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Re: Sources of Vacuum Leaks[140-160/73] posted by Don Foster on
Wednesday, 21 October 1998, at 11:02 a.m.
Craig --
Two quick thoughts.....
First, a serious leak should make an easily heard "hisssssss", so you could snoop around with a hose in one ear (as a stethoscope) looking for the leak.
Second, an old trick for detecting slight leaks around intake manifold gaskets or injectors is to spray the suspect area with something like carb cleaner or WD40. The liquid temporarily seals the leak and the stumble smoothes out.
I would guess that running rich and a vacuum leak are at opposite ends of the spectrum -- a leak usually implies lean. I seem to remember that the old Bosch FI systems had a common failure of the vacuum sensor causing a rich condition -- perhaps a vacuum leak right at the sensor would have this effect. I'm sure another Brickster can expound.
You should get the manual I sent in a day or two.
Wednesday, 21 October 1998, at 11:50 a.m.
Your logical approach is wise, rather than the "Let's start replacing stuff until it runs right" method. Per Don's suggestions, try to eliminate any possible vacuum leaks, including the little hose running from the intake to the vacuum sensor (or Intake Pressure Sensor as Volvo calls it) on the RH inner fender. If that sensor doesn't read the vacuum properly, it will think the car is in a "heavy load", or more open-throttle situation than it really is. That, in turn, will cause the control unit to demand a richer mixture.
If, after eliminating any possibilities of vacuum leaks (and making certain your ignition system is in good shape and proper adjustment), it still runs rich, you can try the following: Carefully drill through the brass plug in the center front of the pressure sensor with about a 3/16 drill. It might be a good idea to remove the sensor to the bench, turn it so you are drilling upward, and shake or suck out any shavings that might have entered accidently. Insert a very small slot screwdriver straight in and you should feel an adjusting screw inside. Reconnect every thing, and with the engine running, adjust the screw a little at a time. As I recall, clockwise = leaner, but if I'm wrong, there's only one other choice. Keep track of how far you crank it. Don't adjust too far without test driving to make sure you don't over-lean and start to ping.
If the sensor is waaayyy out of whack, you probably won't be able to bring it back into adjustment, and will have to replace it. If you know of another car with that system that runs right, maybe you can borrow it's sensor.
--Chris