92 s10 4.3 tbi with 250k miles ... So my heater core was leaking, changed it, no issue there. Now since then it gets too hot on the freeway and warmer than normal around town. Digital gauge starts blinking after about 10 minutes at 60mph or 1500 rpm. always stayed one notch above halfway on the gauge before the heater core never hotter. Flushed and back flushed both radiator and engine separately. Changed stat but didn't drill an air bleed hole in it. Changed gauge sensor but not ecm temp sensor, it's barely two years old. Measured ohms on that when hot and was just above 200 ohms. Changed pump and upper radiator hose. That sucks in just slightly when I Rev the engine but not much. Tried running without the stat and of course stays way too cold but no feeling of the hose collapsing. Water gushes through the radiator without the stat looking with the cap off (also replaced that). With the stat in flow isn't very noticeable and the level seems go up and down watching with the cap off, sometimes gushes over. Tested that in boiling water and it seems to open all the way . Thought maybe head gasket but did block check to see if emissions going in rad and came out clean. No loss of coolant and none out the tail pipe or in the oil. Coolant clean. Flow is all that makes sense with it getting hot on the freeway. Could I be missing something? I believe most all air is out of the cooling system. Doing some serious head scratching at this point. Hope someone has an idea.

 

air in system or fan clutch? but because you had the system open and only replaced the heater core it really sounds like air in system

 

Fan clutch seems OK. Air is the only thing I can think, like a pocket moving around because it seems to act erratic. If it's over heating amd I rev the engine to about 3500 the temp will drop a good 40 degrees in a matter of 5 seconds or less. But now of course since the weather hasn't been over 50 degrees lately the temp stays normal. Stays around 200, sometimes up to 240 on the freeway but then drops back down on its own even at a steady speed.

Not sure how is going to act again once the weather starts warming back up. I would think since I posted this the air would have worked out by now, so it makes me think it's not necessarily air that's the issue.

 

have you checked the radiator for external clogs? if it only gets hot going down the highway its usually an airflow/radiator issue if you can leave it idle for a long time without overheating it would eliminate the fan, thermostat and air pocket possibilities.


Are the radiator and hoses getting hot physically? Have you checked into the ect sensor at all? The gauge/sensor may just be acting up if its not getting hot

 

Since I originally posted I replaced both sensors. Tried 3 different thermostats, replaced upper hose, flushed and back flushed radiator and engine. Upper hose does get hot lower of course a little cooler. Can't remember if I mentioned replacing the pump. Ran without stat and flows real fast looking with the cap off, so it doesn't seem like restriction. Temp won't even lift the needle on the mechanical gauge I hooked up, it starts at 100 I believe. No debris blocking air flow. Only thing not replaced is the lower hose (spring inside still intact) and the radiator (it's maybe 4 years old and coolant never been dirty since. Also flowed water out as fast as it would go in with a garden hose, took out the radiator to flush it).