A long while ago, I had the heater core in my 2-Door, 4x4, S10 Blazer crack, and since then I have been running the heater hoses in bypass. This happened about 6 months ago, and I've been waiting for the chance to decommission my Blazer for a few days (because it's my only daily driver) so I can take time pulling the dash apart.

I took the dash completely out. What an adventure that was... Probably unnecessary, but I wanted to do a clean job without bending or breaking plastic.

I finished installing a new heater about 2 months ago, and since then I left the heater hoses in bypass until I can look into replacing all the coolant hoses. The previous owner never did proper coolant flush when it had the old dex cool in it, and all the old hoses were literally crumbling apart on the inside. The heater core was clogged when I bought it, and I unclogged it.

This past weekend I finally replaced the coolant hoses and the thermostat because of code P0128 - Engine taking too long to reach operating temperature. So, I thought its stuck open. I'm in Florida, so I don't have to worry about extreme cold.

I gave my Blazer a good test run today, about 20 minutes to and from my destination (city driving), and noticed the temperature gauge is noticeably running hotter. I have a bluetooth OBDII adapter I use regularly, and I clocked about a 20 degree increase in coolant temperature. For this test drive I was running max A/C for the whole duration and clocked the temperature at high 180s, low 190s. About a tick and a half from the center 210 marker. The first quarter mark is around 160, and my usual operating temperature when the heater hoses were in bypass and the faulty thermostat is just after that first quarter mark, high 160s low 170s, when running no A/C on a cool enough day, and high 170s running the A/C on an average hot day.

Today was 83 at peak, so I'm concerned about the temperature when Florida gets into it's hotter months in city driving.

I checked the coolant levels when I got back. Reservoir level were fine, and after cooling off radiator levels were fine. So no leak.
Just wondering if this normal operating temperature when all the coolant systems are working as intended. I'm weary about running it close to 210 in fear of over heating. The previous owner already run it hot before. All the plastic in the engine bay is so brittle it falls apart when touched.

 

What is the temperature of your thermostat? These came with 195 ones so they tend to run close to that temp. Some years will throw a code if the temp doesn't get hot enough in a certain time frame.

 

Is that suppose to be the threshold temperature. I wasn't aware.
This is what I bought.

I bought that one because I know AC Delco is the OEM manufacturer, and Amazon says it fits my 2002 Blazer..

After driving for about an hour today, it seemed to hold stable in the high 190s, not getting any hotter.

 

Does the 2002, 2-door, 4x4 model with the thermostat just above the water pump need a gasket? I saw a couple YouTube videos on this where they didn't install a gasket with the new thermostat, but that was for a different kind of S10 where the thermostat is off to the side under the airbox.
I'm asking cause I noticed a few wet spots around the thermostat hosing today. No hard leaks yet.

 

No gasket I know of, but there is a rubber ring around the thermostat itself.