Thermostat Issues

[Black99]

Tried searching and found similar issues but not quite this exact situation.

-1999 tahoe 5.7
-on cold start the temps will get up to 210 then fall to where they are in the pic (that seems low to me since she used to run 1 tick below 210)
-running temp is where they are in pic, sometimes it dips to the fat tic mark but never lower
-however heat gets HOT within 10 minutes of driving even at this temp

Is this indicative of a bad thermostat? I'm concerned because the truck always used to run 1 tick below 210. It seems like something's wrong with it and the trucks running too cool because my gas mileage has absolutely tanked, it's getting 9-10 mpg.

What's confusing is if it was stuck open, it wouldn't even hit 210 right? Think it's worth replacing it to see if it fixes anything?

Any insight appreciated!

Thanks

 

[swathdiver]

Could be, could be. Might be cold weather and winter blend fuel too no?

 

[Black99]

I was thinking maybe cold weather but I've had the truck 8 years and never seen the coolant temps that low or the mpg that bad, even in winter. I recently had a shop drain and refill my coolant, I really don't think it was doing this before that which is weird

 

[swathdiver]

I was thinking maybe cold weather but I've had the truck 8 years and never seen the coolant temps that low or the mpg that bad, even in winter. I recently had a shop drain and refill my coolant, I really don't think it was doing this before that which is weird

Well, that could have done it or maybe there's an air pocket? Did they keep the Dexcool or use the green stuff? I would check the coolant level and then do a thermostat.

 

[east302]

I’d confirm the actual temperature with a scanner - read the ECT (engine coolant temperature) sensor output. This is the sensor by the thermostat and is the temperature value that the computer uses.

The gauge on the cluster uses the sender on the driver side of the block. The tick marks are not linear so it probably shouldn’t be considered that accurate (quick...look at your photo above and tell what the temperature reads). It’s largely just to alert to a problem. If it helps, my two 98s consistently read where your photo shows but the ECT reports 190-195 usually.

You can try bleeding it in case the shop didn’t get all of the air out. Check YouTube for how-to videos, it’s sometimes messy but not difficult to do.





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[Big Poppa Pump]

This is how my gauges read at 177,000 miles and has read this for as long as I can remember. I replaced the waterpump at 100,000 but I'm not sure if I changed the thermostat but I'd be surprised if I did not. My temp gauge reads the same as yours. This is at idle after 20 minutes driving and the heat will burn me out of the vehicle. I do not have anything that reads the true temp other than my gauge. Please pardon the dust it's wood cutting season.

 

[Wedge]

This is where mine runs after thermostat change and temp sensor. Seems correct to me and I'm plenty warm. It will sweat you out of the vehicle.

 

[Sean James]

This is exactly where my 99 tahoe 5.7 4wd 185K miles sits when driving. It can be 10 degrees F. or 95 degrees F. Always runs at the same place. I put my OBD scanner on it and it says "coolant temp" is right at 190 degress in this position. Had the truck almost 10 years, no change. Changed out the quick connect heater hose on the intake manifold, to a standard 1/2" to 3/4" hose barb last year because it had crusty leakage around the stupid "quick connect" fitting. Installed a Stant 190 Degree "super-stat" and drained and flushed the whole system 3 times, then used regular green coolant while I was at it. No issues so far, and still runs the dash temp gauge in the same position. I would not worry about it. My wife says it used to read higher when it was new, but I don't remember. Could be that with age, the gauge does not read as high as it used to. Buy a scanner or go to autozone and have them put one on it and get the actual reading. Again probably not an issue. Better cool than hot.

 

[McDermut]

I have a 99 Yukon 5.7, what is the typical operating temp on the gauge for city driving or on the highway?

 

[east302]

The computer uses a different sensor than the dash gauge. Dash gauges usually read about halfway between the 100 and 210 marks on my three 98 models.

To me, they’re pretty much just for decoration or to alert you of an oh-crap moment and not for actually reading a specific value.


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