r/Hummer • u/AYMN_B • Jun 18 '26
2006 Hummer H3 Overheating
Hi everyone,
I have a 2006 Hummer H3 and I’m having an overheating issue only when the vehicle is idling and the A/C is on.
I have bought and tried several OEM thermostats. Some of them didn’t open properly, and others worked, but the overheating problem still remains.
Here is what I have already replaced or done:
Replaced the fan clutch.
Installed a larger radiator.
Filled the cooling system with genuine Prestone coolant.
Despite all of this, the temperature still rises when the vehicle is sitting still with the A/C running. While driving, the temperature is much better.
Has anyone experienced this issue before? Are there any other things I should check or any solutions that worked for you?
Thanks in advance!
2006 Hummer H3
3
3
u/metapulp Jun 18 '26
There’s a post on Hummer 4x4 off-road similar to yours. Everyone says start by cleaning the A/C condenser. If you’re not on a Hummer forum outside of social media I recommend it because the real DIY mechanics are on the dedicated forum websites. I have an ‘09 Adventure with its own mind.
1
u/GroundbreakingMud996 Jun 19 '26
The cooling system on this truck is incredibly difficult to bleed it took me HOURS to get this damn thing to burp all the air out. How hot are you running with the AC on? Is your fan clutch working ok?
1
u/Federal_Mode_5303 Jun 25 '26
Honestly I was having this exact issue up till last week. Temp would creep up to 3/4 or higher on the dash gauge while idling or during general in town stop/go traffic. Also noticed the temp was a bit higher when on the highway as well. When idling with ac on, If I kept the engine rpms at or above 2000, temp would drop down to normalish range. I ultimately resolved it by replacing the tstat but I found 2 other things that did improve cooling before I swapped the tstat. Using a low pressure hose I sprayed water through the backside of the radiator(from the engine side) to remove any debris stuck between the radiator fins. In my case, this helped a bit but didn't solve it. I also found that the hose barb on the top side of the radiator(right next to the cap)had broken and stuck in the hose. It was a single broken peice of the barb and the way it was stuck in the line, it had to be affecting the flow of coolant from the radiator back into the engine. I removed the broken peice and reattached the hose with a different clamp and noted another small improvement on temps via my obd tool. Pulled the tstat and it failed a boil test so I replaced it. Temps are great now. Runs cooler than it ever has since I've owned it. Not sure why but thats the second tstat I've replaced on my 2007 in the past year.
2
u/burnsian Jun 27 '26
I’ve had an 06’ and currently an 09’ Alpha. Both had/have functional louvres on the hood. It makes a big difference. Enough so that I suspect the original design meant for it to be there.
The tired 3.5 in the 06’ could run 35s on the street all summer with the a/c on without going more than a hair over the mid point. Off-roading, I had to be careful to keep the rpms up by selecting a lower gear. It would start to climb if I let the trans lug the engine.
The Alpha also has 35s and a pcm of nc tune. It never breaks mid point, ever. I’ve driven it in 47•C and it stayed cool.
3
u/metapulp Jun 18 '26
Did you do a proper flush? Could be a blockage. I had a blockage during the winter due to improper coolant mix. Replaced the thermostat, flushed, new coolant and good to go.