1991 Volvo 24O(sedan) non/turbo.

Having issues which believe are vacuum related! involving Cruise Control, Brakes, & engine bogging-down (but NOT stalling)...

Cruise Control stopped working ; can hear sometype of noise when attempting to engage cruise control, as thO it is making the attempt @ setting desired speed, once release button NOTHING. Also hearing a hissing sound from under dash when brakes are applied , gas milage as gone dramatically lower along /w it! Have checked both 'check valves'

- 1.) front of brake booster

- 2.) top of motor running horizontal when looking @ motor /w hood opened

Both checked out fine [BLOW TEST = open one direction-closed the other.]

if any advice as to what could be the issue, and/or how to fix these problems i'm having , would be GREATLY APPRECIATED! i contemplated possible issue /w charging/alternator being as høw car will bog-down and seem to almost die when applying brakes, but i honestly don't knOw! replaced alternator just last year when chasing down a strange electrical/charging problem that ended up being the Harmonic Balancer needed replacing. Also, there is NO change in a/c when having these issues with loud hissing when applying brakes (have read some post which mention such)....

ANY/ALL relies will be GREATLY APPRECIATED! Hope to get this figured out & fixed AS QUICKLY & as soon as possibly possible! THANKS!

 

The hissing noise is more than likely a leak somewhere in your climate control system. All the flaps that open/close to direct heat/AC to the various vents are driven by vacuum driven actuators under the dash. There is a vacuum reservoir under the dash as well. Usually the culprit is the vacuum check valve on the top of the engine but you have already checked that. I would try to get a MightyVac and pull a vacuum on the line that runs out of that check valve and through the firewall. It will take some pumping on it to get a proper vacuum but once there it should hold the vacuum. If it doesn't then you need to work your way down the line (under the dash) until you find your leak.

 

Hi, thanks for taking the time to reply.... the hissing (or air escaping) noise is ONLY when applying brakes, in which makes the motor big-down and sounds to almost stall but stutters & gargles like until let off brake. Still could be within A/C-Heat System? Along /w the cruise controls failure to work since all these things began also... read where the cruise control has its own vacuum pump (in which i think i've located to the left of the MAFF & intake area , is this correct?)
so, follow the vacuum hose from the check valve atop of engine ? Which will go behind firewall i'm assuming and hope to find a leaking hose or connection before gets too deep? What parts of the dash needs be disassembled 1st , any suggestions?

 

If the hissing sound only happens when applying brakes then it may be a leaky brake booster. I've never had one go bad and don't know how to go about diagnosing a leak there. There is a single vacuum hose that goes to the brake booster so either the leak is in that hose or the diaphragm in the booster.