97 Honda Civic. Non-VTEC
OK, here is my dilemma.
New radiator-check
New Thermostat-check
New Fan Switch-check
Tested Fan, it works.
New 50/50 coolant and air bled out to the best of my knowledge.

Car will run fine at operating temperature on highway and basic driving for 30-40 min.

After said time, car begins to overheat the drop quickly back to operating temperature randomly. If car starts to get really hot, heater stops blowing hot air. Sometimes when it reads very hot, radiator fan does not even come on.

Do I need to replace coolant temperature sensor? If not, what the Hell is causing this?

Thank you all. All advise is welcome.

 

Sounds like the coolant temp sensor might be to blame. If you have a volt meter you can test it. http://www.doityourself.com/stry/how...ature-sensor#b

Did you install the thermostat backwards?

Have you ever had the cooling system pressure tested to verify that there is not a leak some where that can not be seen?

How many miles are on the current water pump?

Have you tested the coolant for hydrocarbons? Prior to changing it would have been best.

 

Pretty sure I put the thermostat in right. Spring end towards engine.

Water pump is original I think. But no leaking or winding sound.

Never tested the pressure or hydrocarbons. What the price like on that?

I rarely take my cars to a shop but this symptom blows my mind.

 

Spring end towards the engine is correct.

How many miles are on the water pump? I know of people running around with 300,000 miles on their original ones, which I concede is perfectly fine provided the blades are checked and the gasket replaced every time the timing belt is changed. The only thing that concerns me a bit in your case is if a previous owner didn't care much about the car and ran just straight water or never changed the coolant for many years. Doing so can corrode the blades on the water pump to the point that they can no longer effectively move the coolant through the engine.

I can't honestly give you a solid $ amount on what a shop would charge just to pressure test a cooling system or just test a cooling system for hydrocarbons. Typically those procedures are done in the process of diagnosing a problem. People generally don't come in specifically to get their cooling system pressure checked or tested for hydrocarbons. If you are a home mechanic or a diy kind of person it might be worth your while to purchase this kit: http://www.harborfreight.com/radiato...kit-69258.html and just return it after you are done. Think of it as renting a tool for the cost of gas to get to the store and back twice. I love harbor freight for that reason. 90 days no questions asked return policy. Great for diy mechanics that need a specialty tool one time every 10 years.

Testing for hydrocarbons will be a waste of money if the vehicle has brand new coolant and you don't have the old coolant.

I would start with testing the coolant temp sensor via the method I linked in my first post. If you don't have a volt meter, borrow one from a neighbor for 10 mins.