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Pressure Build Up in Gas Tank

Discussion in 'Daihatsu Hi jet' started by tdrich7, May 8, 2021.

  1. tdrich7

    tdrich7 Member Supporting Member

    Usually when I unscrew my gas cap there's a slight hiss and pressure relief, but nothing major. Yesterday after I got back from a ~30 minute drive with a big uphill climb right before home I unscrewed the cap and there was a ton of pressure. Gas was boiling up almost to the point of spilling out of the tank, and fumes/vapor continued flowing out for over a minute or two.

    I've done some quick research, and it appears some similar themes are vacuum lines and the EVAP canister. I definitely think the big black canister by the fuel tank could be clogged, and I believe that's the EVAP canister. I have been noticing a weird hiss/wheezing coming from there when the truck is off. I also re-routed my vacuum lines recently. They were configured in a way inconsistent with the diagrams and other members trucks, including a nozzle on the bottom right of the carb not being connected to anything at all, and from what I can tell everything is now as shown in the diagram. However, I suppose this other configuration was a workaround for whatever problem I may be seeing now (clogged EVAP canister??). Just spitballing here. The truck always ran fine like that, although I do feel like I have more high end power now (I live in a hilly area - it's a struggle to get this thing to 80 km/h unless I'm going downhill).

    I also know it has been mentioned there is a one way vent on the gas tank but I have never seen this/don't know where it is?

    Other things of note:
    -Truck has been starting reliably but also reliably taking a lot of cranking. I usually need about 15 seconds of cranking in 2-3 intervals to get it to go. I get a very low rumble at first and then it picks up steam and gets going. It can stall on reverse if I don't give it enough gas until it warms up. Been meaning to look into this but have been swamped using the truck to get stuff done.

    -My exhaust tailpipe rusted and fell off while driving last summer and I have yet to get it fixed

    -When I replaced the fuel filter recently, I noticed I had a steady stream of gas coming from the tank, even when I opened the gas cap and released the pressure. It kept going and filled up multiple gallons in a container until it slowed down. Is this normal?
  2. Roadster

    Roadster Active Member

    The gas tank is mostly higher than the normal fuel filter location, so taking the line off the fuel filter will, if the tank is more than say, half full, run out of the disconnected line. With the tank mostly full, gas will run out almost to the top of the engine height on mine.

    Hard starting: the choke might be set wrong? - now that the weather is warmer, the line and the roller on the carb should line up at 20*C with the engine cold...

    I get a little pressure release after running and opening the gas cap, but never what you are describing. I too would suspect the EVAP canister as others have pointed to it being problematic...
    tdrich7 and Limestone like this.
  3. Limestone

    Limestone Well-Known Member

    That was one of the problems that I encountered also! In my experience, with different vehicles, over the years, it may be on issue, but between making sure the tank vent,(first choice), was clear, or replaced. I cleaned the Charcoal canister,(EVAP), with starting fluid, and let it dry out good in the sun! Starting fluid is a great cleaner, and especially in a fuel application, doesn't introduce contaminants, to the system! On a few occasions, the vent in the fuel caps vent, needs to be working properly! I have replaced caps before on vehicles, although you would think if the cap, isn't venting properly, that the tank vent would solve that problem, but not always the case! These older units develop these same, or similar problems, and everything that you can do to eliminate problems down the road, as preventative maintenance will be worth it! Keep us posted!
    tdrich7 likes this.
  4. tdrich7

    tdrich7 Member Supporting Member

    Thanks guys for the responses. Few questions...

    Where is the vent on the gas cap and how do I see if it needs replaced?

    Where is the vent on the gas tank? The only thing I can see that maybe looks like one runs to the EVAP canister...

    How to properly check/clean the canister? Do I take it apart somehow or just spray the fluid in there. I've seen a couple different ways to test the canister online. One says to block the biggest barb while blowing air into the smallest barb...then air should come out the third one. This did not happen for me. Is this a valid test for these? I don't know how they work at all.

    Finally, what are my options if my canister is clogged? I think @Jigs-n-fixtures said he bypassed his. An online search doesn't really seem to bring up any replacement options.
  5. ttc

    ttc Active Member

    Build enough pressure in the tank and you will have fuel forced into the carb float bowl then fill the base with fuel. Check your engine oil level and smell/taste for fuel.
  6. Jigs-n-fixtures

    Jigs-n-fixtures Well-Known Member

    The problem I had with mine being clogged was that when it pressurized the tank, the return line from the fuel pump to the tank, also pressurized. The fuel pump regulates the discharge pressure to the carb inlet, by flowing fuel back to the tank, through the return line when it is generating more than the carb can handle. With the return line pressurized, the fuel line to the carb went up above the 2-psi set pressure by however much pressure the tank had in it, and would flood the carb.

    Happens worst in the spring, or early summer, as teh temps out side warm up. The fuel cap, has a one way valve in it to allow air into the tank, as it cools off, so the tank doesn’t collapse from the vacuum, as the fuel cools and shrinks in volume. But, it doesn’t let the air go out when the tank warms up and expands.

    When the charcoal canister is working right, the excess pressure vents through the charcoal, the charcoal absorbs the volatile organic compounds, and then when the truck is running the engine apples vacuum to the charcoal canister to pull the organic compounds out of the charcoal, and into the engine to be burned.

    I will probably hot knife the canister apart this summer, and repack it with new activated charcoal.
    tdrich7 likes this.
  7. ttc

    ttc Active Member

    Mine filled the engine with fuel on its trip from Japan and once in the summer when I hadn't used the truck for a month.
    tdrich7 likes this.
  8. shogun

    shogun Active Member

    Not a Daihatsu, but gives you some ideas, see page 3,Charcoal canister refurb http://www.km5tz.com/BMW 850i.htm
    tdrich7 likes this.
  9. tdrich7

    tdrich7 Member Supporting Member

    Thanks for the information guys. I'm hoping to come up with a temporary fix so I can still drive it safely in the meantime, but don't want to just disconnect everything and having excess air or crud going into the tank. Any way to safely remove the filter for now? I suppose I could also just drive with the gas cap partially unscrewed?

    In the long run taking the canister apart and repacking sounds like the way to go. That link should be a helpful starting point, thanks shogun.

    Hopefully my engine hasn't flooded with fuel yet, but I will check the oil to see.
  10. ttc

    ttc Active Member

    Unplug the line from the tank to the charcoal filter and put a filter on the line let the tank breath
    tdrich7 likes this.
  11. Jigs-n-fixtures

    Jigs-n-fixtures Well-Known Member

    I added a filter to the tank vent line, and let it get air. Pretty much what TTC recommended.
    tdrich7 likes this.
  12. tdrich7

    tdrich7 Member Supporting Member

    Thanks guys. Any recommendations on a filter? I've tried a google search for something logical but not seeing anything obvious and not really sure what to search/look for. Also - what would I do with the other 2 lines?
  13. tdrich7

    tdrich7 Member Supporting Member

  14. Jigs-n-fixtures

    Jigs-n-fixtures Well-Known Member

    Way to big. I used a Ricky-Racer rebuildable one I had laying around, because I had it laying around, and a mounting bracket that fit it. I had to go to the hardware store to get a nipple that fit the vent hose.
  15. tdrich7

    tdrich7 Member Supporting Member

    Hmm...that's the only thing I could find that seemed like it would be what I'm looking for. I'm assuming I want a very fine air type filter to keep dust and what not out?

    Anyways, think I'm going to attempt the rebuild of the canister pretty quickly here. Hoping with some new activated carbon from the aquarium store and a screen/filter I can be back in business with gluing the canister back together.

    Before I do that - is there a way to remove the one way valve/vent between the gas tank and the canister? Mine appears to be embedded in the frame?
  16. tdrich7

    tdrich7 Member Supporting Member

    Was able to blow air through the one way valve at the lowest setting on my air compressor, so that wasn't the issue.

    Finally just went for it and took a hacksaw to the canister to open it up (I tried a dremel first, don't do that - hacksaw is much better). Went to a local aquarium store and bought some way overpriced activated carbon and a cheap filter pad I could cut up to stick inside. Replaced 2/3 filter pads and put the new charcoal in, then used epoxy to glue it back together. There's a spring in there holding everything down so hopefully all goes well with the epoxy - it's currently curing being held together by a clamp. Would be nice if they used a little more rigid plastic as it wasn't easy lining everything up perfectly, but it is what it is. Will report back with how it works.
  17. tdrich7

    tdrich7 Member Supporting Member

    For now rebuilding the canister seems to have done the trick. I haven't experienced any hiss whatsoever when opening the gas tank, although I haven't taken it for more than a 5 minute drive yet. Also finally got around to a bit of a tune up - changed the oil, replaced the rotor and distributor cap, and changed out the air filter.

    What's concerning is that I am now seeing some white smoke at start up/idle. It seems to disappear as soon as I back out of the barn, and there is no sign of it after coming back from a trip. Is it possible with my gas tank pressurizing I created a serious issue here? Engine temperature is right in the middle via the dash gauge.

    Also found this gem of a surprise when changing the air filter...

  18. tdrich7

    tdrich7 Member Supporting Member

    Truck has been running great, other than a CV axle boot that slipped off. Put some more grease in there and put a hose clamp on - back in business. However, we just took it out this evening to pick up some sawdust and upon returning I heard a rattle coming from the back of the truck. Unscrewed the gas cap and a good amount of pressure let out - not as much as before but still a lot. I just recently cut open the EVAP canister and packed with new activate charcoal, then resealed with epoxy, so it shouldn't be that. If anything I was worried my epoxy job didn't fully seal and I had a small vapor leak. Any ideas what's going on here?

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