Hello again! I am prepping my 2008 H3 base model to drive down to the bottom of Baja with a heavy load. I installed a Scan Gauge II so I could monitor my engine and transmission temperatures along the way. I’ve suspected for a little while that my Hummer has been running hot, engine compartment seemed overly warm and slight smells of coolant. In stop and go traffic around 90° the scan gauge tells me the engine is running over 220° and the transmission temperature slowly rises to above 200. I loaded the Hummer with most of what I am going to transport and did some test driving on the freeway thinking maybe the higher speeds would help it to run cooler. I got the same kind of temp readings as I did around town, maybe slightly lower coolant temps. I had a top end rebuild a couple years back and that’s when I started seeing the higher temps. And I installed an aftermarket radiator because the OEM one cracked. Here are my questions:

1. Has anyone found the aftermarket radiators don’t perform as well as OEM?
2. I am installing a transmission cooler with a fan. Should I bypass the cooler in the radiator or hook it up in tandem? I’ve heard that the plastic radiator can crack and intermix the coolant and the tranny fluid causing severe problems. Will this safeguard against us and give me better cooling?
3. Has anyone had good luck with the aluminum radiators that are offered. Did you see coolant temperatures go down significantly? Any particular brand where fitment was good?

Tomorrow I will begin diagnosing the overheating issue. I’ll be doing the following:
1. Pressure testing the coolant system for leaks
2. Flush the coolant system
3. Replace the thermostat with a 165
4. Check for collapsed hoses
5. Check the fan clutch
Is there anything else I should look for? I was planning to do the drive this coming Thursday so I only have a few days to get this issue resolved. I don’t suspect the water pump as it has been replaced. My Hummer has 145,000 miles. Transmission is rebuilt and has maybe 20,000 miles. Thanks for whatever advice you might share.

 

I cannot speak for aftermarket radiators, as I have only installed AC Delco radiators. If you are installing a transmission cooler, I'd recommend bypassing the trans cooler in the radiator. Completely separate the systems. You can go a step further and get the AC Delco radiator used in the H3 with a manual transmission. That radiator doesn't have the integrated trans cooler. That is what I did. I used a Hayden Rapid Cool Trans-cooler 676. I had planned to upgrade the cooler to a larger one, but it seems to work pretty well. That was installed in 2018. I have a ScanGauge and monitor trans temp. Generally it's in the 170-180 range. If you've ever had mud in your radiator and condenser, make sure it is rinsed out well. There's barely any room between the two, so the mud gets caught in there.

 

I tackled all these issues as well during my rebuild. I have a build thread here. So far everything I've done has made the truck efficient under extreme loads. There going to be a ton of people that say good or bad about both but more negatives in the aftermarket side. If you understand cars. You will be fine if you can fix the sometimes little mistakes from aftermarket parts.

 

Leave the OEM T Stat in place. Rinse your radiator front and back to remove crud, bugs and residue. If you smell coolant you may have a pinhole leak which you might find with your pressure test.

 

Thanks Doc Olds…. I didn’t think I had any leakage because my coolant level never dropped. Is it possible to have a pinhole leak and lose pressure and not lose coolant? Maybe at the head?

 

If you smell coolant and are sure about that, then there has to be a leak.

It may be days, weeks, maybe months, but leaks do not fix themselves and eventually get worse, leaking coolant on the ground. Some leaks are so small and only allow let out enough coolant to coat things in a slime that then attracts dirt and get cooked on over a long time period. When you inspect under the motor you can see when/where that happens.

 

I went through this on my alpha about 18 months ago. I reached out for help chasing an overheating issue and was berated for it. People told me to ignore it, that the temps I was seeing was not overheating. I was in Texas with air temps in the 80s and was running about 215-220, climbing further when idiling. I was worried what it would be like in 100+ degrees. The cooling system isn't rocket science so I just started with standard prcedures like reverse flushing, new tstat, etc. The radiato only had a tiny seeping leak and built pressure well so the leak wasn't much of an impact.

Those things had no noticable impact. In fact on a 96 degree day my scan tool showed 248 degrees and still climbing. Trans was at 220ish. I was again told this was not overheating. At this point I started ignoring those stupid comments and decided to just do a full cooling system upgrade. Here's what I did, all of it easy:

Chinese aluminum 3 core radiator (not spending 1000+ on one and figured I'd see how it went)
High flow water pump (helps with idling flow mostly)
High flow 180* stat
New HD thermostat fan clutch
Fit shroud shroud neatly to the new radiator

I then bypassed the radiator trans cooler and installed a big remote one with a fan under the cab. This removes the trans cooling load from the radiator and gets the hot, airflow-restrictive cooler away from the radiator. This was the most difficult part because I had to access the trans line connections at the side of the trans which was not fun. I cut them to about 6" long and flipped em 90 degrees to run hoses rearward.

It now heats up quickly and settles at 194 in most weather. In extreme cold (10 degrees F or less) its stays at about 188. In 105 degree weather it stays about 200-205, creeping to 210 when idiling. The transmission cycles between 175-190 all the time. Couldn't be much better.

So far the cheapo radiator is working great with no leaks. Fingers crossed but I don't expect an issue at this point.