Volvo: CI fuel system frequency valve, air fuel mixture, rich mixture

Question
Hi Jason,

I am working on my 1982 240 with a b21ft engine with ci injection and o2 sensor. I have done extensive diagnosis of the ci system, I have two questions: Should the frequency valve buzz continuely while the engine is running? The valve stops buzzing after the engine is running for approximately 15 seconds and then cycles on and off.  All the electical circuts appear to be working correctly, up to the frequency valve.  Even when the valve stops buzzing 12v is present at the valve.  My second question is: How does the computer control the grounding circut to the frequency valve?  Does this problem indicate the CI system computer is defective?  My car is on its death bed, and I'm trying not to spend to much money on it.  Thank you for your help.

Eddie

Answer
The frequency valve should buzz continuously while the engine is running.  It regulates the air/fuel mixture and timing.  The ECU computer controls the voltage to this value through feedback given by the oxygen sensor and air mass meter (if you have one on your 82).  The voltage changes should be very very slight to the valve under normal operation, so depending on your VM meter and it's accuracy, it might be hard to tell if it works properly by looking for the right flucuations.  Another question I would ask you is how the engine runs when the frequency valve shuts off.  In my experience, the engine loses a lot of power and runs horribly (usually rich mixture).  Are you experiening any of this?  Another thing I usually check first is all the electrical connections between the ECU and the valve.  Sometimes they can get corroded and cause issues such as this.  Running sandpaper over the contacts can fix a problem like this.  Finally, it might be worth your time to pick up a used frequency valve from a junk yard, just to test it.  They will only charge a few bucks.  Good luck!