Any time I put fuel in my truck, the fuel tank is negatively pressurized. Every time I take the cap off I hear air rush into the tank. I've checked to make sure it's not air rushing out. Any ideas? Can the ventilated cap be bad?
CK5
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2000 OBS 6.5TD Fuel tank pressurizing.
- Thread starter GWeakland620
- Start date
Your getting "vacuum"not "pressure ,right ?..sounds like its letting vacuum build up,only source of that would be the lift or injector pump sucking fuel from the tank,I assume..
Most gas caps do have a pressure & vacuum release built into them..
Here they have a machine to test them during a yearly inspection,they have to pass or you cant get an inspection sticker..
I have no clue if a diesel uses the same gas cap as a gas powered truck would...looking up what part number is listed for your truck and doing some googling will likely show what the specs are ,when it should open to release pressure,and break the vacuum..
I would think the engine would starve for fuel eventually,if no air can get into the tank and let the fuel flow to the lift pump and injector pump...it would be like dipping a straw into a glass of water,putting your finger over the end,and lifting it up--the water cant flow back out until you break the vacuum by lifting your finger off..
I had a puzzling stalling dilema on one of my riding mowers,it would start and run great for about 5 minutes,then stall,and not re-start unless I let it sit,it would just crank over..
After cleaning the gas tank,replacing fuel hoses,and cleaning the carb,with zero change--I finally found the gas cap had a vent on it with a "cap",and a lot of mud was blocking the vent,probably mud dauber wasps...soon as I cleaned that out,it never stalled again..
Most gas caps do have a pressure & vacuum release built into them..
Here they have a machine to test them during a yearly inspection,they have to pass or you cant get an inspection sticker..
I have no clue if a diesel uses the same gas cap as a gas powered truck would...looking up what part number is listed for your truck and doing some googling will likely show what the specs are ,when it should open to release pressure,and break the vacuum..
I would think the engine would starve for fuel eventually,if no air can get into the tank and let the fuel flow to the lift pump and injector pump...it would be like dipping a straw into a glass of water,putting your finger over the end,and lifting it up--the water cant flow back out until you break the vacuum by lifting your finger off..
I had a puzzling stalling dilema on one of my riding mowers,it would start and run great for about 5 minutes,then stall,and not re-start unless I let it sit,it would just crank over..
After cleaning the gas tank,replacing fuel hoses,and cleaning the carb,with zero change--I finally found the gas cap had a vent on it with a "cap",and a lot of mud was blocking the vent,probably mud dauber wasps...soon as I cleaned that out,it never stalled again..
A diesel should have a non-vented cap I believe and some kind of vent on top of the tank I think. There has got to be a way for air to come in when fuel is being used. I think my blazer does the same thing when I crack the cap open but I never paid attention if it was vacuum or pressure. No worries until it starts giving you issues with running out of fuel or something.
Correct Bob, it is vacuum not pressure. I worded that wrong.
Luke, I haven't really noticed any issues from it yet. And it's been that way since I bought the truck. It amazing how much air moves when the cap is loosened.
My '82 with a 6.2 has a factory siphon/drain arrangement made into the sending unit--at first I thought perhaps that was also a "vent",but no,it has a valve you must manually open to drain out water from the tank...my tank & sending unit has no other external vent--in fact,the tank in it now is from a gas powered truck..
I know the filler neck has that large vent hose,but the gas cap seals both that and the filler hose from the atmosphere...I wonder how air DOES get in the tank--I'd guess the cap HAS to have some sort of vent made in it...otherwise the tank would get "sucked in" eventually by the vacuum I would think..
I know the filler neck has that large vent hose,but the gas cap seals both that and the filler hose from the atmosphere...I wonder how air DOES get in the tank--I'd guess the cap HAS to have some sort of vent made in it...otherwise the tank would get "sucked in" eventually by the vacuum I would think..
The big truck stuff I'm used to all use non vented caps and they have a fitting with a hose on top of the tank. There is a check valve in the fitting that lets the tank build pressure but then when it cools off or uses enough fuel it allows the tank to suck in air. I haven't messed with enough trucks like yours to know how the tank is setup.
Yeah, I may just need to crawl under it and poke around.
Mine used to do that a lot, never saw any signs of bad stuff happening..
Til you abandoned the diesel world....
Yeah, who's this redbmx guy?
black dawg
1/2 ton status
I have seen one of those tanks caved in because of this...old cap would not vent any vacuum
That's what I am afraid of. There is tons of pressure.
Buy a cap, they probably cheap! Doesn't somebody around here work for a salvage yard? I'm sure they just throw those away when chopping up a truck lol
They usually crush the truck with the diesel engine intact also around here...
Brother in law had a 6.5 suburban I think is was a 1995... tank collapsed...bad cap.
yeah, cap it is. I go look now.
Hope that solves it for you.
Let me do some out loud thinking here.
Is it possible that while the tank is negatively pressurized, that once the truck is shut off that is could drain the fuel out of the injection pump and fuel lines?
Is it possible that while the tank is negatively pressurized, that once the truck is shut off that is could drain the fuel out of the injection pump and fuel lines?
Hard to say--it "might",but I would think the vacuum may also "pull" fuel to the tank from the injector pump return line too..
