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  #1  
Old 10-18-2006, 11:59 AM
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Hissing sound in dash

There is a significant hissing sound (obviously vacuum related) in my dash that varies according to engine rpm. It is especially noticable just after a shift and sounds like a 'release' of vacuum at that time. Also, if I hold the accelerator such that I am just before a shift point, a constant hiss is heard. This is on the '84 300TD.

Any suggestions on what this might be before I tear into the dash?

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  #2  
Old 10-18-2006, 12:21 PM
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Strange you mention that. I have the EXACT same thing on my 84 Euro 300D (They stuck a turbo on it in Germany).

I always wondered what it could be.. I assume it has to do with either the transmission pressure release, or the Turbo, but I have no clue.
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Old 10-18-2006, 03:53 PM
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Quote:
Originally Posted by Zerohour3k View Post
Strange you mention that. I have the EXACT same thing on my 84 Euro 300D (They stuck a turbo on it in Germany).
Is your Euro an auto? My guess is that it is transmission related... question for forum members: is the 'dash pot' (aka surge damper) located under the dash? I thought this was under the hood...
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  #4  
Old 10-18-2006, 05:26 PM
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Quote:
Originally Posted by pdxwaker View Post
Is your Euro an auto? My guess is that it is transmission related... question for forum members: is the 'dash pot' (aka surge damper) located under the dash? I thought this was under the hood...
Unfortunately, yes.. it's an auto.
I *WISH* it was a 5spd. =\
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  #5  
Old 10-18-2006, 06:03 PM
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Quote:
Originally Posted by Zerohour3k View Post
Unfortunately, yes.. it's an auto.
I *WISH* it was a 5spd. =\
Stuck a turbo on it? Its not a turbo engine? What kind of boost?
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  #6  
Old 10-18-2006, 06:19 PM
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I have the answer:

The hissing sound is normal, thats the transmission shiftpoint-valve-vent-line. Its a little black vacuum line that just ends inside the dash (so it sucks in cleaner air) and when you let off the throttle it causes the valve to draw air through that line, thus the hissing sound. Its normal, mine does it too when I am coasting. I kinda like it though.
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  #7  
Old 10-18-2006, 08:10 PM
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Originally Posted by winmutt View Post
Stuck a turbo on it? Its not a turbo engine? What kind of boost?
Very, VERY low boost. Whoever did it appears to have been intelligent enough not to screw up the engine. From what I understand, they modified the valves properly, and toyed with the IP, before sticking the thing in there. Even then, they dont have it cranked up very high.. since the car is still only slightly faster than a 240D.
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Old 10-18-2006, 08:36 PM
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Quote:
Originally Posted by pawoSD View Post
I have the answer:

The hissing sound is normal, thats the transmission shiftpoint-valve-vent-line. Its a little black vacuum line that just ends inside the dash (so it sucks in cleaner air) and when you let off the throttle it causes the valve to draw air through that line, thus the hissing sound. Its normal, mine does it too when I am coasting. I kinda like it though.
Hmmm... makes sense but my '84 wagon sure is _a_lot_ louder at hissing than my '85 sedan. In fact, I've never noticed this in my '85. Its so loud in my '84 that it drives me kinda nuts. Where is this vent line located? Mabe I can move it slightly to change/reduce the sound.
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  #9  
Old 10-18-2006, 09:09 PM
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Quote:
Originally Posted by pdxwaker View Post
Hmmm... makes sense but my '84 wagon sure is _a_lot_ louder at hissing than my '85 sedan. In fact, I've never noticed this in my '85. Its so loud in my '84 that it drives me kinda nuts. Where is this vent line located? Mabe I can move it slightly to change/reduce the sound.
Usually its just laying amongst the wires/tubes inside the dash behind the instrument cluster. Our '83 also makes no hissing sound, and mine doesn't sometimes, but does most of the time. Moving it would probably affect the sound is my guess.
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'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)
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  #10  
Old 10-19-2006, 01:13 AM
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I suggest you do a test or two...

Quote:
Originally Posted by pdxwaker View Post
There is a significant hissing sound (obviously vacuum related) in my dash that varies according to engine rpm. It is especially noticable just after a shift and sounds like a 'release' of vacuum at that time. Also, if I hold the accelerator such that I am just before a shift point, a constant hiss is heard. This is on the '84 300TD.
Any suggestions on what this might be before I tear into the dash?
... by taking each vacuum line that comes through the firewall and close them off one at a time to see which is the source of the vacuum that is causing what I think might be something broken or disconnected.

IF it turns out to be the "Vent" line from the Vacuum Control Valve [VcV] mounted on the rear of the IP... IF this is the source of this noise, I would most certainly put a cheap paper fule filter on this line where it terminates under the dash... this way it will not suck in dust/dirt and cause a problem with your Engine/Tranny vacuum control system under the hood.

I have the kick panels off both of our 1980(s) and you canNOT hear the vacuum when shifts occur on either of these cars.
Regards,

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