1994 850 Compressor Bypass Valve failure CBV

[northernlights]

I've been chasing down erratic boost for a while and as I was going down the list of possible culprits, I decided to test the CBV. I attached my MityVac hand pump to the vacuum can, and it wouldn't hold any vacuum.

So, I ordered a nice repair kit (MHI K5T09671), and pulled the cover/diaphragm assembly today. The diaphragm looked fine, but testing on the bench confirmed it wouldn't pull in.



I looked a bit closer...





I replaced the diaphragm/spring/retainer, and now it works on the bench and on the car.

If you are troubleshooting boost problems, I'd say a test of the diaphragm is an easy check worth making. And, while annoying, it can be replaced on the car without pulling the turbo.

[chrafael]

Thanks for the info and pics. I think mine is doing the same thing. Did you notice if the diaphragm also holds positive pressure? Mine doesn't. I'm hoping for info before I order parts and tear it down for nothing. Any help would be greatly appreciated.

[northernlights]

I didn't have an easy way of testing for pressure, but given the presence and number of holes, I'm certain it was leaking both ways.

What I wonder is how much leaked from outlet to inlet when the boost was on. At the least I think this would add extra heat to the air, maybe confuse the BCV (boost control valve) logic because the wastegate control to airflow rate wouldn't match what the ECU expects, and probably limit the maximum boost.

[erikv11]

One good way to check out the diaphragm is to hold it up to a strong light. Very easy to see thin spots where light (and air) comes right through.

[chrafael]

You talked me into it. I'll post back later and let you know the results.