H3 New Thermostat – Now gauge reads at 3/4 during idle

My 2008 H3 3.7 (93,000 miles) have been dealing with the A/C Off message for about a year. At first, the issue was intermittent and was more of a nuisance than a disaster. If I ever needed to have the A/C on right away, I just disconnected the PCM-B fuse and plugged it back in. Most of the time, the message came and A/C shut off, only to come back on again by itself after ~10 minutes of driving. It just recently started to have the issue every single day, so I figured something got worse over the year. Finally took it to my local mechanic and had them replace the thermostat as that's what I read as the first thing to try with my issue. I will say that when I didn't have the A/C Off message previously, my temp gauge in the dash read at slightly below 1/2 and sometimes even closer to 1/4. Thought that was odd as well so just paid the $260 to have them replace the thermostat. I bought the ECT sensor ($16 on Amazon) and plan to swap that out myself since it's a bit easier to get to.

Got the car back today and noticed that my temperature gauge now reads fairly high. Drove for about 8 minutes today and pulled into a Drive-Thru and sat for ~5 minutes. I noticed that my temperature gauge at idle was now almost to 3/4. Any ideas or anyone else have the same issue after a thermostat replacement? I know that the gauge should not be relied on for actual temperature readings, but I would expect it to be somewhat accurate, maybe! I've also heard/seen multiple places that H3s tend to run a bit hotter. Not sure if there's truth behind that.

I did call the shop back and spoke with the mechanic that actually worked on the car. He said that he noticed the temperature reading higher on the gauge as well and hooked it up to their computer to see the actual temp and everything checked out just fine. He says that with this new thermostat, the temperature reading will always be higher now but it isn't something I should worry about. He also said that I shouldn't need to replace the ECT sensor. I was planning on doing that anyways. Do those parts somewhat go hand-in-hand? In other words, would it be better to have a brand new thermostat as well as a brand new ECT sensor? I kind of thought so. Any similar stories/suggestions would be great. Mechanic did say that the thermostat housing was leaking and that was probably causing the faulty P0128 code. Anyways, I will hook up my OBDII scanner later and see if I can get a temperature reading. Is it true that the H3s should run 190-220 degrees normally? What temperature should I expect?

What was the temperature outside? Also, if it's getting hot while sitting at a standstill you should check the fan clutch. 3/4 "could" be normal at a when sitting for a while but I would expect that on a hot day.

If your temp gauge was often reading close to 1/4 before I would suspect a thermostat that was stuck open. Earlier H3s would typically show a "normal" temp at a tick above 1/2. Later H3s (I believe) had the gauge recalibrated to show a tick below 1/2. I don't know when that recalibration was performed in the H3 production cycle.

Yeah today is pretty cool in Kansas. High is 48 degrees and at 8:00 this morning while i was in line, it was probably 40 degrees. Went and hooked the OBDII reader up with the car running and I was at 213 degrees. Still don't know if changing the ECT sensor will give a different reading. Thinking I might go ahead and do that this weekend anyways.

I don't think that sensor is a common failure on our engines but that doesn't mean it couldn't be an issue. You could also have a brand new stat that's stuck closed - make sure the hoses have pressure and are getting hot. I'm still thinking you're looking at a fan clutch issue too.

I replaced my t-stat (dealer part) earlier this year and the same thing happened. I went back to the parts guy and he said GM upped their T-stats by 10 or so degrees for emissions reasons.

Quite a while ago I changed the t-stat, ECT sensor and it cleared up a problem I was having. This was some years ago. The ECT was part of the failure that my H3 was going through @ that time.

Fast forward to this year and I replaced the radiator (again), as much coolant as humanly possible and the fan clutch. Still at just slightly above 1/2 to 3/4's mark. So, I'm not so sure about the 3/4's being all that concerning lol. The idiot guage on the dash seems pretty relaxed.

Also 213-218 degrees seems about right. I know my Jeep Cherokee xj runs 180-220 degrees.
I've not felt the need to check the H3's temp.

the gauge is just a suggestion as to how hot the engine is, you really need to see the actual values coming from the computer.

Jayhawk1525 said:

My 2008 H3 3.7 (93,000 miles) have been dealing with the A/C Off message for about a year. At first, the issue was intermittent and was more of a nuisance than a disaster. If I ever needed to have the A/C on right away, I just disconnected the PCM-B fuse and plugged it back in. Most of the time, the message came and A/C shut off, only to come back on again by itself after ~10 minutes of driving. It just recently started to have the issue every single day, so I figured something got worse over the year. Finally took it to my local mechanic and had them replace the thermostat as that's what I read as the first thing to try with my issue. I will say that when I didn't have the A/C Off message previously, my temp gauge in the dash read at slightly below 1/2 and sometimes even closer to 1/4. Thought that was odd as well so just paid the $260 to have them replace the thermostat. I bought the ECT sensor ($16 on Amazon) and plan to swap that out myself since it's a bit easier to get to.

Got the car back today and noticed that my temperature gauge now reads fairly high. Drove for about 8 minutes today and pulled into a Drive-Thru and sat for ~5 minutes. I noticed that my temperature gauge at idle was now almost to 3/4. Any ideas or anyone else have the same issue after a thermostat replacement? I know that the gauge should not be relied on for actual temperature readings, but I would expect it to be somewhat accurate, maybe! I've also heard/seen multiple places that H3s tend to run a bit hotter. Not sure if there's truth behind that.

I did call the shop back and spoke with the mechanic that actually worked on the car. He said that he noticed the temperature reading higher on the gauge as well and hooked it up to their computer to see the actual temp and everything checked out just fine. He says that with this new thermostat, the temperature reading will always be higher now but it isn't something I should worry about. He also said that I shouldn't need to replace the ECT sensor. I was planning on doing that anyways. Do those parts somewhat go hand-in-hand? In other words, would it be better to have a brand new thermostat as well as a brand new ECT sensor? I kind of thought so. Any similar stories/suggestions would be great. Mechanic did say that the thermostat housing was leaking and that was probably causing the faulty P0128 code. Anyways, I will hook up my OBDII scanner later and see if I can get a temperature reading. Is it true that the H3s should run 190-220 degrees normally? What temperature should I expect?

Click to expand...

The problem with your ''AC OFF'' message and reading 1/2 or 1/4 in the dash could be directly because of the ECT sensor, my first step would be replace that inmediatly if you have the part already in your house. Continuing with the quote about high temperature in the Hummer H3 i will say something a bit important to everyone here:

FIRST: The Hummer H3 doesnt have ANY problems with temperature, or tends to run high!! Each part of the cooling system is more than necessary to cool down the i5 platform, 3.5 or 3.7, the problem with high temp reading are:
-THERMOSTAT: GM redesigned the original 180ºF thermostat, the new thermostat is #12622316 and 195ºF, with smaller hole for water passing than the original one that comes with the 3.5 and 3.7. GM made this change because of emission and something more that is useless for us and our normal use.

-DEXCOOL: If you have an original hummer h3 and his original thermostat and you notice a increase in water temperature during long highway trip in inclines, the problem is not your ''cooling system'' or the design of Hummer factory, make the test, replace the original dexcool coolant and replace it with Coolant organic 50% red colour of good brand but not G12 from Vag Group or the infamous Dexcool, you will notice directly that the temperature never increases from 1/2 or if the temperature increase during summer is only one milimeter over 1/2 reading.

IN RESUME: If you are dealing with 1/2 to 3/4 temperature reading in the dash and you already replace thermostat with one new, then you received the #12622316 part (whatever brand, other brands take the original thermostat and erase the original number part, but is the same, despite they say their thermostat open at lower temperature, the temperature is 195ºF), then your Hummer H3 are operating completely normal, dont worry about that and live with it (or not, see P.D.2).

If you still have the original thermostat and usually you see the 1/2 temp reading but sometimes in highway or city see the 3/4 mark, then your problem (almost for sure) is Coolant dexcool, flush it with something good from another brand, one Organic Coolant 50% in red colour, and you are going to see always the 1/2 temp reading.



P.D.1: There is NOTHING wrong with the original temp gauge in the dash, actually is wonderfull because shows you the truly temperature of the engine, not like other brands like BMW that show a 1/2 reading from a range of 85ºC to 110ºC that is almost overheat for me. The H3 temp gauge show the real temp all the time. 1/2 temp is 190ºF (where a 180ºF thermostat estabilizes the coolant in a coolant system), 3/4 temp is 220ºF
It is always better to buy a Scangauge II that show the exact value, and I recommend that, but the temp needle is not ''innacurate'' like some people say.

P.D.2: There is not ''stock'' parts solution for the temp gauge in the Hummer H3 reading higher after a thermostat change (but yes if you have the stock 180ºF thermostat, flush the coolant to another one) BUT there is other solution that was posted in CLUBHUMMEROFFROAD forum, is to remove the original housing and remove the thermostat inside, leaving only a metal pipe without restriction and put it again in the truck, then install a INLINE HOUSING AFTERMARKET THERMOSTAT HOUSING AND 180ºF THERMOSTAT, and mount it directly into the inlet coolant hose in the upper part of the engine, that way you remove the original thermostat and leave an aftermarket housing of your choice and a cheap 180ºF thermostat (or other temperature thermostat) of your choice tool. With this mod there is always a 1/2 temp reading regardless of the exterior temperature, driving or AC use.

https://www.jegs.com/i/JEGS/555/53260K1/10002/-1

Hope this help you and everyone out!
Guerrero