1982 380 sl Frequency Valve

[NewmanP]

Can some one explain the function of the Frequency Valve.
I have been having starting and idle issues. I have to crank on the start more then normal once it starts very rough idle till it warms up then drops to 500 rps
and this is where I amy seeing a rough idle as well

I have changed the Fuel filter, Fuel Distributor, warm up regulator, Idle control Valve. and the cold start valve .

Any suggestion will be helpful

newmanpc2@verizon.net

[Frank Reiner]

The function of the Frequency Valve (FV) is to keep the fuel/air mixture in the stochiometric range, so that the catalytic converter can operate efficiently.

The FV changes frequency in response to the signal from the O2 sensor, a signal that is available after the O2 sensor reaches operating temperature.

Before chasing fuel system problems, ensure the perfect operation of the ignition system, and that all cylinders have good, and equal compression.

[97 SL320]

First a bit how CIS / K jetronic / KE Jetronic works. ( basic )

A spool valve is connected to a flap that measures air flow. One end of the spool valve has fuel pressure on it , the farther it moves with increasing air flow, the more fuel flows to the injectors. The other end of the spool valve has Control Pressure pushing it in the closed direction.

So , total this up and the lower control pressure, the more fuel that flows to the injectors.

The old way of generating Control Pressure was to have a Warm Up Regulator that altered pressure according to engine temperature ( there is also a electric heating element in there as well ) .

Later a Frequency Valve / Buzz Valve

EDIT __Was added to__ ( NOT replaced )

the Warm Up Regulator allowing a computer to alter control pressure. ( K jet ) This valve is sent a Pulse Width Modulated ( PWM , basically rapid on / of with the on time altered to suit ) to allow flow.

Even later, a EHA valve was integrated on the fuel distributor, ( KE Jet ) This uses a non pulsed signal that bends the internal valve to control pressure. I'm pretty sure it slowly swings positive and negative with nothing being mid fuel.

Remote diagnostic can be difficult, but the typical issues are old fuel , clogged injectors, low fuel pressure.

[rowdie]

The 380SL still has a WUR. I really don't know but I think the frequency valve does not control until the O2 sensor is heated. It has a one wire O2 sensor while the KE Jet has a three wire electrically heated O2 sensor and no WUR.

[Frank Reiner]

"I really don't know but I think the frequency valve does not control until the O2 sensor is heated." rowdie

Originally Posted by Frank Reiner

The FV changes frequency in response to the signal from the O2 sensor, a signal that is available after the O2 sensor reaches operating temperature.

[rowdie]

Originally Posted by Frank Reiner

"I really don't know but I think the frequency valve does not control until the O2 sensor is heated." rowdie

But in the same post he stated it replaced the WUR on the K-Jet which it did not. I was merely pointed out that it does not control at start up when cold.. The WUR was not eliminated until the KE Jet.

[97 SL320]

I had some bad info in my prior post. At least on my 1980 SAAB with CIS there was a FV and WUR, the FV would buzz even with O2 sensor disconnected.

[Pex]

I made a write up when i troubleshooted my fuel-system including FV-valve...

Check this link to my explanation, maybe something gets a little bit clearer..
Finally made my decision - Page 6 - Benzworld.org - Mercedes-Benz Discussion Forum

// Pex