clem zahrobsky

that is what we found out back then in the 60s when this problem of filling the crankcase with gasoline and causing these BBC corvette engines to spin bearings. we never saw this problem with any other GM cars with these 427 BBC engines. i think this i why there are very few 67,68 BBC corvettes with the orignal engine. we just drilled holes in the cap to let out the pressure.i told people to check their oil every time they were going to start these 427 BBC corvette engines to make sure the level in the pan had not risen because of gasoline.

SWCDuke

Is that a cap with or without relief valves. Both were used in '63 and both are considered "vented".

Duke

nassau66427

The above picture showed the cap "venting" under slight pressure. Applying greater pressure [blowing harder] to the hole on the under side of the cap would engage the pressure valve and cut off the air flow. There is no "pressure relief " function incorporated into the cap. Why? Because it is a bloody vented cap! The proper working of the cap prevents pressure from building up in the first place. The pressure side of the cap would shut off the fuel flow in the event of an upset. Why would the fuel be free to flow out otherwise? Because it is a bloody vented cap! POS or NOS if your vented [it even says so right on it] cap isn't venting properly, you might consider getting another on. SWCDuke, just noticed your question. Cap is from my '66 it has valves.

clem zahrobsky

the pressure builds up after you shut off the engine because of the heat from the mufflers expanding the air in the fuel tank. all i know is drilling the hole in the caps stopped the carb flooding

nassau66427

Part #3843576 late '63-'67 vented gas cap
The large spring provides pressure to seal the gas cap gasket to the fuel tank filler neck. The valving is in the cup at the base of the spring.

nassau66427

Air enters and exits through the notches in the bottom of the little inverted cup at the bottom of the picture. This is the piece that rattles when you shake the cap. If you turn the cap [or car] over, it contacts the rubber gasket in the piece above it and cuts off flow. There is a hole in the gasketed piece for air flow. The spring merely holds the gasketed piece in place. The top piece is pressed into the cylinder on the right and keeps all the parts in place. It too has a hole for airflow. On the right we see the bottom of the cap with the cylinder that holds the valving.

SWCDuke

Very interesting information. For those of you who have Nolan's Vol II. look at the early and late '63 cap drawings on page 93. The early cap (3820865, which is on my car) drawing does not show the big spring that applies force to the gasket, but I just verified that my cap has this spring because I can depress the bottom of the cap toward the top against an obvious strong spring force.

The late cap, 3843573, drawing shows both pressure and vacuum relief valves, yet the photos show what appears to me to be just an anti-siphon valve that will prevent fuel from flowing out the vent if the car rolls over, but neither pressure nor vacuum relief valves.

It appears that the only difference between the two caps is the antisiphon valve, and actual parts don't match the drawings. Go figure!

Duke

P.S. On second thought there does seem to be a pressure relief valve. If the anti-siphon valve seats, increasing pressure could lift the rubber gasketed part against spring force and create a flow path around the periphery of the rubber gasketed part, but in normal service the fuel vapor pressure wont rise fast enough to close the anti-siphon valve with the cap on the tank, so it must have another purpose.

In the event of a rollover and the car remained upside down for some time, hot weather could cause a pressure buildup in the tank and without some protection the tank could burst and send fuel flying everywhere. Could be that the purpose of the pressure relief valve is to prevent eventual tank bursting in the event of a rollover by providing a pressure release when the anti-siphon valve is closed.

It appears analogous to the safety valve on a steam boiler. If the anti-siphon valve closes for any reason - rollover, malfunction, etc. - which will precude normal venting, the pressure relief valve will open below burst pressure.

Any other ideas?

nassau66427

I am sure that a lot of confusion exists about these caps because Mr. Adams erroneously stated "The second type cap was not open to the atmosphere,..." [vol. 2, pg 94]. Your thoughts on pressure relief in the event of malfunction or upset work both physically and logically. I stand corrected in stating categorically that there was no pressure relief incorporated into the cap. I should have said that in normal operation the cap did not need to rely on a pressure relief function in order to provide venting. My apologies to all concerned.

Matt Gruber

i ONLY use my air compressor to check a press/vac cap for pressure release. never even considered the lip method where is monica lewinsky when u need her?

JohnZ

Just checked mine again, and it provides no pressure relief with the cap in car position or inverted, whether the inner portion of the cap is compressed against the spring load or not. Go figure

SWCDuke

I see two ways that pressure could be relieved if the car was upside down, which would close the "safety valve" (I think this is a better name than anti-siphon valve) if the car inverted or if the safety valve closed due to a malfunction. First is the Internal valve with the rubber seal shown in the photo. The second is lifting of the center section to create a pressure escape path past the fill neck seal.

Assuming it takes about ten pounds of force to press the center section up toward the top cover, it would take about 2 psi since the area is about 5.5 sq. inches. Tough to gage the force, but's it's considerable. If it's 20 pounds it would take about 4 psi break the fill neck seal.

It won't take much pressure to lift the safety valve in the normal cap position since the only force holding it down is its own weight, but it essentially provides a leak path for fuel vapors. If pressure suddenly increased, which it probably won't in service, the safety valve could close and then at some higher pressure either the internal pressure relief valve will open or the center of the cap will rise and break the fill neck seal.

How much pressure did you use on the vent hole to try an unseat the internal valve?

Duke

Loren Smith

Slightly off topic (C1), but here's my question: Before I started the bodyoff resto on my 59 and actually could drive my car, the gas fumes would give my wife migranes. Obviously, I'm going to have to fix this. The fuel lines were all new, as were the seals and sender, so I doubt it was a leak. But my gas cap was a vented gas cap.

Weren't most gas caps for C1s non-vented? If I simply switched out the vented cap for a C1 non-vented cap, could it help cut back on the gas smell? Are there any dangers or potential pitfalls in using a non-vented cap with a stock C1 fuel tank and system?

6T7L71CPE

My cap experiment showed flow at low volume/pressure as wonderfully demonstrated by nassau66427's "Smoking Cap" and "Destructive Analysis". Yet, "in service" the flow must be enough to lift the "safety valve" and hold it closed and therefore continue to build pressure. As initially noted, my pressure was noted after a short ride on a relatively cool day and the first time I'd noticed the tank pressure when removing the cap.
My thoughts are:
A) The inside of my cap is now as rusty as nassau66427's and the small springs' base is supposed to move under pressure but is rusted in place.
B) The normal pressure needed to overcome this small spring is more than needed to overcome the carbs needle valves as noted from Clem's experience.

Veet 66

I may be missing something here. My cap is doing the same thing as JohnZ. I can blow in on the bottom of the cap and the air will not go out the top of the caqp. Yet when I suck in the air goes in to the tank. Also I thought if a tank inside is pressurized it would force the gas out of the tank better than no pressure and it would be just gravity pushing the gas out. This is a 1966 car.

SWCDuke

Blowing through the vent hole will likely force the safety valve up, and at this point it will take pressure to release the internal valve. You will have to blow pretty hard to unseat the pressure release valve.

In service the tank will never see such a quick increase in flow. The smoke test shows that vapor can escape at a low rate - not fast enough to seat the saftey valve.

I tested the cap on my Cosworth Vega. It has an evap system with typcial vacuum and pressure relief valves built into the cap. I teed in my vacuum/pressure gage and used oral vacuum and pressure. It took little effort to generate the 2" Hg (about one psi) vacuum to unseat the vacuum relieve valve, but I had to "blow" pretty hard to achieve the 1.5 psi required to open the pressure relief valve.

Duke

Matt Gruber

i use a stant press/vac cap, but non-vented is stock. to greatly reduce fuel slosh thru the vent line try a tiny "jet" at the tank nipple. mine is .025"

JohnZ

Yes, C1 caps are non-vented; the tank is vented separately through the rubber vent line from the top of the tank out into the filler door cavity. There's another thread further down from yesterday that deals with all six problems that cause "gas smell" in C1's and how to fix them.

Loren Smith

Thanks, John.

IGO200

I bought a cap from Paragon to fit my new Quanta tank and it allowed pressure to build in the tank and actually ran fuel up to and out the carb supply line (I had the carb off). I checked the cap and found that it had a free flow "into" the tank but resisted flow out. I set up a small regulator and gauge (from my airbrush) and determined that it had about a 5psi pop-off before it would vent out. I'll see if Paragon knows anything, but expect I'll end up drilling it.

6T7L71CPE

Since the time of the "VENTED gas cap" discussion I've determined my problem can be attributed to a steep driveway with a dip a the end. If the outside temp is hot, the car and gas is hot, the tank probably has a sight constant positive pressure relieved by the valve. However, as I pull into the drive, provided enough gas is in the tank, the cap/valve is splashed by fuel and it quickly closes the valve and the slight pressure holds it closed. I always pop the cap after I pull into the garage now. It is almost always wet with fuel. This same scenario can happen while driving esp. with a full tank. $0.02