Not like every other H3 running hot thread, hear me out.

[HumbeeH3]

Although it's been quite hot lately 110-115F outside, my H3 3.7l seems to run a bit too warm than I am used to.

On the street and idling my coolant temp will be around 205-210F, 213 at most. However on the highway I am at 217-220F with transmission temp pinged at 233-235F. I have checked and replaced the following:

-Top and Bottom Seals (Between Radiator and Condenser Coil)
-Front Baffle
-New Belt, Tensioner, Idler Pulley
-Radiator is fairly new (previous owner had it replaced)
-New 15psi Radiator Cap
-New Water Pump
-New Fan Clutch
-New Coolant Hoses
-Chemical Flush
-Dex-Cool Coolant 50/50 (filled and burped)
-180F Inline T-stat (installed at the top between water outlet housing and radiator inlet) original T-stat housing gutted.
- New 180F inline T-stat is operational, checked in boiling water and jiggle valve was installed properly to allow system bleeding of air.
-Checked over flow tank lines to make sure there's no obstruction.
-AFR is good along with STFT & LTFT, timing looks ok too 8 degree advance during idle and about 38 degrees at 2500rpm
-IAT seems a little high 135-140F (stock intake box with new filter)
-ATF level, color, taste, and smell are good. Was replaced 8K miles ago along with filter.
-New Plugs and Coils

Not sure where to go from here, everything involving the cooling system has been checked and replaced yet I can't keep my truck cool. I want to operate between 195-205F, definitely concerned about the transmission running at 235F

I am also reading on the forums that this is normal, yet no one can provide actual source. All I got was "trust me bro" type of response. In fact only thing I was able to find on the net is a troubleshoot guide for H3 overheating and 225F was the number indicated as being the baseline to go on to the next step.

I also searched this forum up and down for suggestions, how I came across the inline tstat idea. Only difference is I had to get the outlet housing machined to make room. Photos below for your viewing pleasures. Any other suggestions are greatly appreciated

 

[08H3]

I think you answered your own question. Diagnostic tree in the service manual lists 225 as the temperature to continue diagnosing an overheating condition. You're under that so I wouldn't worry.

 

[HumbeeH3]

I think you answered your own question. Diagnostic tree in the service manual lists 225 as the temperature to continue diagnosing an overheating condition. You're under that so I wouldn't worry.

Good point, when I started this journey it was higher up to 230F especially sitting in traffic. Adding a top seal and bottom seal (the flap that hangs down from the radiator and tucks back under the skid plate) brought temperatures down. What about transmission running at 235F, is that normal? I am using OBD adapter and Torque Pro to monitor.

 

[Happy Hummer]

So, why didnt GM just place the t-stat on top like you've just done in your img? That just makes so much more sense. I've already replaced the t-stat once in my H3 and man what a crummy place to put it. The Jeep i6 has it literaly right out in the open on top and it can be gotten to quite easy. But NO lol GM decides to put it under, behind and d@mn near impossible to get to hahah!

I'd be a bit concerned with running the 180 deg. t-stat. Simply because it may now have other difficulties with emissions and what not.

 

[alrock]

Do you know what radiator was put in your H3 - was it OEM or aftermarket? I had an eBay cheapie and it did not perform well.

I feel your frustration on the "trust me bro" but sometimes that has a bit of merit and I think it does here as the temp issues are an oft-repeated and oft-observed standard for the H3s. While I have not confirmed my actual temps (I just trust the idiot gauge) I do think you have some concern on the highway temps, at least with the transmission. I moved my transmission cooler to an external cooler and went to a manual transmission radiator that does not include the inline transmission cooler. That, in my opinion, keeps some of the heat separate from each other. If you go that route, definitely oversize the transmission cooler as most of the charts you read show the cooling ability when paired with the OEM inline cooler.

Does the transmission seem like it's working well and that the torque converter locks up when it should? Are the condensor and radiator clean or perhaps filled with bugs and debris?

While my H3 has seen many days over 110°, it's never seen this many consecutive days and I never paid as close attention to the temps as I did this summer. I'm sure you saw my other post about this stuff. My H3, while high miles, is well-maintained like yours and I do not suspect any mechanical deficiencies to be causing the high readings. I believe the I5 H3s can't handle stop and go traffic above 110° and I keep the RPMs at 2000 or above to keep the engine (and a/c) cool when stopped. I've also turned off the a/c at some of these stops when the temps get extreme.

Back around 2009, when my truck was still pretty new, I might've been going well above the speed limit on I-8 east of San Diego and it was 112° out that day. I was trying to catch up to my buddy that was cruising at over 90 mph in his H2. When I looked down at the gauges the temp gauge was pretty much pegged. I had to slow down, and I suspect that was the day I snapped a couple of exhaust manifold bolts. Point being that with a high temp load and even with a lot of airflow, my truck was overheating even when it was close to new, still under warranty and everything likely operating to spec.

You've done all the right things so far. You could also consider relocating the trans cooler underneath as well and putting an electric fan on it.

BTW, what does good ATF taste like vs. bad ATF?

 

[HumbeeH3]

So, why didnt GM just place the t-stat on top like you've just done in your img? That just makes so much more sense. I've already replaced the t-stat once in my H3 and man what a crummy place to put it. The Jeep i6 has it literaly right out in the open on top and it can be gotten to quite easy. But NO lol GM decides to put it under, behind and d@mn near impossible to get to hahah!

I'd be a bit concerned with running the 180 deg. t-stat. Simply because it may now have other difficulties with emissions and what not.

Was wondering the same. Apart from PITA location I am also not sure why it would be placed at the engine inlet instead of outlet, wouldn't it sit there in cooler coolant given its at the bottom after the radiator? Not sure if that would matter considering its installed into the block where hot coolant is located but still. Curious to learn the engineering behind that.

Great thing about those inline T-stats is now I can swap from cooler to warmer T-stat without draining the entire cooling system if needed

Do you know what radiator was put in your H3 - was it OEM or aftermarket? I had an eBay cheapie and it did not perform well.

I feel your frustration on the "trust me bro" but sometimes that has a bit of merit and I think it does here as the temp issues are an oft-repeated and oft-observed standard for the H3s. While I have not confirmed my actual temps (I just trust the idiot gauge) I do think you have some concern on the highway temps, at least with the transmission. I moved my transmission cooler to an external cooler and went to a manual transmission radiator that does not include the inline transmission cooler. That, in my opinion, keeps some of the heat separate from each other. If you go that route, definitely oversize the transmission cooler as most of the charts you read show the cooling ability when paired with the OEM inline cooler.

Does the transmission seem like it's working well and that the torque converter locks up when it should? Are the condensor and radiator clean or perhaps filled with bugs and debris?

While my H3 has seen many days over 110°, it's never seen this many consecutive days and I never paid as close attention to the temps as I did this summer. I'm sure you saw my other post about this stuff. My H3, while high miles, is well-maintained like yours and I do not suspect any mechanical deficiencies to be causing the high readings. I believe the I5 H3s can't handle stop and go traffic above 110° and I keep the RPMs at 2000 or above to keep the engine (and a/c) cool when stopped. I've also turned off the a/c at some of these stops when the temps get extreme.

Back around 2009, when my truck was still pretty new, I might've been going well above the speed limit on I-8 east of San Diego and it was 112° out that day. I was trying to catch up to my buddy that was cruising at over 90 mph in his H2. When I looked down at the gauges the temp gauge was pretty much pegged. I had to slow down, and I suspect that was the day I snapped a couple of exhaust manifold bolts. Point being that with a high temp load and even with a lot of airflow, my truck was overheating even when it was close to new, still under warranty and everything likely operating to spec.

You've done all the right things so far. You could also consider relocating the trans cooler underneath as well and putting an electric fan on it.

BTW, what does good ATF taste like vs. bad ATF?

Good call out on the radiator, I think its a cheapo from Ebay. Found some numbers on it, searched for them on the web and ebay listings come up. Radiator is the only thing I didn't replace. I will be replacing it for piece of mind and get a separate transmission cooler.

Also forgot to add, I checked for exhaust gases in the coolant via block tester, both at the overflow tank and radiator. No exhaust gases were found. Also checked compression got 179-181 psi across all cylinders, not the recommended 215psi but I think for the age and mileage that's within acceptable range. What do you think?

One other thing I forgot to do when I did my oil, was to take a sample and send it to Blackstone for oil analysis. Will have to do that next time.

Transmission works great, shifts smooth while parked or moving. Only thing is when I push the gas after coasting you feel a slight thud, but I think that's normal from what I read throughout the forum. As for the ATF taste I just threw that part in there for laughs and giggles, now I know who actually read my long post Thank you sir!

 

[alrock]

Check your the rear bushing for the front differential in regards to that clunk. It's known to wear out and that could be the thud. Or it could be the normal slop in a GM driveline.

 

[HumbeeH3]

Check your the rear bushing for the front differential in regards to that clunk. It's known to wear out and that could be the thud. Or it could be the normal slop in a GM driveline.

Definitely will, thank you for the suggestion. I thought it might be just the normal slop that others have experienced, it only happens from coasting and when you push the gas pedal.

Here's something new I've noticed today after installing the inline t-stat. On a cold engine, temperature will reach about 220-221 before suddenly dropping to 190. Happened two days in a row. I am figuring its either an air bubble, or the area where the T-stat is doesn't reach to 180F until where the sensor is reaches 220F. Not sure what to think of it, T-stat is installed correctly. Now I am really wondering which way does the coolant flow on the 3.7L?

 

[rascole]

Sensor is in the engine so they will not see similar temps until after it has opened. Have you pulled the t stat since it has been installed? Does it have a bypass hole to keep circulation going? might be worth checking.

 

[Guerrero]

Was wondering the same. Apart from PITA location I am also not sure why it would be placed at the engine inlet instead of outlet, wouldn't it sit there in cooler coolant given its at the bottom after the radiator? Not sure if that would matter considering its installed into the block where hot coolant is located but still. Curious to learn the engineering behind that.

Great thing about those inline T-stats is now I can swap from cooler to warmer T-stat without draining the entire cooling system if needed


Good call out on the radiator, I think its a cheapo from Ebay. Found some numbers on it, searched for them on the web and ebay listings come up. Radiator is the only thing I didn't replace. I will be replacing it for piece of mind and get a separate transmission cooler.

Also forgot to add, I checked for exhaust gases in the coolant via block tester, both at the overflow tank and radiator. No exhaust gases were found. Also checked compression got 179-181 psi across all cylinders, not the recommended 215psi but I think for the age and mileage that's within acceptable range. What do you think?

One other thing I forgot to do when I did my oil, was to take a sample and send it to Blackstone for oil analysis. Will have to do that next time.

Transmission works great, shifts smooth while parked or moving. Only thing is when I push the gas after coasting you feel a slight thud, but I think that's normal from what I read throughout the forum. As for the ATF taste I just threw that part in there for laughs and giggles, now I know who actually read my long post Thank you sir!

Hello HumbeeH3!

I am the one who did the Inline Thermostat Mod a long time ago and created the thread: https://hummer4x4offroad.com/forum/threads/guerreros-take-on-h3-cooling.14490/

About your issues I think is a combination of factors:
One thing to consider firstly is everything done is done right, so here is my list of what could be:

1º_ Thermostat Installation: Just check the picture attached about installation and particulary the part of the Thermostat position, need to be installed like the picture.
2º_ Alrock I think pointed very good, the radiator known to work good is the OEM GM one, not aftermarket or chinese ebay radiators, so maybe to consider replacing it with a AcDelco 22000 unit (oem one)
3º_ And more probably for me, the Dexron Coolant. Dexron is not a good flowing coolant, its a coolant that ensures a lot of time without breaking apart, but I would install another 50% good branded coolant, not dexcool. If you want to replace it, you need to flush it, using distilled water, filling up, running and removing process, usually it takes 2 or 3 fill-remove distilled water to completely eliminate the old coolant and you can use a new coolant (30-40% of distilled water will be on the system but could be mixed with the New coolant, just that would be not 50% coolant, would be less.
4º_And super important, air inside. Works amazingly good about bleeding system to fill the system using the upper radiator hose, and fill the engine firstly and slowly. This remove all the air possible inside the engine and radiator. Once coolant reaches top of radiator upper hose, you install again top hose and finish filling the system through the radiator cap.
5º_ TRANNY COOLER doing bypass of the stock system. If your transmission cooler is actually the stock system, you are having 235ºF fluid through radiator trying to cool down, making actully coolant to run hotter. Solution is installing a Tranny cooler in the front of the Condenser, doing bypass of the stock system so you avoid heating the coolant, and possible mixes (catastrophic failure) between coolant and transmission fluid. I recommend the Tru-Cool 40000 gvw. The only tranny cooler that keep transmission cool no matter what happens outside or how you ride your truck. In winter is not a problem to have a cool fluid, you just need to change fluid and filter in the transmission more often, each 30000-40000 miles. I use a fluid additive in the transmission too.


Hope you resolve your issues, and this could help you out. I am sure that if you follow this list, you could resolve the issues you are having. Remember that 180ºF thermostat should give you a 1/2 gauge almost always.

Guerrero

 

[Guerrero]

After you try some solutions, post back to know if you resolve those issues

 

[HumbeeH3]

Sensor is in the engine so they will not see similar temps until after it has opened. Have you pulled the t stat since it has been installed? Does it have a bypass hole to keep circulation going? might be worth checking.

Makes sense, I figured that could be the case. I haven't pulled the T-stat since the install, but made sure the jiggle/bypass valve was at 12 o-clock to allow any air bubbles to pass. I have heard of people drilling those out in certain applications, maybe worth doing.

I don't think its a bubble at this point, when I filled it I used the funnel method and watched the bubbles come up, topped it off as levels dropped slightly. It's just unsettling to watch cooling temp climb to 220 and then drop so suddenly, hope the change in temp like that doesn't damage anything. Only happens after the truck been sitting overnight.

Hello HumbeeH3!

I am the one who did the Inline Thermostat Mod a long time ago and created the thread: https://hummer4x4offroad.com/forum/threads/guerreros-take-on-h3-cooling.14490/

About your issues I think is a combination of factors:
One thing to consider firstly is everything done is done right, so here is my list of what could be:

1º_ Thermostat Installation: Just check the picture attached about installation and particulary the part of the Thermostat position, need to be installed like the picture.
2º_ Alrock I think pointed very good, the radiator known to work good is the OEM GM one, not aftermarket or chinese ebay radiators, so maybe to consider replacing it with a AcDelco 22000 unit (oem one)
3º_ And more probably for me, the Dexron Coolant. Dexron is not a good flowing coolant, its a coolant that ensures a lot of time without breaking apart, but I would install another 50% good branded coolant, not dexcool. If you want to replace it, you need to flush it, using distilled water, filling up, running and removing process, usually it takes 2 or 3 fill-remove distilled water to completely eliminate the old coolant and you can use a new coolant (30-40% of distilled water will be on the system but could be mixed with the New coolant, just that would be not 50% coolant, would be less.
4º_And super important, air inside. Works amazingly good about bleeding system to fill the system using the upper radiator hose, and fill the engine firstly and slowly. This remove all the air possible inside the engine and radiator. Once coolant reaches top of radiator upper hose, you install again top hose and finish filling the system through the radiator cap.
5º_ TRANNY COOLER doing bypass of the stock system. If your transmission cooler is actually the stock system, you are having 235ºF fluid through radiator trying to cool down, making actully coolant to run hotter. Solution is installing a Tranny cooler in the front of the Condenser, doing bypass of the stock system so you avoid heating the coolant, and possible mixes (catastrophic failure) between coolant and transmission fluid. I recommend the Tru-Cool 40000 gvw. The only tranny cooler that keep transmission cool no matter what happens outside or how you ride your truck. In winter is not a problem to have a cool fluid, you just need to change fluid and filter in the transmission more often, each 30000-40000 miles. I use a fluid additive in the transmission too.


Hope you resolve your issues, and this could help you out. I am sure that if you follow this list, you could resolve the issues you are having. Remember that 180ºF thermostat should give you a 1/2 gauge almost always.

Guerrero

The man himself. Your thread is actually what inspired me to install the inline t-stat, great write up. I installed the t-stat exactly like the diagram that came with the pack and jiggle/bypass valve at 12 o-clock position to allow any air bubbles to escape, once bubbles are gone I believe it's designed to shut.

Do you notice temp going up before suddenly dropping after the car been sitting around for a while?
Instructions suggests to leave OEM internal by-pass passages or hoses, but I don't think we have that. Wondering if there's any work around that or fix.

Next stop ACdelco radiator and a tranny cooler. I think those two items will definitely help.

Appreciate all the insight and help.

 

[HumbeeH3]

For those of you with the inline t-stat at the upper hose, do you notice your temp climbing up on a cold engine before suddenly dropping?

 

[HumbeeH3]

Hate to keep bumping this to the top, but figured you guys want to hear the solution to this madness.

Drilling a small hole in the t-stat helped equalize the temp on cold engine. Coolant temp no longer climbs way up before coming back down. Engine is also running much cooler now that the inline T-stat is in place. The small 1/8" hole I drilled was to allow coolant to continue to flow past the T-stat and open it, before coolant must have been stagnant and t-stat was operating via conduction instead of convection. By the time coolant got warm enough inside the hose where t-stat is located engine temp was at 220F and then having a rush of cool coolant entering the hot engine can't be good, especially when outdoor temps finally drop. @Guerrero this wasn't mentioned in your original write up, what is your opinion on that?

What I also discovered and maybe my understanding is incorrect, but getting rid of the t-stat all together was what caused my truck to run that much warmer on the highway. Because there was absolutely no restriction, with higher RPM coolant was flowing too quickly for proper heat transfer at the radiator. With the inline t-stat now in place there is some restriction which allows for better/more efficient heat transfer, especially at highway speeds where fan does nothing. A cooler T-stat at the original location would have worked also, but I don't like having to drain my coolant again and having to take my wheel and liner off. I like the new location much better.

 

[Jeepwalker]

Did anyone mention the possibility of the lower radiator hose *sucking in* on itself at highway speeds and restricting flow (if there's no spring in it)? I've seen in on other vehicles in the past. Just a thought.


Sorry if it was alreagy mentioned. I skipped through some of the threads due to having a short break.

 

[HumbeeH3]

Did anyone mention the possibility of the lower radiator hose *sucking in* on itself at highway speeds and restricting flow (if there's no spring in it)? I've seen in on other vehicles in the past. Just a thought.


Sorry if it was alreagy mentioned. I skipped through some of the threads due to having a short break.

Yes that's definitely a possibility, hose collapsing and restricting flow. Wasn't mentioned in this particular thread as a possibility. My hoses are brand new and were observed @ 2500k-3000k rpm range to rule out that possibility but it did cross my mind. I think it was even mentioned somewhere on here, maybe another forum that these vortec engines are known for that to happen.

 

[HumbeeH3]

Got the bypass cooler mounted, stopped short because my flaring tool sucks. Any suggestions on flaring these lines? Some of the write ups I saw seemed to have no issues double flaring.

 

[alrock]

Now watch your outside temp reading in the mirror go all awry with that new heat source right in front of the sensor. Not really an issue at speed, at least for me.

 

[650Hawk]

On the street and idling my coolant temp will be around 205-210F, 213 at most. However on the highway I am at 217-220F with transmission temp pinged at 233-235F. I have checked and replaced the following:

Although everyone s focusing on the engine temps and radiator/thermostat/etc., I'm drawn back to two things. 1st, it should run cooler when moving than when sitting at idle (more airflow). But 2nd (and this is important to note) is the trans temp. Never, ever, have I had a vehicle's trans temp run hotter than the engine temp. My Hummer, my Tahoe, even my 32' class A motorhome pulling a heavy trailer over a long grade in the summer... every single one runs trans temps at least 10 degrees cooler than engine temps, or more, depending on the load and OAT. The increased temp when moving plus the fact that your trans temp is 15 degrees above your engine temp points to a tranny issue. At least that's what I think.

 

[06 H3]

While my H3 has seen many days over 110°, it's never seen this many consecutive days and I never paid as close attention to the temps as I did this summer. I'm sure you saw my other post about this stuff. My H3, while high miles, is well-maintained like yours and I do not suspect any mechanical deficiencies to be causing the high readings. I believe the I5 H3s can't handle stop and go traffic above 110° and I keep the RPMs at 2000 or above to keep the engine (and a/c) cool when stopped. I've also turned off the a/c at some of these stops when the temps get extreme.

Back around 2009, when my truck was still pretty new, I might've been going well above the speed limit on I-8 east of San Diego and it was 112° out that day. I was trying to catch up to my buddy that was cruising at over 90 mph in his H2. When I looked down at the gauges the temp gauge was pretty much pegged. I had to slow down, and I suspect that was the day I snapped a couple of exhaust manifold bolts. Point being that with a high temp load and even with a lot of airflow, my truck was overheating even when it was close to new, still under warranty and everything likely operating to spec.

I am truly hoping that my ECP radiator solves this problem, the added 1 gallon of coolant to the system and thicker passages allow for more cooling and a better duty cycle. I will deal with (5 to 7) 100-105 days a year but nothing like AZ. Id love to see one of the AZ guys try that ECP setup with a severe duty fan clutch.

 

[EndeavoredH3]

I am truly hoping that my ECP radiator solves this problem, the added 1 gallon of coolant to the system and thicker passages allow for more cooling and a better duty cycle. I will deal with (5 to 7) 100-105 days a year but nothing like AZ. Id love to see one of the AZ guys try that ECP setup with a severe duty fan clutch.

I’ll bite, with the ecp requiring much less modification to install with an i5 I’ll throw one on in the coming months.

Temps for me in the summer time, stop and go, are normally 213-225. Going up/down the 101 freeway I’ll be between 204-213. All during that 30 days of 110+ . Ac was on unless temps climbed above or near 225

I have a severe duty fan clutch, updated fan, but the body lift doesn’t help due to the shroud not working at peak efficiency.

Alex, you should be running cooler. You are 4 inches higher off the ground now

 

[alrock]

Alex, you should be running cooler. You are 4 inches higher off the ground now

The thinner air at altitude counters the increased height