Thermostat question

Mirror

Original poster
Member
Jan 27, 2014
335
I recently replaced my thermostat. My temp guage was reading properly and i was getting decent heat. I had my check engine light come on again the other day and it was my thermostat again. Been noticing my truck isnt pushing out hot air anymore but warm air. Cleared the pcm and light has stayed off but still only warm air. Either the thermostat is already f'd or could it be the temp sensor?
 

Blckshdw

Moderator
Nov 20, 2011
10,665
Tampa Bay Area, FL
What does the dummy gauge on your dash say now? Do you have a scanner so you can see the exact temp being reported for comparison? Maybe your heater core is clogged, and isn't getting good circulation of coolant after your truck gets warmed up?
 

Mirror

Original poster
Member
Jan 27, 2014
335
The needle reads halfway to 100 now. It used to read 100 when i replaced the thermostat
 

Blckshdw

Moderator
Nov 20, 2011
10,665
Tampa Bay Area, FL
Definitely sounds like a bad T-stat. Was the replacement OEM or aftermarket?
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
Have you checked the coolant level since you changed the t-stat?
 

Mirror

Original poster
Member
Jan 27, 2014
335
Ya its fine. That was my first thought. Even the overflow is fine
 

redmaro42

Member
Sep 25, 2014
70
Mounce said:
Failsafe version? Please explain.

My money is on bad thermostat too. There's nothing else that would make it run that cold.
failsafe versions are designed that when it fails it sticks wide open and allows coolant to flow through all the time. A normal t-stat can fail and not open at all and cause car to overheat.
 

Mounce

Member
Mar 29, 2014
13,667
Tuscaloosa, AL
Gotcha.

Every failed thermostat case I've ever heard of has been stuck open. Never heard of one being stuck closed.


-so, I guess I've never heard of a failsafe version because they all fail like that.
 

triz

Member
Apr 22, 2013
746
Did you burp the system properly after the thermostat replacement?
 

Sparky

Member
Dec 4, 2011
12,927
Mounce said:
Gotcha.

Every failed thermostat case I've ever heard of has been stuck open. Never heard of one being stuck closed.


-so, I guess I've never heard of a failsafe version because they all fail like that.
The thermostat on my old 93 Ford was sticking closed. Not fully closed so it wouldn't really overheat, but ran hot until I replaced it.
 

Sparky

Member
Dec 4, 2011
12,927
Ah gotcha, I agree.
 
Mar 15, 2012
45
I am getting ready to replace mine because I have the code for engine does not warm up quick enough. I think my issue is not the T-stat but the sensor as I am still getting heat. I am going to replace the T-stat and the sensor at the same time.

I have a question, though; I do not see if your properly purged out air from the system. Just running the engine to operating temp will not normally purge the system as the air is trapped in the highest area of the cooling system. Since the T-stat is low on the 4.2, the purge process would need to be followed. This the information I found on the database I use for repair of my truck: run the engine at 1,000 to 3,000 RPM in 30 second intervals until it reaches operating temperature with the reservoir cap off. Let the engine idle for 3 minutes and let it cool. After the engine cools, add to the desired level. If your temp sensor is lying to the computer, you would never know. I can't believe that an engine even with the T-stat failed open would run that cold unless you were in arctic conditions.
 

nagyiii

Member
Apr 26, 2014
73
Sparky said:
The thermostat on my old 93 Ford was sticking closed. Not fully closed so it wouldn't really overheat, but ran hot until I replaced it.
Exactly the same with me on my 86 Pontiac Fiero. T Stat stuck closed, caused it to overheat. It was an easy replacement on that car, not so much on our TB's, just had mine done at the shop a month ago, luckily it only ran me $130
 
Mar 15, 2012
45
Yikes!!! We have been around the old goose egg Fahrenheit here. Too cold to work on the 'voy until it gets around 40.
 

DAlastDON

Member
Apr 6, 2014
5,550
Kentucky
This is what i use to burp any cooling system i have worked on. Take 10-15 minutes. Jack the front up or park on some ramps or a steep hill. Install the funnel on the radiator and fill. Idle the engine with the heater running until it gets up to operating temperature. You will see the bubbles come out. Add more fluid to funnel as needed. I usually do some 3k rpm revs with no mess. When you are done shut the engine off and top the reservoir off with what is left in the funnel.


https://www.youtube.com/watch?v=ajKkpFUKQfs
 

Mirror

Original poster
Member
Jan 27, 2014
335
Well changed out the ect and the thermostat. The ECT is a pain in the arse to remove, I lucked out and had a deep 12 point socket that fit (no idea why i have this socket since it's the only one but i did) Had to put electrical tape on the two wires when done since the stupid design cause the rubber on the wires to strip a bit.

Everything is back up to 100 degree and blowing nice and hot.

While I had everything apart I also put in a new block heater cord.

Why did GM have to design such a stupid engine to work on.

The 3.8 is way easier to change all these parts on.
 
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