matt111399

1990 Jetta 1.8L auto - 122k miles, idles ok when in neutral, seems to idle rough while in gear (stopped at a traffic light). All tune up items have been replaced including fuel filter - any ideas? Does this thing have a idle air control valve/sensor? What about this idle adjustment screw - does my fuel injected model have one of these?

boschmann

Make sure the air boot between the filter box & intake manifold doesn't have a crack in it first. I'd also clean the throttle body valve with a toothbrush & TB cleaner. There is an auxillary air valve that can be easily removed & cleaned. It is the approx. 6" long cylindrical thing with 1" vacuum hoses & 2 pin electrical connector located above & slightly behind the valve cover. If all that fails then you can try turning the adjustment screw on the backside of the TB, don't turn the hidden screw on the air flow meter.

matt111399

thanks for the info boschmann - i'll check that. you were saying that the adjustment screw is on the back of the TB? i haven't seen that one, but i'll check it too. i have checked the air boot and found no cracks, but i did find that there was oil in the bottom of that air intake boot - ? is that telling me my pcv valve or something is blocked and not working correctly? thanks

boschmann

I was thinking of a CIS-E system's idle screw in the back of the manifold, your's won't have that, it should be Digi II. It should have an adle air by-pass screw in the throttle valve. There is also an idle switch & full throttle switch on the valve. Check them for continuity under closed and wide open throttle respectively.