First, get a very inexpensive code scanner to help guide you - the small
ones for EEC-III will also scan your MCU system.
Computer-Controlled Carburetor with Stepper Motor Air Control System
Other computer-controlled carburetors have a stepper motor that controls
the air flow past a tapered valve on the motor stem into the main
system. This stepper motor is popular on some variable venturi
The stepper motor has four field windings, and the travel on the motor
stem and valve depends on which field winding the computer grounds. As
the computer grounds the various field windings, the motor stem and
valve move horizontally in steps with a maximum travel of 0.400 in (1.01
A lean O2 sensor signal causes the computer to move the stepper motor
stem and valve inward to close off some air flow to provide a richer
air-fuel ratio. If the O2 sensor indicates a rich air-fuel ratio, the
computer moves the stepper motor stem outward and allows more air flow
into the main system to make the air-fuel ratio leaner. Since a certain
amount of fuel is moving through the main system with the engine idling
on a Ford variable venturi carburetor, the stepper motor controls air
flow only into the main system.
This should point you in the right direction, anyway.
ALSO: the common failure: from:
The Motorcraft Variable Venturi carb, which was used on various '77
through '82 Fords, gives great response and efficiency. That is, when
Its main weakness is that big vacuum diaphragm, which tends to rupture.
That results in ultra-rich running, a fouled O2 sensor, and perhaps even
an over-filled crankcase from gasoline dilution. An approved
modification that'll make the diaphragm last longer is to drill a drain
hole in the bottom of the cavity 1/8 in from the gasket surface with a
#46 (.081 in.) bit. This eliminates the nasty fluids that eat up the
Another common problem with the 7200 version is a jammed stepper motor
that won't respond to computer commands. You may be able to free it up
with electrical contact cleaner.
An R&D engineer at a company that makes carburetor repair components
gave me a good tip: "When you pull the top off a VV, don't disassemble
it and throw it into the solvent. Instead, use spray cleaner so you
don't have to remove the enrichment rod levers, etc. That way, there'll
be much less to adjust." Actually, the idea of doing no more disassembly
than necessary can be applied to any carb.
PS... the police 5.8 had the VV carb through 1991...
Thank you Rob.
This is surely helpful since I've just got rid of that stepper motor
thinking it was the choke :-)
Haven't looked at the plugs since. Hope it is some solution until I get the
stepper motor running correctly.
The point is that I had the carb rebuilt (using the kit) and it looks pretty
now. I had the stepper motor checked and it should run fine BUT it doesn't.
It might be the computer steering it causing problems or the O2 sensor.
Another thing that comes to my mind is the float level. Do you think it
might be too high?
Thank you for your opinions as it it virtually impossible to get any
constructive help from any wrench down here.
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