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Loud 740 Turbo Wastegate/ bypass valve..... 700 1990

posted by  Lincoln 72 on  Thu Oct 28 17:22 CST 2004
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I have a 740 with 156k miles on it and all was well at 145k. Now It lacks acceleration, and I can hear the bypass on the out put end of the turbo open under heavy acceleration. The turbo gauge on the dash reads at about the 2 o'clock position, and the performance is less than desired. Much less than it was about 10k miles ago.
I never heard the noise (blast air) before and dont really know where to begin. I can hear the turbo spool up just fine, but when the pressure builds up and the car starts to accelerate I hear the noise and the power just dosnt continue to rise. If anyone can lend some advice I would greatly appreciate it. If there are any diagrams that anyone knows about, that would be great! Thanks again. Go Navy!







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    Loud 740 Turbo Wastegate/ bypass valve..... 700 1990

    posted by  anastasialk on  Sat Oct 30 04:06 CST 2004
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    last visit: Thu Mar 8 09:09 CST 2012
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    Hello,

    A few other places you can check: Check the hose that connects to the intercooler(outlet), small hose that clamps onto the intercooler and onto the larger metal (on my car) pipe that is on the intake side of engine, feel the underside, make sure there are no cracks, etc. Also check the air intake hose from your AMM to your turbo, make sure there is not a crack by the nipple.

    Best Regards,

    Anastasia
    1986 740Turbo Wagon 385,000 miles








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      Loud 740 Turbo Wastegate/ bypass valve..... 700 1990

      posted by  Lincoln 72 on  Tue Nov 2 14:36 CST 2004
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      Thank you for the info. I have since gone over the proble areas and after further consideration have come to the conclusion that the bleed air is coming from the bypass valve on the clean side of the turbo. It is the one that has a vacuum line coming off the front of it and controls a plug type valve on the compressor outlet that is inline just after the turbo, and before the intercooler. Do you happen to know what that valve does? or what controls it? I want the pull that I miss so much! Well thank you for the insight and the advice, and any future tips that you might have! Happy driving! Lincoln.








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    Loud 740 Turbo Wastegate/ bypass valve..... 700 1990

    posted by  johnstanleyho on  Fri Oct 29 12:28 CST 2004
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    last visit: Tue Aug 5 23:38 CST 2014
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    Dear friend, check two places first. The diagphragm that is connected to the head on the front of the engine ahead of the turbo sometimes works loose at the bolts that clamp it together. Make sure this is tight. It makes a whoosh blow off sound when it leaks. Also on pre 1990 turbos the rubber 1.5" diameter hose that connects the diagphragm to the turbo can get holes and slits that are not obvious until under pressure. Get a strong light and carefully look behind this rubber tube for holes. My '89 had one that prevented boost and made performance skitterish. The previous owner thought it was a bad transmission since the power failed upon rpm change. I was really happy when I found it was only this tube. It was not obvious as you have to carefully check the back side of the tube-look for weakness or abrasion. Good luck








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      Loud 740 Turbo Wastegate/ bypass valve..... 700 1990

      posted by  Lincoln 72 on  Tue Nov 2 14:43 CST 2004
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      Thank you John, I appreciate the info. I have checked the lines and components for security, and all seem to be in pretty good shape. I have concluded that the air is coming from the bypass valve that is just after the compressor, inline before the intercooler. It is the one that has a vacuum line coming out the front of it that opperates a "plug" looking valve that bleeds clean air from the compressor. Do you know what the purpose of this valve is and what controls it? I think that I am close to solving the riddle and greatly appreciate the help! Happy driving!







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