924Board.org

[awilson40]

Heres the deal, The car runs great until its been running for a few min. Then its doesnt accelerate as smooth. I've traced it to something in the lambda control circuit. The Freq valve quits working after the car has been running for a while. I can disconnect the connectors from the O2 control box or the enrichment control box, reconnect them and the freq valve will work for a min, then get quieter and finally shut off. I replaced the O2 sensor and checked its signal to the control box, that looks good. I opened both boxes looking for bad connectons, all looked good. I checked all switches, grounds and jumped out the thermo switch to force it to run "closed loop" at all times. Looks like a bad freq valve, accel box or control box.
Any way to isolate or fix. What would I need to do to tune the car like the 70's before the O2 sensor.
Anyone have a set of carbs and a manifold for sale??? Or the 2 control boxes ??
Thanks

[Richard]

I think you may be confused on a few issues. I am no expert on Bosch-k,(mine runs great) but with that warning I will try to help.

The frequency valve, which Bosch calls the timing valve, is nothing more than a solonoid in the fuel line to the fuel distributor whose on/off pulses change the pressure in the lower chamber which affects the final mixture at the injector. The pulse ratios are controlled by the oxygen sensor and lambda control box. If there is a problem with the sensor, the timing valve gets a fixed on/off pulse instead of a variable one. As far as I can tell, "jumping out the warm up regulator to make it go open loop" cannot occur. It can however change the fuel pressure, which is what it does during warm up, to enrichen the mixture and compensate or mask some other fault. The part of your diagnosis I don't like, is that I think the Lambda control circuit would not change acceleration so noticeably. You also did not mention if jumping out the warm up regulator makes accleration better.
As far as the timing valve not making any noise after a few minutes, it may simply be a bad valve that gets warm and then seizes up. Is it still getting a signal? That really need to be found put before you condem the control boxes.
Good acceleration is controlled, amongst other things, by the amount of overswing the air plate has as the rush of air goes by. This overswing acts like an accelerator pump and enrichens the mixture. Did you take a look at it? If not, check it out. Should be clean and move freely, also centerd in venturi.

I apologize if all this is basic for you; it sounds like you are light-years ahead of me in understanding this system. Just thought another perspective might help.

[gohim]

I think you're making this way more complicated than it is.

The Frequency Valve will not buzz when the car is properly tuned, in fact, when it is operating correctly, and the car is in tune, and warm, all it will do is click occasionally. Since it causes minor changes to the fuel pressure to correct the mixture as reported by the O2 sensor, when the exhaust is satisfactory according to the O2 Sensor, the Frequency Valve will not buzz.

When the engine is cold/not warmed up, the Auxilary Air Regulator may be open (allowing additional air into the engine), and the Warmup Regulator (may be lowering the dampening pressure, allowing the Air Sensor Plate to rise riser, enriching the fuel mixture). This would cause the O2 Sensor to signal the Frequency Valve, and cause the Frequency Valve to Buzz, as it tries to lower the fuel pressure, and lean out the mixture.

When the engine warms up, the auxilary air regulator is supposed to closed, and the Warmup Regulator should stop lowering the dampening pressure, and the mixture should lean out. This should again be read by the O2 Sensor, and the Frequency Valve should stop buzzing constantly.

If the Frequency Valve buzzes constantly, whether the engine is cold or hot, you have a tuning problem.

In your case, I would guess that you have the mixture too lean. What may be happening is that the Frequency Valve is overheating, and stops working. Sounds like the Auxilary Air Valve is probably working correctly (you cah check it be operating the car till the engine is warm, then disconnecting one of the hoses connected to it, and shining a light into it with a mirror to see whether it is open or not, or remove it and take a look). The Warmup Regulatory is probably working okay as well.

Did you eliminated all vacuum leaks (vacuum lines, and the injector seals) before you messed around with the Idle Speed and Fuel Mixture? Did you adjust the Idle Speed and Fuel Mixture per the Factory Manual with the Idle Stabilizer disconnected?

The Sensor Plate on the Air Flow Sensor very seldom goes out of adjustement. Usually they get messed up by people screwing around with them. Take off the rubber boot covering it, and check to see that the fastener securing it is not loose, it is not bent, and it appeared to be centered on the casting, then leave it alone.

If you do not adjust the Mixture, and Speed with the engine warm, and the Idle Stabilizer disconnected, you would have symptoms exactly like you describe.

When I bought my 81 924, my car acted exactly as you describe. The PO (previous owner) or the shop where he was having the car serviced corrected problems caused by leaking vacuum, with the Idle Speed and Mixture Adjustments. This will make the car idle correctly when parked, but does terrible things to the engine performance, when the engine is cold, or when you are trying to drive the car.

[awilson40]

I hear what you guys are saying...It was my understanding that the Freq valve vibrates at all times. If I hook a dwell meter up to the test connector and adj the mixture (idle stabiliser disconnected and engine warm) I can adj the 'duty cycle' to 45 deg dwell.(50 % duty cycle) ..Which is where I have alwaysed had it. The valve will 'buzz' hense the slang term 'chatter valve' Only lately has it stopped. Then yesterday...it started staying on again ??? Sounds like an intermittant valve or control box. The temp switch I was talking about is the water temp switch that closes when the water is warm to tell the Computer to go 'closed loop' My sensor went bad, so I jumped it out as a temporary fix.
I really do believe the freq valve is suppose to 'chatter' at all times, just vary in its duty cycle (time spent on vs time off)