2006 Chevy Tahoe Coolant temperature low

[Mfriend3]

I got a 2006 Tahoe that needed an engine.  found a remanufactured (z code engine) had installed with new water pump, radiator and thermostat ( have replaced 3 times with factory 187 degree stat). Now the engine runs about 165 to 175 on the road. When idling it will get up to about 210 and will stay there all day idling fans turning on and off working normally. But when I drive temp drops back down to 165-175. Has anybody sean this and know what the issue might be? I’m checking coolant temperature with ODBII scanner and gage. 

Edited May 16, 2021 by Mfriend3
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[Sharpz]

If you are positive you have no air in the system and the gauge doesnt jump past 210, you are within specs.  Your heater works as it should I am guessing since you didnt list that as a symptom.  Since you have went through and did all this work, you may want to stick another coolant temp sensor in, for what they cost, it would eliminate at least one possible issue to whats going on with your gauge.  

[Mfriend3]

I have replaced the coolant temp sensor and harness when replacing the engine. The heater gets warm not as hot as I would think but is putting out heat.  As far as bleeding all the air I have been trying to find the best way to make sure it’s all out but it’s hard to know. As many times as I have drained and filled the system you would hope I don’t have any air pockets. 
Thanks for help alway open to suggestions. This is driving me crazy. 

[Sharpz]

It should get hot, a clogged Heater core might be giving you issues.  Get a pipe and bypass the core and see if this resolves your issue.  

[Mfriend3]

I will do that it does make sense. I will have to figure out how to bypass heater hoses but the hose to the surge tank (return)connects to the heater hoses. 

[Sharpz]

3 hours ago, Mfriend3 said:

I will do that it does make sense. I will have to figure out how to bypass heater hoses but the hose to the surge tank (return)connects to the heater hoses. 

2 things you can do.  First get the HVAC coupler removal tools, and remove the heater hoses where they plug into the HVAC.  Then get a pipe that they will fit (its only to bypass the Heater core for the test.  The other thing you can do, if the hoses are old, and its been a while since you have changed them, go ahead and just cut the hoses on inlet and outlet about 8 - 12 inches from where they plug into the HVAC and get a pipe to just slide them over using screw down clamps.  Keep in mind that a known issue is the plastic coupler for the inlet is a common fail point, so you may just want to plan on replacing that coupler, anyway.   This should allow you to bleed out the system again, and get a reading on your temp issue. Obviously if the temp is stable it would have been because of air in the system or your heater core.  Flushing it out with the lines off with a garden hose on the outlet side without high pressure sometimes helps to free any clogs. 

[Mfriend3]

Well I decided before I bypassed the heater.  I would try and restrict the return hose from the coolant surge tank to the water pump/ heater hose. Then started it up and let it run came right up to temperature about 197 degrees and then I drove it stayed around that temperature fluctuates a few degrees up and down. Not sure if this is a fix but I will drive around some and test. I’m sure I will need to adjust how much flow and see how it does. I’m still looking for a fix and other suggestions or ideas. 
Thanks everyone for your advice!

[Mfriend3]

I believe it is fixed and it turns out that the radiator has a restrictor on the small hose that goes to the surge tank. But what I think is that the restrictor was letting to much coolant flow to the surge tank and bypassing the thermostat and keeping the engine cool. I added a brass shutoff to that hose and the temperature I’d now running around 195-200 and I can adjust the flow and temperature. Thanks for everyone’s help. I will let you know if I have any other issues.