PSA - Dealing with Extreme Heat Soak

[TeCKis300]

Following up on a trip post where I experienced extreme hot conditions - ambient temps 107°F, crawling an arduous, rocky, and steep slow speed trail from 2k ft to 10k ft. Wheeling in the southwest, hot conditions in the summer are the norm, between Vegas, Death Valley, wheeling and towing high loads. This was something else completely. Setting was the High Sierras in August, around 5k feet elevation (thin air) where heat soak really started setting in.

Intake air temps (IATs) crossing 200°F tells the story of just how hot things were getting. The intake is in the fenderwell, just above and behind the fender liner of the passenger side front tire. Usually it's no more than 30°F over ambient when rock crawling. Closer to ambient at highway speeds.

Low range the whole time which is super important for cooling fluid circulation and not riding the torque converter (which generates extra heat). This time, that wasn't enough.

PSA - Like most Toyota body on frame models, the 200-series radiator fan is engine driven. Fan speed is tied to engine speed. If experiencing heat soak, use low range 1st gear, not because terrain or wheel torque demands it, but to drive the fan to increase airflow.

I've never seen the engine coolant temps much beyond 205°F on OBD-II. The dash gauge doesn't move. This time it did. Interestingly 230°F is about 2/3rds on the dash gauge. Still not red, but moving the wrong direction.

I was doing a lot of low speed crawling in Lo 1st gear. After technical sections, I was going into lo 2nd and 3rd gear. I could see the response equating engine speed to coolant and intake temp. Where I came to the realization I needed to boost engine speed (2.5-4k rpm) regardless of driving conditions to force more air through the radiator and engine bay. With that understanding, I was able to manage temps bringing things down to a still toasty, but more reasonable 160°F IAT, 215°F coolant, with tranny temps following downward.

Shutting off the A/C is a good move too, and turning on the cabin heater full blast. With higher engine / fan speeds, things were returning to normal. This same strategy can help with improving A/C performance as the radiator fan is drawing air over the condenser.

No harm, no foul, but lesson learned for another strategy to deal with extreme heat.

Related for these types of high heat and high altitude is fuel boiling. Discussion here.

Might be interesting, but my buddies Gen 2 R1T tri-motor max pack (410 mile range) spent a shocking 20% of its battery over the 15 mile journey, to actively cool its battery. That changed our reserve and backcountry route.

 

[gasman4u]

Following up on a trip post where I experienced extreme hot conditions - ambient temps 107°F, crawling an arduous, rocky, and steep slow speed trail from 2k ft to 10k ft. Wheeling in the southwest, hot conditions in the summer are the norm, between Vegas, Death Valley, wheeling and towing high loads. This was something else completely. Setting was the High Sierras in August, around 5k feet elevation (thin air) where heat soak really started setting in.

Intake air temps (IATs) crossing 200°F tells the story of just how hot things were getting. The intake is in the fenderwell, just above and behind the fender liner of the passenger side front tire. Usually it's no more than 30°F over ambient when rock crawling. Closer to ambient at highway speeds.

Low range the whole time which is super important for cooling fluid circulation and not riding the torque converter (which generates extra heat). This time, that wasn't enough.

PSA - Like most Toyota body on frame models, the 200-series radiator fan is engine driven. Fan speed is tied to engine speed. Use low range 1st gear, not because terrain or wheel torque demands it, but to drive the fan to increase airflow.

I've never seen the engine coolant temps much beyond 205°F on OBD-II. The dash gauge doesn't move. This time it did. Interestingly 230°F is about 2/3rds on the dash gauge. Still not red, but moving the wrong direction.

I was doing a lot of low speed crawling in Lo 1st gear. After technical sections, I was going into lo 2nd and 3rd gear. I could see the response equating engine speed to coolant and intake temp. Where I came to the realization I needed to boost engine speed (2.5-4k rpm) regardless of driving conditions to force more air through the radiator and engine bay. With that understanding, I was able to manage temps bringing things down to a still toasty, but more reasonable 160°F IAT, 215°F coolant, with tranny temps following downward.

Shutting off the A/C is a good move too, and turning on the cabin heater full blast. With higher engine / fan speeds, things were returning to normal. This same strategy can help with improving A/C performance as the radiator fan is drawing air over the condenser.

No harm, no foul, but I now have another strategy to deal with extreme heat.

Related for these types of high heat and high altitude is fuel boiling. Discussion here.

View attachment 4191729

Might be interesting, but my buddies Gen 2 R1T tri-motor max pack (410 mile range) spent a shocking 20% of its battery over the 15 mile journey, to actively cool its battery. That changed our reserve and backcountry route.

View attachment 4191740

Is your buddy going to start packing a small genny and a 30A cord for future trips?

 

[Jason Andrews]

Following up on a trip post where I experienced extreme hot conditions - ambient temps 107°F, crawling an arduous, rocky, and steep slow speed trail from 2k ft to 10k ft. Wheeling in the southwest, hot conditions in the summer are the norm, between Vegas, Death Valley, wheeling and towing high loads. This was something else completely. Setting was the High Sierras in August, around 5k feet elevation (thin air) where heat soak really started setting in.

Intake air temps (IATs) crossing 200°F tells the story of just how hot things were getting. The intake is in the fenderwell, just above and behind the fender liner of the passenger side front tire. Usually it's no more than 30°F over ambient when rock crawling. Closer to ambient at highway speeds.

Low range the whole time which is super important for cooling fluid circulation and not riding the torque converter (which generates extra heat). This time, that wasn't enough.

PSA - Like most Toyota body on frame models, the 200-series radiator fan is engine driven. Fan speed is tied to engine speed. If experiencing heat soak, use low range 1st gear, not because terrain or wheel torque demands it, but to drive the fan to increase airflow.

I've never seen the engine coolant temps much beyond 205°F on OBD-II. The dash gauge doesn't move. This time it did. Interestingly 230°F is about 2/3rds on the dash gauge. Still not red, but moving the wrong direction.

I was doing a lot of low speed crawling in Lo 1st gear. After technical sections, I was going into lo 2nd and 3rd gear. I could see the response equating engine speed to coolant and intake temp. Where I came to the realization I needed to boost engine speed (2.5-4k rpm) regardless of driving conditions to force more air through the radiator and engine bay. With that understanding, I was able to manage temps bringing things down to a still toasty, but more reasonable 160°F IAT, 215°F coolant, with tranny temps following downward.

Shutting off the A/C is a good move too, and turning on the cabin heater full blast. With higher engine / fan speeds, things were returning to normal. This same strategy can help with improving A/C performance as the radiator fan is drawing air over the condenser.

No harm, no foul, but lesson learned for another strategy to deal with extreme heat.

Related for these types of high heat and high altitude is fuel boiling. Discussion here.

View attachment 4191729

Might be interesting, but my buddies Gen 2 R1T tri-motor max pack (410 mile range) spent a shocking 20% of its battery over the 15 mile journey, to actively cool its battery. That changed our reserve and backcountry route.

View attachment 4191740

That's surprising. What's the GVWR rating on the aux tranny cooler you have? I'd have thought pulling your Airstream would be a tougher task than this even with those ambient air temps and lack of airflow at super slow speeds. Would a dedicated electric pusher fan on the trans cooler help?

 

[Artie]

You hit some impressively high temps. The coolant temp of 230° on your OBD fusion gauge vs the dash gauge makes me curious at what the temp must achieve to get that dash gauge into the red. I can vouch that 238° won’t do it and I don’t have any intentions on surpassing that. While under pressure I’m imaging 250° or more….oof!

I run a snorkel and it really helps at keeping that intake air temp low, especially while moving. However, given your described scenario I’m guessing the heat soak would make it a wash at those low speeds.

At what coolant temp are you calling for a break to let things cool down?

 

[GrouchyTech]

278°F is what the coolant temperature value will read when the circuit is shorted…
I’ve seen the needle move to about 3/4 on the gauge in a Tundra. I THINK that was at around 240° (but don’t quote me).

 

[sdnative]

Interesting. I just got back from running (I think) the same trails under the same conditions. Our highest point was 11600'.

I did not have a scan gauge or OBD scantool to monitor temps but the coolant temp needle was rock solid at halfway the entire trip. I was in 4 low once I started climbing and stayed there the entire time. I use lower gears for more controlled response in the technical stuff but also for engine breaking. RPMs in any type of climb, decent or technical terrain was frequently above 2k, sometimes 3k and above.

I was thinking I wish I had something to monitor temps. This post gives me some motivation to get something setup. Thanks for the PSA.

 

[LTSR]

I was just looking at my photos from august 2025. I was also in the sierras at this time, climbing up to 10k feet. It got a bit hotter than this photo. I recall the trans temp hitting 235F. Low range, 1st gear, though i kept the AC on. Fuel boiling, I recall we were crawling for at least 2 hours due to a slower group up ahead. I stopped to let the LC cool after the long climb.

I did two transmission drain and fills after this trip. I didn’t like that the trans temp got as high as it did.

Good PSA for those of us who find ourselves in these conditions.

 

[TeCKis300]

Is your buddy going to start packing a small genny and a 30A cord for future trips?

We've done quite a bit of off-roading with my Rivian buddy, but this was his/our first trip deeper into the backcountry. Between the severe heat (cooling battery and cabin) and crawling at motor stall speeds, it added a giant unanticipated variable. Not a bad idea for a genny for worst case-scenario. We were discussing towing to regen. What we did anticipate is that so long as he was able to get to the peak under his own power, there would be lots of regen down. We left his car at the peak while continuing to explore.

On the way back from 11.5k to 4k ft, he regened 7%.

That's surprising. What's the GVWR rating on the aux tranny cooler you have? I'd have thought pulling your Airstream would be a tougher task than this even with those ambient air temps and lack of airflow at super slow speeds. Would a dedicated electric pusher fan on the trans cooler help?

My aux aux cooler doesn't have a rating. The tranny temp wasn't driven by loads as much as it was the engine coolant radiator exchange. Transmission pan temps (227°F) was within a degree of coolant temp (228°F). Once pushing more air through via engine rpm, it pulled both down nicely.

You hit some impressively high temps. The coolant temp of 230° on your OBD fusion gauge vs the dash gauge makes me curious at what the temp must achieve to get that dash gauge into the red. I can vouch that 238° won’t do it and I don’t have any intentions on surpassing that. While under pressure I’m imaging 250° or more….oof!

I run a snorkel and it really helps at keeping that intake air temp low, especially while moving. However, given your described scenario I’m guessing the heat soak would make it a wash at those low speeds.

At what coolant temp are you calling for a break to let things cool down?

This did get me thinking about a snorkel that would breath fresh ambient air rather than heat soaked air from the engine bay. It would also help my engine air filter change interval.

Interesting. I just got back from running (I think) the same trails under the same conditions. Our highest point was 11600'.

I did not have a scan gauge or OBD scantool to monitor temps but the coolant temp needle was rock solid at halfway the entire trip. I was in 4 low once I started climbing and stayed there the entire time. I use lower gears for more controlled response in the technical stuff but also for engine breaking. RPMs in any type of climb, decent or technical terrain was frequently above 2k, sometimes 3k and above.

I was thinking I wish I had something to monitor temps. This post gives me some motivation to get something setup. Thanks for the PSA.

This actually occurred on a much harder southern route from Big Pine with rock crawling at idle speeds and deep sand. Wouldn't recommend it on anything less than 35s. The Rivian on 35s took some minor rocker damage. I ran the Bishop route the next day to guide a buddy up and that one went nice and smooth without even boiling fuel.