Oxygen sensor or ground wire problems. Check to see if there are brown ground wires on the coolant outlet flange on side of head. I used to move them to the intake manifold. Also some were on the manifold and would break. Check them close.
Hi. I have a VW Jetta (gasoline not diesel) '91. 2 days ago, black
> smoke suddenly started blowing from the exhaust. Also, idle became > very rough.
>
> I know this indicates a "rich" intake. Or it indicates at least that a
> large proportion of the fuel is not burnt. I tried changing the coil
> and wires. The distributor cap and rotor are pratically new. The spark
> plugs are fine. I tried this a first because the car experienced
> trouble to start when its humid or raining.
>
> I though that maybe there is a short in the wiring somewhere and the
> spark plugs do not fire consistently causing unburnt fuel problems. >
> However this "solution" changed absolutely nothing. My problem
> remains. Could someone help me on that one. Technically-wise and > price-wise.
>
> Thanks!! AL