This winter i've had issues with my truck taking a long time to warm up and sometimes start to cool(below point where tstat would be closed) while idling depending on what setting I have the heater at. Motor is a bone stock 04 4.8.

Once fully heated the temp averages 198*, however on a cold, usually below 40* day the engine will take a longer than usual time to heat up. If I come to a stop light or sit for awhile at idle and the heat setting at 3 or above the engine temp will fall to as much at 160*. I did a search and can't find anybody with this specific issue. So far i've replaced the Tstat with another stock 195* stat and it did nothing. Try another?

The only light I can shed is the upper radiator hose will get warm long before the engine hits 195*

 

Thermostat maybe stuck open, do you have a clutched fan? Is the cab heater on full blast?

 

clutch fan, usually wait till the temp is on its way up before full blast, but once its warm full blast it will bring temps back down.. Maybe this is normal given its a small v8, however my last truck which was a vortec 350 could be run full blast all day at idle.

 

Same with mine but in this cold weather it never sees over about 170 and takes 20-30 minuets of driving to get it close.

I think im going to go ahead and put a new t stat in and see what happends. I hope its not the clutch stuck on.

If the fan clutch was stuck on you would hear that right?

 

You'd know if the clutch was stuck, souns like a jet taking off. Sometimes you'll hear it when you first start it up but then it'll quiet down.

Sounds like your having the same issue, maybe worse. Once you change it let me know, did nothing for me so far.

 

Hey I just changed it and now its working and warms up to about 200 then drops to about 195. And the T-stat is 187 so.

Now ill see if it affected my timing, I might be getting more knock now but havent looked on the scanner yet.