[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]

Re: Frequency Valve

   > I need to know what the frequency valve on my '84 Coupe GT does.  Mine is
   > not running.  How big of a problem is this?  I believe that I may be missing
   > a relay.  Could this stop the Frequency valve from operating?

   The fuel frequency valve adds extra fuel over-and-above the capabilities of the
   fuel distributor, under, I think, WOT conditions. Without it, you might hear
   some detonation (pinging) under load.

Ummm . . . not quite. The FV acts to bleed down the fuel system pressure
(the active fuel system pressure driving the injectors, not any of the
dozen other "fuel system pressures" you can measure). I think (it's been
awhile) that it, in conjunction with the CPR (Control Pressure Regulator)
act to control backpressure on the fuel distributor valve assembly, thus
controlling the fuel pressure feed to the injectors.

The FV is designed/situated/sized such that running at a 50% duty cycle
(open half the time, closed half the time), with all else being at design
operating points, your engine should see the correct fuel mixture (neither
too rich nor too lean). Raising the duty cycle will enrichen the mixture,
while lowering the duty cycle will lean out the mixture.

In a nutshell, The ECU (Engine Control Unit, aka Engine CompUter, aka
FTU Fuel and Timing Unit, aka...)  reads the O2 sensor and, based on the
voltage read (0.5V is stoichiometric, < 0.5V is lean, > 0.5V is rich),
governs the duty cycle to the FV to control the mixture. Before the
O2 sensor comes online (needs to be something like 500F to operate, so
it takes a little while to warm up), the ECU just runs the FV at 50%.
If it is *cold* (nominally below around 100F), then the ECU will run
the FV at higher duty cycles to enrichen the "cold" engine. The CPR also
acts to enrichen a cold engine. There are also other components (e.g.,
The Auxilliary Air Valve that act to feed more air to a "cold" engine)
that get involved in the game as well... Further, when running under
high load scenarios (WOT or Wide Open Throttle), the ECU will force the
FV to a high[er] duty cycle (70-80% on the UrQ's MAC02) to run rich to
both develop more power and better cool the valves.

Basically, the FV is the handle by which the ECU controls the fuel/air
mix for the engine. Since it should normally be operating in the 50%
range. On my UrQ, at idle, it typically oscillates in a 6-10% window;
I tune the static fuel pressure ever so slightly rich (so that cold
warmup runs slightly rich), so that my FV is typically driven in the
42-48% window.

You should be able to hear the little critter buzzing happily to it-
self at idle. If it is not, check to see if it is being driven by the
ECU. There is ("should" aka "might" be) a two-conductor socket dangling
by the ignition coil ('83 UrQ; dunno 'bout any other car...) which is
a test tap to monitor the FV signal -- just hook your duty cycle meter
across these two wires and read out what is happening. Again, with the
UrQ, a "test" to verify things is holding the WOT switch closed at
idle (i.e., both throttle fully closed and fully open switches en-
gaged simultaneously) -- the ECU (again, MAC02, dunno 'bout others)
will then force the duty cycle to 50% and lock it there. Anyone know
if this works on the normal Coupe?

Later cars use a totally different mechanism wherein the ECU can
directly control each injector and open/close each injector "just in
time" to administer the correct amount of gasoline to each cylinder.
Much less kludgey, and mechanically, much simpler system.

Hope that answers the questions.