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Thread: Excessive Vacuum in Gas Tank - Sputters then Stalls on Acceleration

  1. #1
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    Excessive Vacuum in Gas Tank - Sputters then Stalls on Acceleration

    Hello All,

    I hate to be the guy who's first post is asking for help after just buying the car, but here I am.

    I purchased a 318TI swapped with a 2.5 I6 that ran good enough (lots of vacuum leaks), but is now having issues on a full gas tank cold start. The PO specifically mentioned that the car does not drive well with a full tank of gas, but I ended up putting a full tank in on accident. When filling the tank, I also noticed that there is extreme vacuum in the fuel tank; enough that the plastic tank was sucked in and deformed, and the cap was hard to remove. The fuel pump has also been replaced in the last 6 months and it is very loud (drop in replacement but not OEM). I’m wondering if it is loud because it is on the way out from fighting the vacuum, or if that’s just how the aftermarket pumps are. Below is the troubleshooting I’ve done so far:

    - Started the car with the return to the tank disconnected from the access panel under the seat and dumping into a jerry can. Clean fuel was coming out, and the return barb on the left side fuel level sender was sucking air, indicating that the return line and crossover line (jet pump?) were not blocked.
    - Blew into the return barb on the left side fuel level sender and saw bubbles on the right side of tank, confirming that at least something can cross from the left to right side. PO was also driving ~250 miles on 10 gals, indicating that fuel was getting used from both sides.
    - Removed fuel pump and looked inside tank. Pump assembly looked brand new with nothing on the strainer and the tank was clean.
    - Inspected tank exterior and noticed after removing the gas cap etc., that the shape of the tank has returned to normal.
    - The car will start and idle most times, but when gas is given, it sputters and dies unless it’s almost floored. It will die after idling for a few minutes, though I was messing with the return and letting air return to the tank vs fuel…not sure if that helped.
    - There was a large vacuum leak under the intake mani (that plastic connector that is known to break was broken- pipe to ICV?). I replaced that just BEFORE having these issues, and I believe that made the problem worse…reasoning below.

    I think the TEV/fuel tank evap solenoid is stuck open. I did check the resistance and it was at ~42ohms. Is this an OK value? I’ve seen different opinions shared on the internet on what is an acceptable range. I think fixing the large vacuum leak is causing the additional vacuum to pull more gas vapor (or raw fuel??) through the TEV on cold starts which is why it’s worse for me than the PO. I’m under the impression that this valve should be closed on cold starts and at idle to prevent these issues.

    When the car is turned off, you can hear the blue evap lines by the tank and fuel pump area ticking as if there is a good bit of pressure or vacuum built up in them.

    Suggestions? I’m going to pull a used TEV from the junkyard today, but I’m wondering if the charcoal canister is plugged somewhere?
    TIA,
    Alex

  2. #2
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    Well the simplest test is to blow through that valve when the engine is off, no air should flow.

    - - - Updated - - -

    After you figure out whether or not that valve is good you should also check the valve at the opposite end, the one that allows air to enter the system, it's usually located by the charcoal canister under the spare tire.
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  3. #3
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    just beeing curious, where is that valve that allows air in on cars that don't have a charcoal canister?

  4. #4
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    Check the diagrams on realoem.com for your specific model.
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  5. #5
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    I have a '95 compact and I do not believe there is a valve in the other end according the the RealOEM diagrams.

    Also, is the blue line supposed to be fairly rigid? It looks "formed" and I'm wondering if it will be pliable enough to reinstall a new valve. I was afraid to pull it off to blow through it in case it broke.

  6. #6
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    Quote Originally Posted by nnamssorxela View Post
    I have a '95 compact and I do not believe there is a valve in the other end according the the RealOEM diagrams.

    Also, is the blue line supposed to be fairly rigid? It looks "formed" and I'm wondering if it will be pliable enough to reinstall a new valve. I was afraid to pull it off to blow through it in case it broke.
    Ah that would have helped to know earlier. Then yes in your case it should be in the back. Either way, unplug it and it should stop deflating the tank.

  7. #7
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    Sorry for my misunderstanding. Are you saying to unplug the TEV under the intake mani, disconnect the charcoal canister, or disconnect the evap tank?

  8. #8
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    TEV under intake manifold.

  9. #9
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    Remove the end of the hose that connects to the manifold, it shouldn't be a hard line, then blow through the valve, it should not blow through with the engine off.
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  10. #10
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    Thanks for the tips. I feel pretty silly that I didn't think of just blowing through the valve. I grabbed a valve from a 97 that was in the junkyard on the way home from work. Resistance across the two pins was also mid 40s and I could not blow through it. I was pleased to find that when I got home I could blow through mine with zero resistance.

    Fingers crossed that this is the fix, but seems promising. The 97 valve was shaped differently, but seems like it will work the same.

  11. #11
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    Coming back to this once more. Seems like the e36 came with 2 different styles of TEV. The one I grabbed the other day had the barbs at 90* to each other. The style that was on the car was "straight through" with a barb on each end. Today I grabbed two more TEVs, both of which were the "straight through style."

    The 90* style does not allow air through in either direction (to or from gas tank). The "straight through" style allows air to go to the tank, but not away from it, acting as a check valve. I would think that since the car is always making vacuum it shouldn't matter, but why are these different?

  12. #12
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    If you think your valve is working properly then check the valve at the other end of the system.
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  13. #13
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    Compare the charcoal systems on both e36 styles to see what your missing and need to change, I see you have an engine swap.
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  14. #14
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    Quote Originally Posted by samy01 View Post
    just beeing curious, where is that valve that allows air in on cars that don't have a charcoal canister?
    please add month/year to your profile. Muuch easier to help you, this is not enough, you can also add A after the i, then we know it is an automatic
    My Cars: BMW e36 328i Coupe
    So, then we take realoem, select a EURO E36 328i Coupe, and as you did not mention the M/Y we select a EURO spec automatic, 1994/11
    and even the oldest one has a charcoal filter http://de.bmwfans.info/parts-catalog...filter_tubing/
    and #1 is the tank breather valve http://de.bmwfans.info/parts-catalog...urb_air_valve/

    If the valve is working properly, it is sprung closed and powered open. On my E32 the fuel tank breather valve should have a resistance of 45 ohms ± 20 and be closed in de-energized state.
    https://www.bimmerforums.com/forum/s...0#post19328150
    Shogun tricks and tips for the E32 series are HERE!

  15. #15
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    Still struggling with this and I'm not sure why. Vacuum in the tank is now worse than ever. I'm pretty sure I'm supposed to have a charcoal filter up front, and an expansion tank in the passenger rear fender well. I think the vacuum has also caused the new fuel pump to begin to fail. The "whirring" is inconsistent and surges with the idle.

    I'm going to begin to blow through the evap lines and see if one is clogged. Which direction is the flow arrow supposed to point for the TEV? Since it's a vent, I would assume it flows to the intake mani?

    EDIT: Looks like the arrow is supposed to point towards the gas tank. Also, it looks like my TEV wiring is getting 12v as soon as the key is turned on. I'm assuming this is incorrect?
    Last edited by nnamssorxela; 10-22-2017 at 06:00 PM.

  16. #16
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    Our gas caps are not venting, but I think if you can find one that fits, it may fix your problem.
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  17. #17
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    I have thought of that, but the amount of vacuum is great enough that I believe I'd still have issues despite the vented cap. Additionally, I believe the car would still run poorly as there is still a large vacuum leak of unmetered air entering the manifold.

    It seems that the issue is due to the engine management sending power to the TEV when it shouldn't. Under what conditions should the valve be open? Should the arrow on the valve be pointing to the tank?

    My plan is to remove the TEV line going to the manifold, plug the barb, and temporarily vent to atmosphere. I'm a bit of a tree hugger, so I'll likely plumb it to a charcoal canister then vent that to the atmosphere instead of the manifold, and put a resistor on the wiring if I get a code (OBD1). Not 100% ideal, but better than venting straight to atmosphere and should not impact the car's ability to run correctly.

  18. #18
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    Since your car has an engine swap, have you looking into the fact that it may be the wrong vent valve. So it would do the opposite it does now.
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  19. #19
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    Just for some closure: With the manifold fitting plugged and the valve removed, the car runs pretty good. Still getting a slight stutter at high load, high RPM and I suspect the fuel pump may be damaged from attempting to overcome the extreme vacuum.

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