For a 97 Blazer :

Open Loop is when it thinks the engine is cold. Air/Fuel ratio is calculated using coolant temp and MAP/MAF sensors. It goes to Closed Loop based on O2 temp, coolant temp, or time (unspecified). Air/Fuel can range from 0.8:1 (over 50% fuel) to 16.8:1 (around 5% fuel)

Closed Loop is when it thinks it's "warm". Air/Fuel ratio is calculated using O2 and "other" sensors. It will enter Open Loop again if O2 cools off, or other sensor failure. Tries to maintain 14.7:1 air/fuel.

Having just replaced my cat, and was seeing mpg of around 15, I'll be getting a new thermostat and make sure I see the right temps

 

i usually run 1/4 all the time. Make sure you always have coolant in your tank, and your radiator.

 

Just changed my radiator to an all aluminum yesterday.
Put in Amsoil antifreeze 60/40 mix. By the way I used the Peak mineral free water.
Not sure if it will make a difference, but I figured since it was all new that I would use it..
Put on all new hoses and a Gates 195 thermostat. It ran at a constant 199 degrees,
whether it was highway driving at 80mph or 30 miles per hour in the city
with the air conditioner blasting. Previous I had the plastic end tanks. It was leaking.
In the old coolant system I ran Sierra(the green stuff) with a 195 thermostat.
It always ran around 198. Use to go up to 210 with the air conditioner /30 mph driving
I always hooked my scanner up to verify the temperatures.

 

I'm happy to find thia post while searching other infos about closed loop..

My 4.3 is running between 190 and 225(while climbing a hill a 60 mph on 3rd gear)..Running 200-210 steady at 60 mph on flat road. I was nervous about overheating, but it removed my stress.

 

Yesterday I was driving around and my blazer went from what I would guess around 190 (which is what it always runs at) to around 210 or maybe just a bit higher.... The strange part was when I got out I heard the overflow bubbling like no other and it even spewed a bit???? I have a new radiator new water pump fresh coolant and am going to replace the thermostat today... Why would it seem to overheat at such a low temp??? Or am I just being paronoid and it didnt overheat?

 

Change the cap while you are at it. One of the most important and most often overlooked piece of the cooling system. If the cap isn't holding the necessary system pressure, the coolant will boil at a much lower temperature.

 

One more thing to add to this, it'll go open loop for power enrichment if you go WOT.

As for my temps, right now it's always a tick or so below 210. It went up a bit after the aux trans cooler was installed.

It has whatever coolant was in it when I bought it plus garden hose water. Yes I know it needs to be flushed soon.

 

actually my mechanic told me, when I discussed this issue with him, that these motors normal operating temperatures are around 228~235 degrees, this was also confirmed through a friend of mine who is an automotive engineer with GM. So we are all running low, and I am having same issue, plus low heat. I need to figure mine out.

 

The GM engineers are the ones who specified the 195* thermostat, you know.

 

I had a 1999 Chevy Blazer and now have a 2000 Chevy Blazer, I did a heater core flush on both of them and there were big time blockages and so much crud came out of them and now i at least get heat. The running temp of my truck is around the first quarter line or maybe right after it but the heats there but it doesent seem to be getting hot as any other vehicles my friends/g/f has etc. Now if the truck sits for a little bit at a light or out in front of the house the heat will get warmer im assuming from the fact of there isent cold air rushing in front of the radiator or motor cooling if off. I know the heater core isent clogged up and there isent anymore debris in my antifreeze so i came up with possibly making a cardboard cutout to block off some of my front grill in the winter with a hole in the middle. Anyone have any suggestions? I feel that should be my last resort and trying to see how everyone else is managing