Two Guys Rally

The Volvo Chronicles: CBV Diaphragm II
[ October 16th, 2008 ] By: Mark Ozimek Posted in » Ramblings

It seems like all I post about these days is my own car. Hopefully Charles and I will get our hands on a rally car to work on, but for now, the Volvo will have to do as my source of entertainment. Anyways, a long while back, I ordered a CBV diaphragm to replace the old one in my car that I had suspected to be worn out and torn, allowing boost pressure to leak out.

I finally got around to replacing it last weekend. The old diaphragm was in much better condition than I thought it would be, however, the rubber was definitely worn. The sealing lip that gets compressed against the turbo was leaking, creating the huge mess on the turbo:

(click for larger)

However, there was something very significantly wrong. The hose that leads to CBV that supplies pressure or vacuum depending on throttle position had torn almost all the way off. Unfortunately, I missed this when replacing the vacuum hoses, so a new hose got put on with the new diaphragm.

After driving it for a week, there is a pretty noticable difference in power in the mid rpm range and top rpm range, though more in the mid than the top, which is opposite of what I expected. The turbo flows more air at higher rpm, so perhaps the leak, which would remain constant due to constant pressure, became less and less noticeable.

So in the end, the top rpm range still isn’t making the power it should, and I need to figure it out still. I have a new turbo control valve (TCV) that controls the wastegate actuation on the way, and I’ll try that out too. If the stock unit is fine, the new TCV has improved response over the OEM unit. At the very least, it should be able to control boost more precisely.

However, an important lesson can be learned from all this: Turbochargers are complex systems that have many parts to wear out and cause poor functionality. In a rally car, these components wear quickly due to the demanding conditions, so make sure you stay on top of the little small things in your car!