Frequency Valve, what does it do?

[Thack]

I have a '79 911SC and I want to know what the frequency valve does? My car does not run under 4000 RPM and I have to floor the pedal and pump it to get it to 4K RPM. Once it is there it revs to the moon. If I let it go under 4K it backfires and sputters. I also have to prime the intake runner with fuel to get it to start. I suspect the frequency valve but I'm not sure what it does.

Mike

[kstylianos]

The frequency valve regulates the fuel pressue to richen/lean the air/fuel mix depending on the signals received by the lambda brain and O2 sensor. If you think your F-valve is faulty, the first check is to listen or feel for it buzzing. If its buzzing, its ok. I'd say this is probably the last place that would cause you this problem....there are more obvious places to check first.

I have a feeling your your warm/cold control pressures are out of wack (bad or misadjusted WUR) or your idle CO% is out of adjustment.

Where is is backfiring from (airbox or exhaust)? If airbox, your mix is probably a little too lean....richen up up your idle CO a little and vise versa.

I've also seen this happen when the ignition timing is way off....way off like the distributor is off by a gear. Check to make sure the rotor is facing the notch in the distributor body at TDC #1.

Hope this helps......

There is plenty of info on the stuff mentioned above if you search for specifics.

[emcon5]

If your 79 is stock the frequency valve doesn't do anything, because it doesn't have one.

The Freq valve is pat of the K-jettronic with Lamda system, which was first installed in 911s in 1980.

Otherwise, what Kurt said.

Tom

[Thack]

My 911SC is a California model and it has the lambda circuit and all the 1980 emission modules. It happened all of a sudden while I was driving. I don't think my mixture just "got out of adjustment " while driving down the street. It backfires out of the exhaust. Does my year model have a idle injector or a low RPM idle circuit controlled by an eletrical component?

Mike

[john walker's workshop]

sounds like a major intake air leak, like a blown airbox. pull off the air filter and see if any of the little silver screws are out, or 1/2 way out. you can see the inside ones and the ones down the left side.

[Thack]

I'm not sure if my problem is intermittent but it started tonight. All I did was move the sensor plate up and down. It could have been stuck from carbon build up. I sprayed it with carb cleaner. I'll be able to test everything now that it is running and has a smooth idle. What do you guys think?

Mike