Any of you guys know what this vacuum diaphragm looking thing is on the top of my TBI fuel tank?
CK5
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what is this vacuum diaphragm looking thing?
- Thread starter metalneverdies
- Start date
All I see is a vacuum hose "tee"?
Without seeing the sending unit fittings, I'm guessing it's for EVAP. No idea why there is more than one line, unless it's one of those high altitude things?
Without seeing the sending unit fittings, I'm guessing it's for EVAP. No idea why there is more than one line, unless it's one of those high altitude things?
I cant see the picture that well, Kinda fuzzy. Could it be the rear axle breather?
With two hoses I was thinking maybe the axle breather and EVAP was combined, but without a one-way valve I'd think that to be a potentially bad idea.
Sorry guys. I will try and get a better pic. I took like 6 and that was the best I got.
Definitely not part of the rear axle breather.
I thought the only evap emissions was the gas cap had to pass a seal test.
Definitely not part of the rear axle breather.
I thought the only evap emissions was the gas cap had to pass a seal test.
Ok. I got some better pics of the unit. Its got a rubber nipple that comes out of the gas tank. Then the little plastic fitting.
Thats part of the high altitude EVAP stuff.
No idea how it operates.
No idea how it operates.
Well, shoot. Looks like I will have to drop the tank to replace the nipple either way. If that rubber nipple in the picture that is broke runs into the tank, I don't like the idea of fumes coming out and debris coming in.
It doesn't smell like fuel in my garage though, so maybe the nipple is just part of a vent line that runs in front of the gas tank?
Fumes escape the tank through that line, before the diaphram. The rubber hose that is broken (I assume that's what you are calling a nipple? Everything else looks fine) is almost certainly connected to a steel line that runs up the frame rail to the EVAP cannister. But you are probably right, to get to it, you'd have to drop the tank.
That's an odd place for a line like that to break IMO.
My suspicion is that diaphram somehow limits when the tank purges, probably somehow based on pressure.
That's an odd place for a line like that to break IMO.
My suspicion is that diaphram somehow limits when the tank purges, probably somehow based on pressure.
Ah, ok. I didn't even realize I had an evap canister. There is a vacuum ball near the drives side hood mount and a cylinder that I assumed was for vacuum located behind the drives side headlights.
I kept calling that line a nipple because by just feeling around there it seemed like it went into the tank. A line that runs to a steel line makes more sense.
EVAP cannister is probably on the drivers side of the core support. Round black plastic canister, one of the lines off it will go to the Throttle Body. 4-6" diameter or so. Probably what you are describing behind the headlights. The vacuum ball is what helps keep your heater/AC blend doors "engaged" when engine vacuum drops, since the AC blend door controls are vacuum operated.
Probably with only a few exceptions, everything that has a non-vented gas cap (which was prior even to 1973 AFAIK in most applications) has an EVAP cannister.
Probably with only a few exceptions, everything that has a non-vented gas cap (which was prior even to 1973 AFAIK in most applications) has an EVAP cannister.
454Sub
5/8 ton status
Could be some sort of valve to prevent raw fuel from going up the lines in an over pressurized tank or rollover situation 
It's definitely a high altitude thing, only high altitude destined trucks got that "valve", or an EVAP canister that had the valve integral with it.
I looked it up in the GM manual, as I had not run into that setup personally.
I looked it up in the GM manual, as I had not run into that setup personally.
