I am going to try to keep this as narrowed as possible, as far as all the suggestions and methods to diagnose issues in the cooling system, I have tried them all, and more than once.

The patient, an 87 Civic 4 door, with the engine of the 88-90 generation.It has the old 87 radiator, the one large one. with a brand new fan as well.
Here is my scenario, about a month ago we had to replace the water pump, it was leaking, so we did that and the timing belt. Thermostat is also less than 1 year old. (I even took it out to see if it opened in a pot of boiling water and it did, it is a OEM)

If I leave the car sitting on idle, for even 1 and a half hours, temp gauge needle never goes past the half mark, it stays a bit lower, both fans kick in, the radiator hoses both look good, now if I take the car and drive it in town it behaves normal, but if I enter the highway and go, let's say 60 mph, temperature begins to climb to 3/4. And no matter how easy I take it on her, it does not go down, if I stop and open the hood the fans are working, the reservoir is not boiling or anything abnormal that I can notice (with my limited knowledge)

I wonder, could this be a clogged radiator, a bad temp sensor unit? I am lost and to be honest a little tight with my budget to take it to the mechanic if it is perhaps something that I can do myself. I am not cheap, but times are not the best right now.

And to finalize, I checked for a possible blown HG, with the only methods I could (no milky oil cap, no oil traces in the coolant and no white smoke)

Any suggestions I would very much appreciate, and sorry if I ran around the bushes a lot and for my broken english.

Thanks!

 

Test for hydrocarbons in the coolant. I had same symptoms and a new head gasket cured it.