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Hiss behind head after stopping when hot 700

posted by  Rtilghman on  Wed May 26 12:31 CST 2004
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1985 745t

Title really says it all on this one. When I stop and pop the hood I can hear a hissing noise like air escaping through a small hole from the behind the head. I have attempted to figure this problem out but for the life of me have been left stumped.

Because of the position its really tough to narrow it down specifically. From the passenger side it sounds like it might be coming from over on teh driver's side of teh firewall (a la heater valve), but I JUST replaced the valve with a brand new one in perfect shape when I did the radiator. I've checked the hoses by the heater valve and all seems hunky dory.

From the driver's side it actually sounds like its coming from under the distributor, which I find really odd since the only leak that SHOULD happen back there would be a fluid leak. I've felt the crank sensor (name? under dist...) and the distributor itself and everything seems to check out. In fact, I would almost say it sounds like its coming up from the front of the tranny but I can't imagine what could cause the problem down there (rear main seal would just be a leak, not air... right?).

Recently I found that my crankcase cover was loose on the downside (more in the front with the screw by the CBV hand loose) but I tightened it down and the noise is still there.

Is it possible I blew out the crankcase gasket and am getting a tiny pressure leak from teh back of the cover in front of the distributor? If so I wouldn't be able to see it, but I've never removed the cover (had the indy do a tappet adjustment about a year ago) so I don't know what it looks like.

If anyone knows of a potential source for this I'd appreciate it. I should note that currently I only have two problems with the brick:

1. Clutch worn (slips under heavy boost at lower rpms in higher gears)
2. OD Oddities (OD spontaneously disengaging and failing to immediately disengage when actually turned off)

As far as I know neither one of these should be related to this problem. BTW, heater adjustment/climate control works fine and other gauges that rely on vacuum seem to work fine as well.

Thanks,
rt








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    Hiss behind head after stopping when hot 700

    posted by  someone claiming to be J on  Thu May 27 06:15 CST 2004 [RELATED]

    You didn't answer Ringlees question. Are you loosing coolant? All you have to do is look at resevoir.
    Don't forget the metal pipe that runs from the water pump back around back of block underneith distributor.








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    Hiss behind head after stopping when hot 700

    posted by  david hunter on  Thu May 27 00:39 CST 2004
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    last visit: Mon Jun 30 22:10 CST 2014
    [RELATED]

    Sure it isn't just the AC bleeding freon into the evap? This goes on for about 30 seconds or so, all cars do it.

    Does the noise occur if you have not used AC?
    --
    David Hunter








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      Hiss behind head after stopping when hot 700

      posted by  Rtilghman on  Thu May 27 06:00 CST 2004
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      I'm not using AC now, so I don't know. The AC is the old type used in 1985, and hasn't been serviced in years. The fluid is just too expensive to bother for me, I don't care about AC so much, and overhauling the car isn't worth it (its on a downhill rust slide).

      -rt








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    Hiss behind head after stopping when hot 700

    posted by  Steve on  Wed May 26 15:08 CST 2004
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    last visit: Fri Oct 24 07:16 CST 2014
    [RELATED]

    Leaking coolant? If so, replace both the heater hoses and the water valve. Do it anyway if they are original. See the 700/900 fAQ.








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      Hiss behind head after stopping when hot 700

      posted by  Rtilghman on  Thu May 27 05:56 CST 2004
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      I've looked and I don't see any wetness that would indicate the sound is a pinhole coolant leak. The sound also sounds much more like air than liquid; there's more of a hiss like air escaping, and it diminishes slowly in strength after I've stopped.

      -rt








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    Hiss behind head after stopping when hot 700

    posted by  swede4 on  Wed May 26 14:01 CST 2004
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    Could be the A/C gas dissipating its pressure thru the orifice tube when you shut off the compressor.








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      Hiss behind head after stopping when hot 700

      posted by  Rolyak on  Thu May 27 06:10 CST 2004
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      last visit: Fri Oct 24 08:03 CST 2014
      [RELATED]

      Just for grins try releasing the pressure in the cooling system to see if it changes the sound. Unscrew the cap then screw it back on.

      Another thought: maybe a vacuum leak?








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      Hiss behind head after stopping when hot 700

      posted by  Rtilghman on  Thu May 27 05:58 CST 2004
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      I've cut the belt to the compressor so it isn't even running. The coolant doesn't work and I don't plan on ever replacing it (too expensive for the coolant) or overhauling the AC to the newer standard (not worth teh money in this car), so I just cut the belt so it wouldn't perform a messy failure down the road.

      -rt







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