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2 Things--Fuel Gauge and Thermostat

03 Chevy Venture with 126k. I noticed a couple weeks ago that the fuel gauge isn’t working right. It seems to be okay until it’s at 1/4 tank. Then it bounces back and forth (sometimes more than others) around 1/2. Everything else on the instrument panel seems to be working.

AND, It had a Check Engine light come on today. The parts store said the coolant is running cool and said I probably needed to replace the thermostat. Does this seem likely? I had the intake manifold replaced about two months ago. Would the thermostat be included with the intake manifold replacement? At the same time, I had them flush out the DexCool and put in Green, which is looking very murky and dirty now. Is this all related or is everything falling apart at once?

Most of the time an erratic fuel gauge is caused by the gas tank sending unit. This is likely part of the fuel pump module and if the tank is dropped the entire module should be replaced along with the fuel filter.

It’s quite possible that depending on the code the thermostat could be at fault. If they stick open the engine will run cooler and this can lower the gas mileage.
My opinion is that thermostats are a maintenance item that should be changed every 3-4 years whether they’re bad or not.

The T-stat may not officially be part of the intake job but it should be; just as it should be whenever there is a head gasket problem, radiator or water pump replacement, etc. Many engines have been damaged or totally ruined by an inexpensive T-stat that chose to stick closed instead of open and this can happen at any time.