I found a resistor soldered in-line on one of the wires running to the group of sensors on the thermostat housing. I had been searching for a poor connection possibly causing my car's rough idle, and found that the idle slightly surged when I moved one of the temperature sensor wires. I removed the electricalc tape and found a resistor soldered in-line with the wire. Does anyone know what this resistor is for and how significant it is to the performance of the car's idle? Any advice much appreciated.

The resister was a factory suggested / dealer installed fix to fool the Motronic

The long term inpact is that, if the resister was soldered in with a "cold solder", it will eventualy break contact giving an "open circuit". The open circuit will lead to a VERY VERY RICH condition. (BTDT)

The second reason the circuit may fail, is that the resister most likely has solid wire leads, versus the multi-strand twisted wire in the original wiring. Over time, the vibration causes the solid wire to fatigue and break. Again, causing an open circuit.

The problem here is that the temp circuit wires and insulation are becomming very brittle and are buried behind other coverings. As the various ionsulations breakdown, they may even short-circuit against each other causing all kinds of havac. So about the only way to check them is by dis-assembling the wire bundles and repairing the whole lot, or chasing ohm readings between the Motronic and the sending unit.

Regards

John Stein