i have a 94 gmc k1500 5.7L and it runs around 150 degrees after the thermostat opens. as it is warming the gauge rises to right up around 200 as its supposed to as i am running a 195 thermostat. but after it opens the temp drops around 150 and never goes back up. the heat gets warm but is no where near as hot as it should be. coolant in brand new and theres no leaks....anymore...everything works fine, i just dont understand why it wont stay as hot as it should? ive seen other people with the same issue but havent really heard of any definitive solutions

 

this is a pic i took when rolling 200k but it shows the temp exactly how it normally is when the truck is running all the time after warmed up

<a href="http://s175.photobucket.com/albums/w143/zbkeebler/94%20gmc%20k1500/?action=view&amp;current=IMAG0210.jpg" target="_blank"><img src="http://i175.photobucket.com/albums/w143/zbkeebler/94%20gmc%20k1500/IMAG0210.jpg" border="0" alt="Photobucket"></a>

 

defective thermostat ?

 

i vote Tstat

 

Well I thought it was pretty obvious that it was that too. I've replaced it twice. Both times I made sure that it was a 195 thermostat and it still does it???

 

mine was doing the same thing it turned out being the sending unit for the gauge

 

yea the best way to tell is if you had a scanner to compare coolant temp to the gauge, you could test the senders resistance to be sure. Although the sender should have no effect on heater performance

 

I've replaced both of the sensors already too...such a confusing problem

 

I have the same problem on mine, I'm going to replace my thermostat and if that doesn't help I'm going to replace the fan clutch, I'm starting to wonder if my fan is supposed to run constantly

 

I am also running a low temp, AGAIN, in my 2005 Silverado 5.3. I purchased it from original owner, who i thought told me everything, along with a stack of reciepts! After 6 weeks of below 0 temps and NO HEAT! I started replacing thermostats like they were underwear! In the end I was told by the 4th mechanic ...somewhere along the line the ? (I think I was told water pump? But dont remember, now 3 yrs later) some OTHER part that had been changed prior, required the thermostat to be changed to one that wasnt stock with the vehicle. After believing the first 2 thermostats were defective, and the one guy guessing what to put in, I found someone who took the time to look up the ?? (I wanna say water pump but not sure) and HE FINALLY put the ? for lack of a better word... CUSTOM thermostat (because it matched with the replacemennt part I cant remember.. that was preventing the stock thermostat from working. AS SOON AS HE PUT THE "WRONG thermostat in, I HAD HEAT!! FOR THE 1ST TIME IN 2 MONTHS OF 0 TEMPS!! Another 2 months later moved to Florida and never thought of it again, till 3 years later yesterday. The temp is low at 155° for 2 days now. Its March & only 65-80. So now.. Im not sure whats happening. Again.