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Discussion Starter #1
I've done a lot of searching on this and can't come up with a definitive answer. General car discussions claim that if the lower hose is much lower in temperature from upper, then there's a problem. However, I've seen some Trailblazer specific discussion indicating that this isn't the case with this vehicle and it's normal for it to be cool or luke warm (though some disagreed).

I drained and filled coolant a few weeks ago. Burped the system according to online instructions. Temp gauge shows 210 when driving with heater on and just a hair above 210 with it off. Heater gets good heat and is hot by the time the engine is up to temp (a few minutes of driving). Coolant level has not changed since drain/fill. It increases a little in the reservoir after driving and is back down where I filled it when I check it the next morning.

After a 30 minute drive, upper radiator hose is too hot to grab onto and hold (IR gun shows 190ish) while lower radiator hose is always either cool or just barely has any warmth to it. Is that really normal?

Pretty much any discussion I've found where others are mentioning their lower hose is cool comes with them also having other issues like no heat in the cab or engine overheating. But for mine everything seems to run ok and no overheating.
 

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02 Oldsmobile Bravada, 235,000 miles, vortec 4.2L
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Same problem here, I just put in a new thermostat and now mine runs at 220F (with OBD scanner). The upper hose is boiling hot while the lower one is cool or barely wam. Did you figure it out?
 

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Discussion Starter #3
It has since been totaled in an accident but from reading around, I had found a couple references to it being normal for the lower hose to be ambient temp or just slightly warm on this vehicle. My temp gauge always showed 210-211 once up to temp but I have no idea what the actual temp would've read with a scanner. I recall finding a graph of the coolant flow of this engine and it kind of making sense to me how it works. Something like coolant isn't allowed to flow through the radiator but rather it's flowing through the engine and only small amounts of radiator cooled coolant is being metered into the engine from the lower hose as needed to keep at temp. Something about avoiding shocking the engine by letting free flow of cooled coolant. This method was said to keep it at a more even temp. I would guess eventually enough heat would be generated to move hotter coolant down through the radiator to where the coolant in the radiator bottom would be warmer so more would be mixed in and then the lower hose would be warmer/hot to the touch but I only had 30 minute drives back and forth to work so this never happened while I was keeping an eye on it. I deleted all my files on the vehicle once it was gone or I'd look for that graph/explanation for you. I really started to think the cooler lower hose was indeed normal. I don't know about the 220 temp reading though.
 

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Coolant temp readings before the thermostat and in the lower hose certainly sounds confusing especially if one doesn't understand the concept of the engineering reverse flow design.

These vehicles frequently come with front and rear heater cores and they can dump a lot of heat being generated by the engine. Heated coolant is always routed to the heater core(s) first.
  • That in itself isn't a new idea as it's always designed to offer human comfort before the engine gets all the way up to normal operating temperature.
That the thermostat is on the final end of the flow... from the radiator in this case.... is just different enough in design to create confusion.

So... yes.... the cooler "lower hose" also being the return hose... too is a new normal..... but having a thermostat in that position is a strange concept to some.

The reason why I asked another poster to put his hand around the lower/return hose... and was flippantly dissed for asking the question.... was a teaching point.

Here's where physical observations can solve a lot of mysteries and confusion. For the record, rnranimal is correct... mostly.... it just needs a little fine tuning......


The thermostat could not work if it was just a shut door. Out on the end of the return hose, there'd be no actual coolant flow to open and then close the thermostat.

To correct the failure to have heated coolant at the thermostat...to which a thermostat could react, there is a small passage for a sampling of heated coolant from the radiator. This is usually a small hole in the flange of the thermostat. I am sure the small hole is not modulated; its just a small hole.....

Coolant is always going to have to go past or through the thermostat sensor or it'll have no idea of the temperature of the engine or the coolant.

THAT'S WHY I wanted the OP to hold the hose and feel the different thermal zones happening inside that hose. Heated coolant will usually flow above the denser, cooler coolant.
  • The top of the hose would be warmer than the lower section.
This can be felt by hand .... and as an indicator of different zones of heat, one could prove that the thermostat was operating normally at that point.

Some diagnosis doesn't need a scanner or infrared thermometers... just a hand in the right spot and knowing what one is feeling.... is a pretty good tool too.

Simple.... maybe TOO simple for some persons I guess.

SIDEBAR..... I am of the thought that this reverse, terminal thermostat is a primary source of overheating the transmission when towing. The transmission heat exchanger in the radiator will be operating in a rather static, non-flowing zone of coolant.

This would not be conducive for good heat exchange and it won't protect the transmission during towing or heavy usage.

Research continues.
 
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