Ok on the way home from work today I got the famous P0128 code. I read up and everyone says thermostat. But my question is that u have the had the trailblazer for about a year and always had my scan gauge hooked up to it an it had always read 184 degrees and the guage has always been just a hair under 210. Well on the way home today when the check engine light came on it read the same thing the whole time. And I just filled the trailblazer up Friday and got 19 mpg (not to bad). I had just changed the plugs so I thought maybe that was te reason for the light but I guess not. I turned the light off with the scan guage to see if it will come back on but are you guys sure that it is the thermostat? Any other ideas?
Cooling question
- Thread starter jd8181
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Dad-O-Matic
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jd8181 said:
I think most will chime in that it's definitely the thermostat. It's a 195* 'stat, so there's no way it should be reading 184* if functioning properly, unless it's been replaced with a 160* one. I believe other members report thier operating temps in the 200* to 210* range. And the gauge should be just past the 210* mark, not before.
Edit: If I understand Roadie correctly, the gauge doesn't report the "true" temp, the PCM adjusts it to keep the driver happy. So even if the real temp is 200*, the gauge will still read 210* plus 1 tick to the left. It will only start climbing if things are getting really hot.
Dad-O-Matic
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jham said:
No reason to brag
I'm happy to get 14 mpg (4.10 gears, 4 wheel drive, mostly city driving)
Voymom
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RayVoy said:
You definitely need a new t-stat. However MY truck never ever ever threw me a code when mine was out. I have pictures of how cold my truck was running and not a single code was being thrown. Your lucky to have gotten a code because only god knows how long my truck was driven with a bad one as I bought it running rich. Here are my t-stat before and after pics...
Before new t-stat
Before new t-stat
New t-stat
blazinlow89
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Damn I am happy to get 16mpg, I might have to check my RPO codes again I can swear I have the 3.42.
The t-stat is not very difficult to replace. Biggest PITA is the removing the alternator, specifically the lower bolt. Other than that it is an easy 2-3 hour job, depending on how much convincing you have to do.
We did a full coolant system overhaul, minus the radiator. Took about 9 hours to replace the water pump, t-stat, both upper and lower hoses, temperature sensor and flush the engine and radiator. 2-3 hours of that was spent fighting the fan clutch nut, just needed the right tool. Another hour and half was wasted driving to get a new brake caliper (I put on drilled and slotted rotors, front and back, and pulled a rookie move). Then about 30 minutes to eat and take a short break to get all of the tools cleaned off and organized before we went to put things back together.
It is also usually advised to replace the coolant temp sensor while doing the t-stat, be gentle though another member had to deal with alot of headache due to one snapping in the hole. If you are throwing a code it is for good reason. You can try to reset and see if it comes back, most likely will. Temp on mine is constant 202-208*, before I was at 170*.
The t-stat is not very difficult to replace. Biggest PITA is the removing the alternator, specifically the lower bolt. Other than that it is an easy 2-3 hour job, depending on how much convincing you have to do.
We did a full coolant system overhaul, minus the radiator. Took about 9 hours to replace the water pump, t-stat, both upper and lower hoses, temperature sensor and flush the engine and radiator. 2-3 hours of that was spent fighting the fan clutch nut, just needed the right tool. Another hour and half was wasted driving to get a new brake caliper (I put on drilled and slotted rotors, front and back, and pulled a rookie move). Then about 30 minutes to eat and take a short break to get all of the tools cleaned off and organized before we went to put things back together.
It is also usually advised to replace the coolant temp sensor while doing the t-stat, be gentle though another member had to deal with alot of headache due to one snapping in the hole. If you are throwing a code it is for good reason. You can try to reset and see if it comes back, most likely will. Temp on mine is constant 202-208*, before I was at 170*.
blazinlow89
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jd8181 said:
I bought 3 and only used 2, im sure the rest was water in the radiator or engine coolant paths. I think capacity is like 13.8 quarts, double check the manual. 4 quarts to a gallon.
Voymom
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RayVoy said:
Yeah it was pretty bad lol The coolest temp was after driving it for 15min and then letting it sit while running for another 30min and the warmer temp was highway driving
It was bad! I had a lot of get up and go issues because of it too and my fuel economy ha! We won't even go there 
After it was fixed it ran okay, then our ps pump went out lol and less than a week after fixing the ps pump hubby put engine restore in it and almost killed my CPAS. We had a back pressure test done on the cat converter to make sure we didn't have it clogged from the t-stat and it came out fine. We don't have inspections here, so if it was clogged we would have just drilled out the cat and left it that way lol
Ok I am sure that it is the thermostat. I just went to the store to buy a new one and it was running 182-184 and when I would hit the gas t would drop into the mid170s and it had never done that before. So it looks like I will be changing it after work tomorrow. I also bought a temp sensor but not sure if I should change it or not since the one I have is workin fine. Any thoughts?
NewfieEnvoy
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Looks like I got my Saturday planned as well. With my scan tool I'm reading a steady 185 and if I jump on it the temp will go down into the 170's. Never had a code but I've been getting 12 mpg, and I drive like poppy. Might as well clean the TB while I'm at it and start fresh.
Update... I just changed out the thermostat on my trailblazer. It was no as bad as I thought it was going to be. Took me maybe an hour and a half. Bottom bolt on the alternator was the hardest thing but I was not that bad. I took out the old one and it had no temp on it. Just a gm part number. I guess I'll google it and see what it say. I took it out for a test drive once I got it all back together and it jumped up to 200 degrees (it never got past 190 before) and stayed right around 195-200 on my scan gauge and needle was strait up and down on the 210. It was always a hair to he left before. Now I will just wait and see if my mpg's get any better and see if I notice any difference in the way it runs. Thanks for the help guys.
NewfieEnvoy
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Let us know how the mileage is effected, hopefully better mpg that's what I'm looking for. Just picked up a tstat at the stealership, was actually a couple bucks cheaper than the parts stores. It was still $60 which is crazy because I here their $20-30 in the US, and the Canadian dollar is higher right now! 
Enderbygrandpa
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I have yet to find a 195f thermostat for our trucks. The Stant that i got was 86c and the GM was 88c. Anybody know what brand this 195f thermostat is?
Also don't believe the aftermarket catalogs as the Stant cat. reads 190f .
Also don't believe the aftermarket catalogs as the Stant cat. reads 190f .Enderbygrandpa said:I have yet to find a 195f thermostat for our trucks. The Stant that i got was 86c and the GM was 88c. Anybody know what brand this 195f thermostat is?
The thermostat that I got from autozone (duralast) is 192 degrees. That is the only one they stock.
Enderbygrandpa
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was that the temp stamped in the thermostant or just what was on their web site? I got caught by that with the stant brand.
blazinlow89
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Spend the extra few bucks and buy AC Delco. I bought mine off rock auto and it had no listing for what temp it was. Being the oem factory replacement means it should be the same as OEM. Our trucks are finicky when it comes to engine parts, the t-stat may not matter but for an extra $5-$10 I would buy the OEM parts.
MichEnvoyGuy
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blazinlow89 said:
For everybody reading this thread in the future, the alternator removal is not hard because you do NOT NOT NOT have to remove the lower bolt completely to get the alternator out.
What you do is loosen it all of the way from the block and simply leave it. Then, remove the other 2 upper alternator bolts. Twist alternator out of the way with the lower bolt still in the case (but unthreaded from block). Voila. Make sure you leave the lower bolt in the alternator when you re-install.
Messing with removing the lower alternator bolt completely is going to be time consuming and isnt necessary for alternator removal. You can thank me later
Uncle Blazer
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Does anybody have a write up on how to change this out? I checked articles but didnt see anything. I still have an active account on the OS (last i checked) so if there is an accurate article over there just let me know and i will go digging for it (and do a little recruiting while i'm there).
).
David B.
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This all makes me nervous. I just did my Thermostat this past weekend (my left hand still has the scabs and bruises to prove it) and it's running the same temperature as the old one, no where really near the 210 mark like it sounds like it should be. SOMETIMES it's a tick left of straight up and down, but most of the time it's pretty far to the left and reading around 167-172 on the scan gauge. Did I get a bad thermostat from the parts store? Read one time that you should boil it on the stove first, perhaps that's good advice LOL.
Is the consensus that I'm going to be redoing this on Saturday?
Is the consensus that I'm going to be redoing this on Saturday?
blazinlow89
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MichEnvoyGuy said:For everybody reading this thread in the future, the alternator removal is not hard because you do NOT NOT NOT have to remove the lower bolt completely to get the alternator out.
What you do is loosen it all of the way from the block and simply leave it. Then, remove the other 2 upper alternator bolts. Twist alternator out of the way with the lower bolt still in the case (but unthreaded from block). Voila. Make sure you leave the lower bolt in the alternator when you re-install.
Messing with removing the lower alternator bolt completely is going to be time consuming and isnt necessary for alternator removal. You can thank me later
Well excuse me.
JK
Mine was only a HUGE pain to remove as I did not have a 15MM socket. Had to use wrenches and crows feet for everything. Never realized that until I did this project. IT was actually very easy to get back in as I removed the battery to get some extra moving space. As well as a 15MM socket.
David B. said:
Assuming your fluid levels are good...
1 - Did you replace the sensor when you replaced the t-stat?
2 - Are you certain it was the proper t-stat? As you may have read, there are different ones available, which open and close at different temperatures.
David B.
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joe mama said:
I did not replace the sensor when I did the t-stat, but I know the fluid levels are good. As far as the fact that there are two available, perhaps I got the wrong one? My old one was always operating at the same temperature range, but then it started to stick open, which is why I replaced it. It never threw a code since I owned it, but it sounds like that is typical even though it may be running poorly/inefficiently. Should I be looking to swap the one I have out with the higher temp t-stat and do the sensor while I'm at it?