H3 carpet getting hot after driving uphill a lot

So I’ve taken two trips lately where I head up into the mountains. I’m typically climbing 4,000-5,000 feet on these trips, so to keep my H3 at 65+ mph I’m driving a lot of the time with the I-5 at 3,000+ rpms. Both times once I’ve gotten to the top of the mountain and arrived at my destination (friends cabin) I’ve noticed the carpet, particularly right behind and under the front seats is extremely warm. In fact as I was unloading fishing gear last time I noticed the metal bracket on the bottom of the seat that bolts the seats to the floor was very hot to the touch. Like if you held your hand there you would definitely burn your hand. Like touching a pot on the stove.

Is this an ominous sign of transmission trouble? Or typical for running at higher rpms due to all the climbing?

You're right, it can get hot under that area. The catalytic converter shield attaches to the floor via bolts and they go through the floor. Those bolts are relatively close to the converter and can conduct a lot of heat. And, yeah, the rear seat bolts also go through the floor and are fairly close to the CC. Frankly I was surprised to see how GM did it, it doesn't seem like a good idea, based on what I've seen on most other similar vehicles. It's something I've thought about too, although I haven't felt extreme heat (but I'm sure it 'could' occur). It seems like asking for a potential fire, under worst-case circumstances, esp if the CC shield rusts off (which they eventually do in the rust belt states). But I guess (hope) GM did the heat calcs to show it was ok. I haven't heard of any vehicle fires, but who knows?

I don't know the driving conditions of your journey, but I speculate if a person were driving in the same direction as the wind, and not enough air-flow was cooling that area off, (or some slow driving/idling towards the end), maybe in your case it could have allowed even more heat to build up than might otherwise be the case. I guess you should make sure your catalytic converter shield is in place, which deflects a lot of heat downward (the CC shield seems somewhat undersized too). Most vehicles I've worked on over the years have brackets welded to the underside of the cab ...and the heat shield attaches to those brackets (to reduce conduction of heat through the floor).

Some (approved) reflective foil matting above the converter shield attached to the cab underside would reduce a LOT of heat transfer to the cab floor. Something I've considered doing. A lot of vehicles have been doing that for years, like Grand Cherokees, etc.

You might pull off your rt side lower plastic and work your hand under the rear pass footwell carpet to see if it's wet. You might need to loosen the seat's rear bolts and tilt it up to get your hand further in. A wet carpet would conduct more heat than a dry carpet, making it feel even warmer. Leaks are common, if not through the sunroof drain tubes (if you have a S/R), then via cracks in the caulking under the roof side plastic trims (or the rear cargo top gasket). Once I remove the interior plastic and put a hose to the roof of my hummer, there was water streaming in (not drippping) in two places!

To add to Jeepwalker, who is right on the money, check your trans temps. Im 99% sure that half of your issue is coming from the hot trans sitting in the trans tunnel. Before adding a trans cooler, it used to get so hot in the summer id burn my foot if I was wearing sandals and accidentaly leaned my foot off the accelerator onto the carpet by the firewall for a few seconds!
You can buy exhaust wraps and use them as jeepwalker mentioned. Will sure help

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I'm trying to remember if there is insulation on the transmission tunnel above the tranny? Maybe someone else can chime in. If not you could probably add a piece of rigid foil, or maybe buy/adapt one from a different GM vehicle (Colorado/Silverado??)

If it's truly hotter only when going uphill, I would lean toward the extra heat being from the transmission. I don't think the cat is going to be noticably hotter uphill vs. level driving-- it's crazy hot all the time. I can also tell you that mine does get warm, but nothing at all like you have described and I do not have an automatic.

To answer to previous post, there is no heat shield or insulation in the transmission tunnel, but again mine is a manual so I don't know if autos were different in that regard.

BTW, how does your trans fluid look/smell? Do you have a scanner where you can read the trans temp as you duplicate the route? I *believe* trans temp is part of the available readings.

Trans temp is a readable value of you have the right combo of codes to read it with. My Ultragauge is setup for it. You can also read the exhaust temps at the cats and I will tell you they do go up under load, as high as 1800°+ and towards 1900°, at least on the V8. I think normal running temps are around 1200° to 1300°, or 1500°, I can'treally recall at the moment.

With the V8 and the flooring removed inside it will get HOT inside the truck, especially along the trans tunnel. That is part of why the OEM flooring is so heavily insulated, that and NHV reasons. If I had thought about it when I had the transmission out I would have put more insulation in the tunnel. Even the cargo area gets really warm with the flooring in place just from the muffler underneath.

As long as nothing is melting or making weird smells you are probably fine. But put a scanner on and check your cat temps at least to be sure they aren't getting crazy high which would indicate a problem, though the PCM should pick up on that if the O2 sensors are working correctly and would give you a check engine light.

If your trans is heating up that much then you should also be getting a check engine light and message on the cluster display or at least soft codes logged.