LQ4 Running Too Cold!

Ok, have a ques for the forum nobody seems to be able to shed some light on so far. Have a 2000 Sierra that won't get up to temp. Gauge sits just above 160, infrared on the water outlet says the same thing. Have tested and installed 2 additional thermostats, each open at 190 or so and close at 180. No change in the vehicle. Water pump and radiator were replaced 9 months or so ago, no problems until recently. Temp will occasionally reach "normal" on the gauge, then drop randomly, mostly on the highway. The engine is physically running cold, and the mileage has suffered as a result. After sitting with the engine off and heat soaking, the temp rises to 174, then after 20 seconds of running, it drops to 155 or so. Any ideas?

electric fans?

Fan clutch?

thats what im thinkin but u would know it if it was the fan clutch cause u could hear it.

I have the engine driven fan, it doesn't roar like the clutch is locked up, and even at idle the heat won't build.

Is it cold where you live?

My truck does NOT heat up in winter. I have the entire radiator covered with cardboard, and it still takes 20 minutes of driving before the OD will engage. I have a 160* stat in, and I barely even hit those temps. I have electric fans, and NEVER use them in the winter. Just something I've learned to deal with.

I'm barely hitting 160 myself this winter in the truck I bought.

Granted, I've changed the water pump and radiator since last winter, but there was no issue with heat at all. Gauge was always rock solid one needle-width below 210. It was just fine in the cold (30-40deg) until the last few weeks. I don't think living with a condition like this is acceptable, something has gone wrong and needs to be addressed. Even in the "Snowmageddon" the local whiners have been whining about here last season didn't faze the heater, so I am certain there is a new development of which I am presently unaware.

Probably had a restriction in the old radiator that was letting it get up to temp. Now you have a more efficient system and it is doing its job. Try blocking off a portion of the radiator with cardboard to get the temps up.