09-21-2017, 10:14 PM | #1 |
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2017 340ix Knocking Noise When Gas Tank is Below 1/4 Full
Hi Guys,
I picked up my new 2017 340ix 2 weeks ago and it's making a knocking noise from the undercarriage when the fuel tank is below 1/4 full. (I know that most people don't recommend running with so little gas in the tank but there are times when I am not near a gas station.) I noticed there are NO drivability issues. No Check Engine lights. Engine RPMs are stable. I hear it while stopped in Drive or when the car is in Park. I can hear it slightly as I am coasting very slowly also. So it's NOT the drivetrain. It's clearly from the exhaust or the cats. Here's my a video that I shot of the noise. As I googled this issue, I also found another 340i that had the same issue. Another viewer of this video also chimed in and said his car had the same issue and the dealer said it's related to the cats "cooling off". The poster said that his noise also occurred with the tank below 1/4 tank as well. Does anyone here have this issue? If so, did you ever get it resolved? Last edited by dsc888; 09-21-2017 at 10:19 PM.. |
09-21-2017, 10:22 PM | #2 |
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I have the same issue. I actually noticed it since my first tank of gas from delivery.
For me, it only happens when I'm <1/8 of a tank. Didn't get it checked out. I just fill up my tank and the noise goes away. I've learned the 340 (I suppose the F30 in general) has a bunch of annoying creaks, rattles, and knocks. I just live with it at this point.
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09-21-2017, 10:55 PM | #4 | |
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09-21-2017, 10:57 PM | #5 |
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Great idea! It seems to get louder as I approach the cats near the front/center of the car but it's not in the engine bay or the very back from the exhaust/muffler.
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09-22-2017, 07:25 AM | #6 |
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If you think about it, when the tank is nearly empty, it gets lighter and isn't being weighed down by the fuel. Wonder if it has something to do with clearance between the fuel tank and something above or near it on the chassis or suspension or even the exhaust system? I'd be taking it back to the dealership - it has ZERO to do with the catalytic converters "cooling off" - that is generally heard as a series of popping/creaking noises for a minute or two after the engine is shut off and the cats and pipes start cooling and contracting (perfectly normal for exhaust components). It's definitely not due to "lack of testing by the BMW Engineers".
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09-22-2017, 09:57 AM | #7 |
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I have the EXACT same sound in my 330i when the tank is at 1/4 or below. Since this happens with a different engine and exhaust layout, could it actually be something in the fuel tank that's rattling?
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09-22-2017, 10:17 AM | #8 |
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this is funny and strange because ive always had this issue with my old 1987 VOLVO 740 GLE. that i havent had in over a decade now. But every time the tank got low, it would make strange noises.
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09-22-2017, 10:32 AM | #9 |
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I'd say that when the tank is approaching empty it becomes a resonator, amplifying chassis transmitted vibrations to a higher than normal level.
Aside from strange noises you shouldn't let the tank go below 1/4, as it increases the potential for water condensation in the tank. |
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09-22-2017, 12:04 PM | #10 | |
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09-22-2017, 12:07 PM | #11 |
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Thank you for chiming in! That is very interesting. I wonder now if it is isolated to the LCI F30s only. I've never come across any threads regarding this noise. Seems like all folks affected have an LCI model. Hmmm. I wonder if there were underbody changes or even a fuel tank design change that is now causing the noise when almost empty.
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09-22-2017, 12:10 PM | #12 | |
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I travel for work and sometimes try to get my work done before stopping for gas so I will let it run low just for the day and fill up when I get off work. But I agree that it's not a good idea to let the fuel run too low. And now, it appears I have a audible "reminder" under the car to let me know each time. |
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09-22-2017, 12:11 PM | #13 |
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No noise in my 2014 335. Thankfully! It does seem like it's LCI only? That's a weird issue. Haven't heard much about issues like that before.
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09-23-2017, 07:57 AM | #15 |
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I raised my car on jack stands yesterday and got under it to figure out where it is coming from. I thought it might be the exhaust hitting the gas tank but I can move the latter freely and it doesn?t touch anything.
I took a screwdriver, put it on the gas tank and on my ear. It really sounds like it is coming from inside the tank! Could it be the fuel pump??? I also notice that there is no noise even when the tank is empty when the car is cold. It needs to run for a while before the sound appears. |
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09-23-2017, 08:13 AM | #16 | |
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09-23-2017, 10:05 AM | #17 | |
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09-23-2017, 04:56 PM | #18 |
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Definitely not cats cooling or any garbage like that. I would venture a guess that it's an echo chamber effect in the near empty tank, or even more likely the fuel pump struggling to pump the lower level of fuel in the tank. Perhaps a mix of both, where the fuel pump is more easily heard in a near empty tank versus submerged in fuel. The consistency in the thumps make me believe it's a mechanical noise, hence the fuel pump. Since it seems common I wouldn't worry, but the E90/92 had poor fuel pumps so it's not necessarily shocking that BMW beefed up the units, maybe making it noisier?
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09-23-2017, 05:32 PM | #19 | ||
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I am due for an oil change in a week or so and will tell my SA about it. Will report back. |
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09-23-2017, 05:41 PM | #20 |
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Thanks for the reply. And definitely share your findings with the forum. I hope you can go there with your tank on the low side so they can hear it and give you a diagnosis. I'm sure BMW engineering will get plenty of feedback as customers begin to complain about the same issue. Maybe we will even see a Service Bulletin in the months to come.
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09-23-2017, 05:46 PM | #21 | |
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09-23-2017, 10:29 PM | #22 |
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Well when I posted my thread no one really cared so I took the car to the dealer and its normal sound. Just fuel pump working
http://f30.bimmerpost.com/forums/sho....php?t=1410955 |
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