Transmission overheating

dsm1995gst · July 19, 2019

So I just got a new boat (only 17ft fiberglass) and was on the way to put it in the water for the first time and my 2018 GMC Z71 with 10k miles apparently can’t handle that, as within about 15 miles on the interstate I get a warning saying my transmission is overheating and I need to pull over and idle.

After a couple of minutes I was finally able to navigate through the control panel it said the trans temp was 258 degrees.

I was driving in tow mode.

After it cooled down to 210 degrees I tried again without tow mode and it was back to 241 degrees before I got to the next exit about 2 miles away.

Needless to say I’m pissed off.

Farmerman1379 · July 19, 2019

How heavy is your boat? I can't imagine it overheating from that, especially if you're cruising interstate speeds with that air flow through the cooler.

CamGTP · July 19, 2019

I'd take to the dealership with the boat, drive it with a tech and show them.

Something that would be covered under warranty no doubt.

flyingfool · July 19, 2019

you need to remove the trans thermostat

BlancoSilverado · July 19, 2019

17 minutes ago, CamGTP said:

I'd take to the dealership with the boat, drive it with a tech and show them.

Something that would be covered under warranty no doubt.

Yup took the words out of my mouth, take it to the dealer. the thermostat might not be working.

Farmerman1379 · July 19, 2019

10 minutes ago, BlancoSilverado said:

Yup took the words out of my mouth, take it to the dealer. the thermostat might not be working.

Did a quick Google search and found a thread on this site of another 2 people having the same issue with the thermostat not opening.

dsm1995gst · July 20, 2019

Did a quick Google search and found a thread on this site of another 2 people having the same issue with the thermostat not opening.

Could it still be the thermostat if it doesn’t do it when not pulling the boat?

We ended up unhooking the trailer (and swapping trucks) and my truck drove home without the boat in the 195 degree range.

dsm1995gst · July 20, 2019

How heavy is your boat? I can't imagine it overheating from that, especially if you're cruising interstate speeds with that air flow through the cooler.

I’m guessing like 1600 for the boat, 400 for the motor.

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FlipZ71 · July 20, 2019

Trailer brakes dragging ??

dsm1995gst · July 20, 2019

Trailer brakes dragging ??

No brakes, good idea though

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dsm1995gst · July 22, 2019

Just for reference, father-in-laws 2500 pulled the little boat with the transmission temperature at a cool 185 degrees...

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dsm1995gst · July 22, 2019

Just for reference, father-in-laws 2500 pulled the little boat with the transmission temperature at a cool 185 degrees...

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And thus far today my truck has hit 210 driving around town... supposed to take it to the dealership tomorrow morning.

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HeySkippyDog · July 22, 2019

And thus far today my truck has hit 210 driving around town... supposed to take it to the dealership tomorrow morning.

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That is too high. I live in Central Florida and our ambient air temp is about 85-93F, and I am usually between 185-190F. Your transmission thermostat is probably stuck shut.

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BlancoSilverado · July 22, 2019

On 7/22/2019 at 8:45 AM, dsm1995gst said:

And thus far today my truck has hit 210 driving around town... supposed to take it to the dealership tomorrow morning.

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What did the Dealer say?

Sorry, I jumped the gun.

Edited July 23, 2019 by BlancoSilverado

truckguy82 · July 23, 2019

On 7/19/2019 at 6:37 PM, dsm1995gst said:

So I just got a new boat (only 17ft fiberglass) and was on the way to put it in the water for the first time and my 2018 GMC Z71 with 10k miles apparently can’t handle that, as within about 15 miles on the interstate I get a warning saying my transmission is overheating and I need to pull over and idle.

After a couple of minutes I was finally able to navigate through the control panel it said the trans temp was 258 degrees.

I was driving in tow mode.

After it cooled down to 210 degrees I tried again without tow mode and it was back to 241 degrees before I got to the next exit about 2 miles away.

Needless to say I’m pissed off.

I am the first person I know of to have a stuck closed trans thermostat. You’re doesn’t sound like that.

I also towed and even raced once while towing 4klbs (before I knew my trans thermostat was stuck closed)

-My temperature never increased anywhere close to as fast as yours

-it never got above 230, that’s towing 3k lbs through the city of philadelphia for an hour

So here’s how to tell if your thermostat is stuck.

Thermostat opens at 190-200. when your trans temp is like 140-160 range, pop your hood, and feel the drivers side trans cooler line in the front of the truck. Then feel the passenger side.

If your thermostat is stuck closed, your driver side line will be about as hot as your trans temp says it is. Your passenger side line will be ambient temp until the trans reaches 190-200. Once it reaches target temp the flow gates should open and the passenger side should be nearly as hot as the driver side.

if you have trouble located the lines, you can follow them right out of the transmission. It’s just easier to check them at the trans cooler since you don’t need to get under the truck

the techs had never dealt with this and were stumped. I had to basically figure this out on my own and verify the thermostat wasn’t operating and then tell them to change it.

Oh and btw you need to change all your trans fluid out it’s burnt.

Edited July 23, 2019 by truckguy82