I was going to post a separate thread to solicit second opinions, but this one seems about right.
I'm sprucing up my old 95 Trooper and our 95 Nissan truck this year. I'm currently waiting until next week to take it back to the repair shop; a mechanic there damaged connections in my wiring harness, and I'm holding their feet to the fire for returning the car with operational horn and windshield wiper/washer.
The problem with the wiring harness arose because the same mechanic broke my heater core in process of replacing valve cover gaskets. He replaced the heater core and was careless with the dashboard and wiring harness.
None of this is the fault of the Trooper, nor is it my fault, other than authorizing a valve cover gasket replacement which might have waited another year or two.
NOW is our hot season in So-Cal, and I'm running the AC a lot. All fine with that, but I've begun to notice that the radiator overflow reservoir fills up too much after running the car, parking it and letting it cool off. And when I remove the radiator cap, there is an air pocket at the top of the radiator.
I'm not losing radiator fluid, except when it spits out the top of the reservoir.
The oil is clean, without any contamination from the cooling system. No emulsified stuff seen when peering through the oil filler hole in the valve cover, so no indication there of a blown headgasket.
The coolant is pure and clean as can be expected; there's no oil emulsion in it that would suggest a blown head gasket.
Nothing is coming from the exhaust except a little pure water while the car is warming up, and regular exhaust gases -- again -- no indication of a blown head gasket.
I replaced the radiator cap, but this situation persists. Next week, I'm going to replace the thermostat, once I received my parts in the mail from RockAuto.
When I fill the radiator back to the brim and allow the car to warm up before replacing the cap, I may notice the fluid jump a little bit here and there, but it isn't gas bubbles. It appears more consistent with a thermostat opening/closing erratically. The prevailing wisdom suggests that a head gasket leak causing exhaust gas to enter the cooling system would show "profuse bubbling" at the radiator top, and this isn't happening.
So you can comment at will about these remarks. But here is my main question.
The Trooper OE thermostat is rated at 170F. The factory shop manual spec "valve opening temperature" is 166F to 173F. I ordered two thermostats and gaskets: a 170 and a 180.
Should I try the 180F thermostat? And if I do, would I only expect that idle engine temperatures will only increase slightly so the temperature needle on the dash will peg a tiny bit higher? Or should I stick with the OE-spec 170F thermostat?
Update: Still interested in comment or second-opinion answers. However, it looks as though my symptom was caused by overflow hose deterioration at the radiator cap. The cooling radiator will suck as much air from such a leak as fluid it can draw from the reservoir. Will soon know for sure . . .
So . . . 170F or 180F thermostat?