above where gasket meets inner shell. Tank fill pipe is quite tall. Gas should not spray out unless fill pipe is full and under pressure (expanding from outside heat).

If a next time, I'll check spark. Works fine today but .....

"Dry Gas" ? Ya, remember that around 1960. Worked at a Mobile pumping gas. Boss bugged us about selling it, little cans "Upper Lube" and whatever else Mobile told him he had to "push" to meet quota's. It was like selling popcorn and candy.

[Newc]

Hi all; A '47 COE owner in RG#9 sucked his gas tank in this Summer. A little screw needed to be removed for the cap to vent, no instructions included. Newc

[TJ]

Originally Posted by Drbrown

Thank you for these responses. After 4 months of ignition problems, when engine quit this time I again became a 78-year-old guy panicking in the roadway. I was pretty sure Charlie NY's new points crab had finally fixed my old ignition issues. So I first briefly tried the electric pump but in hind-sight not long enough to matter anyway. Then, although gauge wasn't quite on empty, thought maybe ran out of gas. When I opened the gas cap I didn't notice any suction sound. I added 5 gal and left cap off. Still thinking gas supply problem I was able to hear gas squirting inside carb using linkage.

Based on comments, will give the cap a chance. Possible future fix: Drill a very small hole thru outer shell, above where gasket meets inner shell. Tank fill pipe is quite tall. Gas should not spray out unless fill pipe is full and under pressure (expanding from outside heat).

If a next time, I'll check spark. Works fine today but .....

"Dry Gas" ? Ya, remember that around 1960. Worked at a Mobile pumping gas. Boss bugged us about selling it, little cans "Upper Lube" and whatever else Mobile told him he had to "push" to meet quota's. It was like selling popcorn and candy.

Drill the small hole on the inner side of the cap. Drill it in the retainer ring towards the outer edge.

[Drbrown]

I'm about to find out if my problem is cause by a non-vented gas tank cap. There was a pair of OE NOS Ford gas tank caps on eBay and I expect to receive them next week. Photos of them show what appears to be a vent hole in the center of the interior side of the cap. Will let you-all know. May decide to sell the second cap here on the Forum Swap meet.

[blucar]

A vented cap generally will not have a gasket... No need for a gasket if the cap is vented.

[Talkwrench]

How to tell ? well if it anything like mine vented = fuel spews out , non vented = only a smaller amount of fuel spews out ......

[Drbrown]

PS to my original postings .... I bought what was reported to be an original NOS vented Ford gas cap; photos attached. I believe its a Ford - manufactured by "Stant Mfg Co Cornersville Ind." "Pat No. 1.806224".

The inside of the cap has (1) a small hole in the center of the dome - perhaps shaped that way so that any gas that escapes thru the hole when going over rough roads will "leak" back into the tank, and (2) a dense fiber-board-type gasket - not rubber. I could blow air thru the hole at a slow rate - somehow it comes out around the gasket. It fits snugly on the fill pipe. The fill pipe is quite tall so I don't think it will leak much if at all unless I fill the tank all the way to the top of the fill pipe.

Waiting to see if this solves my apparent fuel shortage symptom.