Re: 75 t-spark motronic 4.1

[Lars Arvidsson <arvid@domain.elided>]

[John wrote]
75 T-spark idles Ok  but takes a couple of over shoots to settle down
however it is then stable.
when opening throttle there is a lag then surging up to desired revs
dropping back to near 1000rpm
anyone with any ideas?
I haven't had time to look at it yet hope to check it out tomorrow

[Lars replies]
I suggest check the throttle position switch, it's located on the the
throttle housing.
That switch tells the ECU when you're in idle and when not.
If the switch says "idle", but the throttle is opened, the ECU tries to
maintain the rpm at 850
by means of adjusting the idle control valve (that silver cylinder,
located at the plenum chamber)
But when the ICV is fully closed, and the rpm still is too high, the ECU
cant cope with it, the result is you get far too little
fuel, then the enginge is surging. The situation is like a air leak at
idle, because the ECU doesn't know that it's the throttle
that's letting the air through.
I replaced mine a few months ago, it was intermittent failing, mainly
when hot. Costed about 120 USD.
If you're brave, try to open it, inside is an ordinary microswitch, wich
costs just a dollar.
The other failure mode is, of course, the opposit; the TPS doesn't tell
the ECU that the throttle is in idle position.
Then you get no idle at all, no extra air through the ICV. If the TPS is
sticking, both types of failure often occour.

It's easy to check, and to replace.
Check: First, with no engine on, slowly open the trottle by hand and
listen. There should be a "clic" just above idle position.
If not, replace the switch.
To measure: remove the air filter cover, air flow meter and rubber
manifold. Disconnect the connector from the TPS.
use a resistance meter, measure between the middle and one of the other
terminals (cant remember wich one, try both). At idle position,
there should be zero ohm, above idle infinite. Repeat the checking
several clicks, they can be eye-servants...

good luck

Lars Arvidsson   Alfa 75 TS 1990
[arvid@domain.elided]

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