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Old 05-26-2013, 07:47 PM   #1
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Join Date: Jan 2007
Location: Vero Beach
Default Why do I have a Saab valve on my car? And/or is this why I have no power under boost?

1991 740, B230FT, 16vT with Garrett T3. I did not build this car and I am fairly new to turbocharging, so please bear with me.

Lately my car has had some trouble with power. It drives almost exactly like a naturally aspirated 740. The compressor is spinning up, it's generating boost (currently at 10 psi) but there's only a teensy little bit of acceleration that dies off almost before you feel it. Prior to a few weeks ago it was just as quick as I could ask it to be.

After some searching and pondering, my first thought is that I have a boost leak somewhere so I put together a tester and hooked it up. The hoses hold pressure up to about 20 psi, and I couldn't find any leaks by sound or feel anywhere.

Unfortunately there was also a loud POP, which it turns out is from a Bosch valve, item # 0280142105. This seems to be a Saab overpressure valve, which seems to have taken the place of the external CBV that the T3 in the car came with. It was venting to atmosphere, so I guess it's sort of a BOV given the way it's plumbed.

In fact, the more I look at it the more I wonder if it was even hooked up to the correct port at all. The car has always had a strange noise off boost that sounded like really quiet compressor surge, but the previous owner said that it wasn't when I test drove it. This thing has some sort of vacuum diaphragm but pressure from that side wouldn't open up the valve at all as far as I can tell.

Is this a common modification? I don't remember ever seeing a reference to this being used on a Volvo before. It looks like I have an oddly numbered variant, I just found a bunch of threads describing very similar parts with final digits off by one or two. Can anyone who's used one of these tell/show me how it should be plumbed?

And since it apparently held pressure until today, what else should I be looking at to find my boost problem?

For now I guess I'll push the MBC down to zero pounds and drive it like I'm scared of it.


Last edited by AdmiralSenn; 05-27-2013 at 02:09 AM..
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