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A while back I posted on an issue I was having with my 1985 745t, usually at highway speeds. What basically happens is that I'll be driving down the highway with cruise control set and teh car will just suddenly stutter. When it happens it feels like the car has reset or maybe an air bubble has passed through the fuel system. It actually also feels like what happens when you overboost and the emergency cut-off switch resets the engine (cuts off fuel delivery?).
Anyway, over months of analysis I monitored it to see what caused it. This was especially irritating given how infrequent and spontaneous it can be. The result was that it seemed most common when I had the cruise control engaged and the cc would gas hard, let off, and then gas hard again for some reason (uphill, just engaged cc, etc.). In these instances the turbo needle would be up in the yellow, drop to low black, and then return to yellow.
After some analysis of the components that might be involved I settled on the turbo system. Looking into it I decided that the CBV had to be invlved somehow. I'm still shaky on teh exact function of the CBV, but I believe it acts to control pressure in the engine and prevent an excess condition allowing boost pressure to back-up in the system.
Original CBV's are frightfully pricey, and I haven't been able to get to a Volkswagon dealer to check out the newer Bosch unit that is supposedly compatible (and much less expensive). In the interim I replaced the delay valve ($29.99, only available from Volvo), the purpose of which is even more of a mystery to me than the CBV. I "believe" it prevents quick boost changes from triggering a false excess pressure reading at the CBV.
Anyway, problem is now solved. I haven't had a recurrence of the stutter in the two months or so since I replaced the delay valve. If you are having this problem pull the delay valve (little red valve that sits in the middle of a vacuum hose from the int. manifold to the CBV on top of the engine) aand blow through it both ways. You should get major SOLID resistance (but not full stoppage) going both directions. If you feel a sudden "thwap" as you blow that means the diaphragm inside is probably shot.
I should note that I believe this "solution" is only applicable to Garrett turbos. The mitsu units have the delay valve built into them (it sits on teh top towards teh front of the car... trace the vacuum hose from the int. manifold to find it).
Hope this helps anyone with a similar problem.
good luck,
rt
p.s. the part number for the delay valve is 1346748-5. it took my volvo parts dept. about an hour to find it... it's listed as a simple valve in the CBV system exploded diagram.
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