Car in bits again...
AO Silver Member
Join Date: Feb 2007
Location: Hampshire, UK
The air gap in the expansion tank is there to ensure that there is sufficient compressed air to force the coolant back into the radiator as it cools, but as Lonewolf says, with the engine hot it is quite acceptable for the level to be over the line.
Different makes of coolant additive may make a difference to the engine temperature. I was sceptical about the use of wetting agents in the coolant, until I saw what it could do - running temperatures can be reduced by several degrees. I wouldn't be surprised if some of the corrosion inhibitors and/or antifreeze agents have a wetting action.
(a wetting agent breaks down the surface tension of the coolant, increasing the heat transfer capability - aftermarket wetting additives are available too).
Change in temperature/oil pressure/fuel level with varying electrical load is a fairly common occurrence - there should be a voltage regulator (sometimes called an Instrument Voltage Stabiliser or IVS) connected to the instrument cluster. Typically it goes on the fuel guage and tries to control the voltage supplied to all the instruments. If the supply voltage fluctuates then the reading will vary with it, even if the sensor is providing a constant output.
I have a non-Alfa set of instrumentation in the Green Machine, with better linear regulation too I might add, but that doesn't stop my oil pressure dropping by 4 psi when I switch the headlights on! Its a voltage drop in the circuit, like Lonewolf said. Changing the instrument won't necessarily make a difference, unless the new instrument has better voltage regulation. This might also be the reason for your observed increase in engine temperature, so it is important to make comparisons under the same electrical load conditions.
As a final thought, the temperature sender itself may be faulty. I have had sender units fail by drifting toward short-circuit (shows as increased engine temperature), so don't panic just yet!
(I've also had oil pressure senders drop to very low indicated value, even though a capilliary guage showed 60 psi at idle!!!)
Alfanauts are GO!!!