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740T goes Psssst! 700

posted by  silverbrick on  Sun May 28 09:40 CST 2006
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last visit: Fri Sep 6 18:52 CST 2013
[RELATED]

Hello to all,

I just replaced the intake manifold gasket on my 91 740T. The replaced gasket was broken and leaked very badly at #1, causing a rough high idle. The plug at #1 showed a lean running condition as well (very white). Found the leak by using a length of plastic line to 'listen' for the leak. Car now idles perfectly and seems to run a lot smoother. Also replaced the plugs, cleaned out the plastic connector 'T' where the small line runs over to the intake manifold.

I believe I could hear a hiss before I did the fix, but now it is really apparent and loud when boost goes above about 2 o'clock on the gauge. It seems to be an all of a sudden thing, where something opens up when the pressure gets high enough. Typically the boost only goes to around three o'clock at WOT around 3-4K rpm (half way in the white zone, which appears typical from what I have read in other posts). Car is running fine otherwise. I checked the major hoses and they appear to be good. Most were replaced when I purchased the car a few years ago. So I'm thinking another line perhaps or the intercooler. So far nothing else looks or feels amiss. I read in the FAQ about pressurizing the intake manifold, and tried with the small compresor I have, but no pressure is held. Not exactly sure how this can be done as air must leak into the engine etc, but I do not have a lot of air volume from the small compressor.

Any insight and ideas appreciated. Any places to start, past experiences etc. would also be helpful. Car drives ok, but it would be nice to have all the boost I can get!!!

Thanks in advance,

Sean

--
91 740T M46






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