Serge, couple comments about C4 cooling system operation.
Its EXTREMELY common to have them all butchered up good.
Reason is, that GM switched locations and setups around mjltiple times in the early cars. Before the time of the interwebz, it was HARD to sort through all this. And so owners would give up, bypass relays, run custom buttons, and generally make a mess out of a perfectly good system.
However, the C4 is a very tight package under the hood. If the cooling systems were not maintained (most were not. these were getting real old and unpopular in the mid 2000’s) the cars WILL overheat.
Numerous overheat causes. Ive had it happen to that 4+3 ironhead (your 85 is ironhead too).
- old fan motor begins to draw too much current, and the fuseable link burns on its way to the relay.
- fan motor dies
- relay gets seized gummed up or burnt up if someone previously removed the fuseable link.
- leaves. needles. plastic bags. nests. grass. gets lodged between the ac condenser and the radiator. grab a flashlight and look in there. get in there real good and see what material lies sandwiched inside the shroud.
- missing front air dam. ive seen this lead to high temps but not to overheats.
- plugged/clogged catalytic converter. (in this case, u will have sky high oil temps as well)
- ive never seen the ecu pinout not working but its possible.
-common to have a faulty rad cap. Even more common to have a rotted/unsealed overflow hose. This allows coolant to escape but not to return due to a vac leak. before you know it, coolant in the rad is low and u boil over. Vac leaks also common at waterpump gasket leaks, and intake manifold gasket leaks. these lead to issues that result in an overheat eventually.
-common to overheat due to needed a few burp/vent cycles after a drain/fill.
- weak water pump or plugged rad (nothing specific to c4 there).
- and ofcourse a blown 7 or 8 head gasket. my 85 had this. that was an adventure.
The factory fan does not even come on until 108d Celsius. that in and of itself gives many the heebie jeebies and causes all these manual switches to be installed to override the factory system. even though 108c is perfectly fine on a c4.
There a quick list of things to check as u r getting to know ur new summer ride.