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Things that go bump in the night
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Joe Westy
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Joined: November 06, 2009
Posts: 75
Location: SLC,UT
Joe Westy is offline 

PostPosted: Thu Mar 07, 2013 8:18 am    Post subject: Things that go bump in the night Reply with quote

Well they actually go bump in the daytime as well. My '90 GL is making a mysterious sound that neither I, nor my local VW specialty guru, can seem to diagnose.

I've been hearing an occasional and seemingly random deep toned 'thump' that seems to come from the driver's side front in the vacinity of the seat or floorboard. Not only can I hear it, but I can feel it at times.

I initially thought it was my gas tank contracting and expanding because I had some work done on the expansion system last summer(new grommets on expansion tanks) and shortly after that noticed that a vacuum was being created in the tank. I would get a 'whoosh' of air when removing the gas cap and that had never happened in the 5 years I've owned the van. Also, after shutting off the van sometimes I'd hear that metallic 'bonk' sound of the main tank popping back into shape. I blew through the charcoal canister and it wasn't clogged in either direction, but the 'whoosh' did stop soon thereafter and so did the bonk sound from the tank so I kind of assumed I blew some blockage out of the way and the problem was solved.

Now I'm getting the sound I mentioned in the first paragraph that happens randomly while driving and sometimes just after shutting off the van. I took it back to the shop that replaced the grommets, a highly regarded VW specialty shop whose owner is a real Vanagon guru, and he suggested that perhaps I was hearing the solenoid at the deep cycle battery under the front driver's seat. I assured him that I know that noise and that this is a new and different sound. He retested the expansion system, including putting a vacuum on the purge valve behind the charcoal canister, and said that the expansion system is working just as it should be. Of course he was unable to hear the sound when he test drove it, but I heard it while driving home! Isn't that how it always goes? Of course he suggested coming back when it's happening so he can hear it.

Now I'm wondering if we're somehow barking up the wrong tree entirely and this sound has absolutely nothing to do with the fuel tank or expansion system. Has anyone else ever experienced something similar to this? Again, it's a random deep toned "thump" that seems to come from the driver's side front somewhere underneath and I can actually feel it sometimes.
'90 Vanagon GL
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