low and irregular operating temp

now that our daily temps are in the teens my 2004 gmc 1500 sierra with the small v8 won't maintain normal operation temp of 200 degrees. It will reach
200 going up hills or running on the interstate but immediately starts to cool down when coasting downhill or driving around town. I got the dealer to install a second new thermostat yesterday but if anything that just made it harder to warm up, I drove around town for 1 1/2 hours last evening at about 170 degrees. Now I know that ain't right and i think it effects my gas mileage too. So what should I do?
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If you are relying on the factory gauge for correct temp: those are unreliable. It may be out of calibration. Since you paid someone to replace the t-stat, do you know if the correct 'range' unit was installed? For good heater operation, at least a 180 stat is needed. If you are in a really cold area, a 190 stat is better. I'd start with making sure the gauge is accurate....NOT(normal operating temp) in a well-maintained cooling system is 210-220. Hope this is of some use.....
Dave S(Texas)
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thanks putt-yah the gauge works and was backed up by the dealers computer, and I'm on the third tstat already. All spring summer and fall it runs about 205 but when the outside temp falls below about 20 the truck won't maintain a steady temperature while I'm driving around. is still under warranty but pissing me off

If you are relying on the factory gauge for correct temp: those are unreliable. It may be out of calibration. Since you paid someone to replace the t-stat, do you know if the correct 'range' unit was installed? For good heater operation, at least a 180 stat is needed. If you are in a really cold area, a 190 stat is better. I'd start with making sure the gauge is accurate....NOT(normal operating temp) in a well-maintained cooling system is 210-220. Hope this is of some use.....
Dave S(Texas)
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It probably is just running efficiently and not wasting a lot of gas making heat. Some gauges will fluctuate depending on where they are in the block. What temperature does the OBDII tester show? That is what is controlling the engine. What temp thermostat is in the engine? It is possible your fan is running all the time causing excessive cooling.

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the mechanic's tool plugged into my truck (I guess that's his OBD II), mirrored the gauge in the dash. you can see the temperature drop while the truck idles (right after I finally got it up to 200 on a long hill). They told me the correct thermostats were installed. I'm especially concerned because my gas mileage drops from about 17.5 mpg to 13 mpg when this cooling problem is going on. It could be related. I'll check to see if the fan isn't shutting off. The service manager is checking to see if somehow the wrong radiator got installed during production. Thanks for your help.

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If an OBD II is the tool the mechanic plugged into my truck it showed the same temp change as my gauge. After I finally got it up to 200 the tool showed it cooling off while at idle too. My mileage drops from about 17.5 mpg to about 13.5 mpg at the temperatures (below 20) when this cooling thing goes on. It might be related, I don't think it should drop that much just because it's cold outside. I think the truck needs to run over 200 for the computer to send the right fuel mixture. I'll check the fan to see if it is not shutting off. They tell me the correct thermostats have been installed and are looking in to seeing if somehow the wrong radiator got installed during production. Thanks for your help.

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geezer wrote:

Engine temp is supposed to be maintained by the thermostat by regulating the amount of water flowing through the radiator. Too large of a radiator should not matter unless too much water were getting past the thermostat into the radiator via the bypass hoses/pipes. I wonder if there should have been a restrictor in the bypass that got left out at the factory?
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there is there a bypass around the thermostat? I wondered because I thought like you about the radiator, but i couldn't imagine this problem going on after 3 thermostats. to my way of thinking there has to be a bypass and it has to be wide open, maybe it's in the radiator, i have no idea. But even just now i drove 30 minutes and 15 miles and it barely got over 160 degrees and it is 33 outside. I'll tell/ask the service guys about the bypass tomorrow- thanks
" Paul " <"=?x-user-defined?Q??= Paul

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