Correct Temp for my 3.1

[Jcdew67]

I have a 195* thermostat in my 1996 Monte Carlo (3.1)

When warming up I can watch the temp needle rise up to 195 and then the thermostat will open and it goes down,it then goes up again and is then fully warmed up.

The problem is once mine warms up it only runs about 180-185 unless I am at a stop and in gear.

I have had it hooked up to my Autoenginuity Laptop and it shows this(I wanted to check my gauge accuracy)

Car runs beautiful but gas mileage is only about 24mpg combined when I used to get 29.

Would that small temp difference affect mileage that much and why would it run a tad cooler most of the time?

The Thermostat is opening at exactly the right temperature, pre O2 sensor is a Bosch and is only about 6 months old.

LTFT are going to negative 6 and 10%

Engine Vacuum is a steady 18" and correct.

[robertisaar]

that small of a temp difference wouldn't account for 5MPG.... but it does sound like the thermostat is getting a bit old/weak. mine did something similar before I replaced it, but it liked to stick closed until ~208 or so, then it shot open, dropped to 190ish, then stabilized around 195.

[Jcdew67]

I originally thought it was the Thermostat so I went with a stant superstat and then a OEM one from the dealer and all do the same. The most I'll proabably do is use another new pre O2 sensor since I can get one from Rockauto for about $19. The car is running the best it ever has so was just curious about the temp.
I have mostly new parts bout within the year like New Radiator,Water Pump,hoses. Ive done a cooling system cleaning and flush and used Prestone 50/50 mix fill,all air bubbles are out.

[robertisaar]

your coolant temp sensor(or PCM) might be suspect then.... a new thermostat should regulate temperatures pretty tightly.

I'm not fond of Bosch O2 sensors with GM PCMs... a lot of people report funny operation with them. oddly enough, the factory units(AFS75 in this case, IIRC) look an awful lot like a Bosch design, but I've had an engineer or two recommend a Denso unit due to their design apparently being able to handle higher temps than what the factory units are rated for.

[Jcdew67]

I went ahead and ordered a Denso from Rockauto since my current O2 is a Bosch. I have a lifetime Warranty Coolant Sensor from Autozone and I will have them swap it.