I had the engine light come on with the P0128 code indicating the monte 2005 SS supercharged 3.8l was not getting up to temp fast enough, or not getting up to temp period. It is quite cold here, maybe 10 degrees. I have had thermostats fail in other vehicles where the heat does not blow hot. However, the heat blows hot and when I check the temp on a scanner it reads around 170-180 degrees. The previous owner had the thermostat replaced about 5 years ago/25k miles ago, on the garage bill it says the thermostat is 195 degree. What is the normal operating temperature of a 2005 monte carlo SS? It has been running at this 180ish temp since I've bought it last summer when it was warm outside. I live in Wisconsin so cold winters and hot summers. I see some people run 180 degree thermos and some run stock around 195, whats best for a variable climate? It also has a K&N cold air intake on it, saw some other threads saying that also can affect operating temperature. Any advice/info is greatly appreciated.

 

Should run right around 200. 180 sounds too low, especially since you said it runs that low in the summer too.

Personally, Id pop the stat and replace it. A few possibilities:
-they stuck in a 180 by mistake (box mislabeled, etc)
-the previous owner swapped it as a 'performance upgrade' after the fact
-they put in a drilled thermostat (used to be fairly common in the 3800s - but too big of a hole can certainly trip that code, especially in cooler temps)
-Something is causing it not to seat / seal properly

If that doesnt work, then Id probably take a look at the temp sensor itself - maybe its got some buildup on it or something preventing it from getting a true reading. Theyre cheap enough it may be worth doing both at the same time since theyre right next to each other so you only have to drain down the coolant once.


I'd stick a stock 195 in there. IMO theres no benefit to messing around with t stat temperature on a practically stock car. Also with how cold it gets there, Id want 100% of the stock heat output.

 

Thank you for the input. I ordered an AC Delco thermostat with gasket. From all the videos I watched, it seems you don't need a paper gasket on the thermostat housing. Is this correct? I am picking one up from NAPA just in case so I don't have a non functioning car.

 

Strangely enough, the thermostat was stamped 193 degrees. I put in the stock ac Delco and had running temps from 190-200 which seems right. It's 15 degrees here today. Heat feels better. What was curious was the old one didn't look stuck open like I have seen in the past on bad thermostats. When I started it up, the upper radiator hose stayed cold by radiator until it got in that 195 range. All is good! Didn't even bother with the sensor since it was somewhat hard to get to.

 

I did put in the paper stick gasket from napa. It didn't fit perfectly so I took a razor blade and shaved off the bit that was in the path of the flow. Old thermostat looked like it just had some paste gasket on it. Torqued it to 20 ft lbs like the shop manual said. No leaks.