Interesting observation of my thermostat/temperature gauge

[MisterB]

When I bought my truck, it didn't have a thermostat. The temp gauge was always way on the cold side and didn't budge. I bought a 160 degree thermostat, installed it, and flushed the radiator and put in new coolant. This brought the temp gauge to the middle, once the engine was warm, but it seemed like my heater wasn't as hot as it should be. I swapped in a 180 degree thermostat and put in a new 16 lb radiator cap just for good measure. Heater now heats up nicely, and temp gauge stays in the middle once warmed up, except...... The first time I drive the truck for the day and after I have ler it warm up, if I get on a highway going about 50 MPH, my temp touches the first part of the red area for about 30 seconds, and then goes right back down to the mid point and stays there.

My theory is that it climbs that high just before the thermostat opens, and once it does cold coolant from the radiator circulates and brings Temps to normal.

Does this all sound normal, or should I change anything?

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[DanMcG]

Just a guess here, but I bet you have an air pocket trapped in the system.

 

[Matt69olds]

I agree on the air pocket. My second guess, maybe the new t-stat is just lazy to open the first time after a cold start? If your positive there is no air in the cooling system, and it continues to almost overheat before the thermostat opens, I’d replace it.

 

[Nonstop]

While I agree with the above comments, before you start chasing things down, use an IR gun or aftermarket gauge (even temporarily) to find your actual temps before you start getting worried. Then Go from there.

 

[WFO]

I'd say your new thermostat is a little sticky.

 

[shiftpro]

New thermostats are hit and miss, like most new things.

 

[MisterB]

Thanks guys. Seems like the air pocket is the easiest thing to look at first.

Is this the procedure? Start truck when cold, remove radiator cap, put heater on hot/high/defrost. Idle until warm and coolant level in radiator drops. Fill coolant to top and put back on radiator cap

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[Matt69olds]

Sure fire way to get all the air out, jack up the front of the truck, make sure the radiator fill cap is the highest point. Leave it overnight, if there is any air in the system the coolant level will be lowere

 

[Hatchet54]

When I installed my new thermostat I drilled a tiny bleed hole in the surface. This was a trick an old-timer pointed me to which allegedly helps prevent air pockets from forming in the system and therefore avoiding problems like you probably have right now.

This video is pretty good, it mentions how to do the bleed hole that I'm talking about as well as shows a good test to see if your tstat is working right, if you're so inclined.

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[highdesertrange]

lately(for the last 20 years) I have had good luck with AC Delco thermostats. I have had terrible luck with Stant thermostats. heavy duty thermostats have the little bypass hole with a jiggle valve. that's a technical term Jiggle valve. highdesertranger

 

[Camar068]

Thanks guys. Seems like the air pocket is the easiest thing to look at first.

Thanks for bringing this subject up, because my LS did the same thing Thursday morning. Gauge went up to 220 and came down when I slowed from 80'ish to 70'ish on the interstate. I was preped to get another water pump or t'stat. Think we got it though with the air pocket.

 

[MisterB]

I got this all fixed. I went ahead and replaced the lower radiator hose and clamps for good measure (already did the top recently). Did a better job this time of getting the air bubbles out, and she's good to go.

Thanks for all the help

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[80BrownK10]

I had that issue one time in an s15 and it was a sticky thermostat. A new one fixed that issue where it went over normal the first time it opened and then stayed in the middle the rest of the time .