Hello,

I was driving my '85 740 Ti to school today, just cruising along (turbo wasn't adding pressure or anything), and all of a sudden, it starts running really rough--like it's running out of gas, only without the knocking or anything. The throttle won't respond--it's too busy trying to keep itself on--and then I hear a pop and a short hissing sound that comes from the rear-left of the engine compartment, around the brake booster. I assumed it must've been a blown head gasket, with the hissing, but it didn't KEEP hissing. Only hisses when the turbo spools up (which it does, perfectly normally; i hear it every time). AND after that happens, I realize, the power brakes stop working.

I stopped for gas and had a look, and nothing appears to be wrong. But when it's idling, which it does very roughly, I hear that same hissing sound, which is as rough and inconsistent as the idle. It goes away when any pressure is applied to the throttle, at least, till the boost meter goes into the yellow range. It was running strangely this morning when I started it; I started it, it stalled. I started it, it stalled. It must've done that three or four times when it started and idled very roughly--much in the same way it does now--but I gassed it and it picked itself up. I figured it was just a simple cold start thing; it was freezing outside.

I have had no problems with this car previously, though I've only owned it for about a month and a half. What's wrong with it? This is pretty urgent. I appreciate your help.

Thanks,

Volfro
85 740Ti

In the first generation of Volvo turbos, the turbo waste gate controls AND your power brakes both work off of hoses attached to the engine air intake system, so I bet it's one of those hoses that are the problem. And, given the age of this car, it's certainly due to have all these hoses replaced anyway. The waste gate uses two hoses the size of a vacuum hose. One runs from the turbo unit (or an air intake duct) to the waste gate actuator (works like an air piston to open and close the gate). Another hose runs from the intake manifold to an overboost pressure switch under the dash - this shuts down the engine if the turbo blows too much boost into the engine. I'll bet this is the problem - you'd hear it leak air when the turbo boost is up + the air leak would mess up the vacuum in your power brake booster. Hope this is it, because its a cheap fix.

A cheap fix if you use the bosch or porche/bosche cbv and not the pierburg, or if you can find a good pierburg to pull... New Pierburgs are over $200!

-rt

Why don't you disconnect the vacuum hose from the brake booster end and plug it off. If your idle is OK then, you're leaking air through the booster.

Rene
--
1988 745 GLE B23FT M46 (J-P Type combo). Toronto. 380000Km. Suspension upgraded and lowered . 1982 242 GLT project (B23ET LPT to be included) Also in charge of GF's 1986 244

Duh.

Why didn't I think of that? Thanks, Rene. I'll check it out.

Your 745 sounds sweet! I didn't used to like the 7-series too much. But then I got one. Nice cars.

Have a good day, and thanks again!

Volfro