[alfa] RE: Bosch EFI backfire on decel

["John Fielding" <johnf@xxxxxxxxxxxxxxx>]

Hi Alan,

The idle switch mounted on the throttle body is a critical item in the Bosch
L-Jetronic and Motronic systems.  To quote from the Bosch Service Manual for
the Alfetta and Spider.

Cut-Off Device (idle contact of throttle switch)
During deceleration, when the completely closed throttle hits the limit stop,
a microswitch sends a signal to the ECU, which holds the injectors closed,
until the engine speed is reduced to the rate computed by the ECU, taking into
account coolant temperature.

Full-Load contact of throttle switch
Microswitch is tripped when the throttle is in full-load position.  The
associated electric signalcauses a further increase in fueling".

Check the idle switch setting on the throttle shaft.  The closed position,
when the engine speed is over 1500rpm, should shut off the fuel to the
injectors to prevent backfiring.  The setting of the switch requires several
steps to get the correct setting.

Remove the air hose to the throttle body and check the gap between the
butterfly plate and the body.  With a feeler gauge of 0.5mm (20mil) inserted
between the plate and body set the throttle stop screw to just nip the gauge.
Lock the throttle stop screw. Refit the hose and run the engine so it is at
full operating temperature, the radiator cooling fan should cut in and out to
signify that the engine is at the correct temperature.  Set the idle speed
with the air bypass screw on the throttle body. Slacken off the lock nut and
rotate the inner to set the idle speed.  Set the idle speed to 850 to 950rpm
and lock the adjuster.

With the engine off slowly open the throttle plate, the microswitch should be
heard to click when the throttle is taken off the closed position.  Slacken
the two screws holding the switch body to the throttle body and rotate the
switch body until you hear the switch close, a click should be heard.  Tighten
the two screws and check the switch clicks when the throttle is cracked open.
If you pull off the 3 pin connector and probe the switch pins with a DVM on
ohms you can confirm the switch is closed at idle.  Also check the WOT switch
is closing when the throttle is opened more than 72 degrees.  The connector is
marked with the switch contacts, pin 18 (the center pin) is the common part of
the switch, pin 2 is the idle switch and pin 3 is the WOT switch.  With the
engine off and the throttle closed you should have continuity between pins 18
& 2 and nothing between 18 & 3.  With the throttle fully open you should have
continuity between 18 & 3 and nothing between 18 & 2.

If the closed throttle switch is incorrectly adjusted this also gives the
surging at constant speed noted by another digestee.

I hope this helps.

John
Durban
South Africa
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