2012 Yukon XL Denali temperature creeping up at idle only.

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ShakinNBakin663

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Hello everyone, former mechanic here. Truck is a 2012 Yukon XL Denali, AWD, 6.2L with just under 150k on the clock. Truck has been very well maintained. This issue has me completely stumped and I’m hoping for some validation before I spend more money throwing parts at it. Timeline/work completed below. Each new line is about a week’s time. Sorry for the long read, was trying to be as detailed as possible.

  1. Saw a basketball sized puddle of coolant on the driveway. Pressure tested, found source of leak at thermostat housing, along with a “sizzle” of coolant at both the upper and lower radiator hoses. Replaced thermostat and both hoses with OEM parts. Pressure tested fine.
  2. Squirrels made a nest behind the main fuse block under the hood, chewed wires. Lost compass, backup camera, fog light indicator light on cluster, and one other convenience item that is slipping my mind. Got insurance involved due to not knowing truly how bad it was, and because I have a $100 comprehensive deductible making it that much sweeter to not have to do the work myself. Trusted mechanic shop (who my family has dealt with for thirty plus years) did the wiring repair, all good.
  3. Driving down road and one of the hoses to the heater core broke off at the heater core connection under the hood. Plastic connector sheared off of hose. Replaced hose with OEM parts.
  4. Engine temperature began creeping up at idle and only idle. Truck has never gone above 210f. Has never gone above 215f to 230f when it’s acting up. Happens regardless of outside temperature and conditions and with A/C on and off. Upon accelerating, temps immediately drop to 210f until stopping again. It doesn’t happen every time but, I would say it’s about 85% of stops for a red light cause the issue. Noticed coolant was low, about a half gallon. Figured I didn’t get all of the air out when I did the heater hose. Filled coolant, all good.
  5. Noticed another small puddle of coolant on the driveway. Pressure tested and found a minor leak at the water pump. Replaced water pump and thermostat (for the second time, I was there so I figured just do it), and truck still continues to heat up more than normal. OEM parts.
  6. Checked operation of fans, pressure test is perfect, and stared at the truck for far too long. Noticed the rubber (?) baffles to the left and right of the radiator are torn and have fallen down. Figured the air is getting sucked around the radiator by the fans instead of being pulled through. Began researching and found that people have had this issue on this forum, baffle replacement has been recommended and fixed the problem for some but most threads are left dead. I’m trying to justify this working before I spend the time and money fixing it. I ran the truck with the pressure tester on it for an hour in my driveway. It never moved above 210f. Hood was up because the pressure tester wouldn’t let it close. Closed hood, and drove to find the issue was still there. My thought is that the air is not being pulled through the radiator and is being recirculated under the hood due to these baffles failing. When the hood was open, the air could easily escape causing the issue to disappear. The truck is currently at the mechanic who repaired the wiring after the squirrel damage as I decided to get a second opinion. They have double checked all of my work, found the radiator to be in good condition, and are stumped as well. Their top guy, who has thirty five years of experience, doesn’t believe the baffles could make a difference but the posts here say otherwise. Even the parts manager at the local GMC dealership said that he has never ordered a set prior to the ones that I ordered. Is there something I’m missing? Hopefully I touched all the bases. Thank you for your thoughts!
 

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Marky Dissod

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Engine temperature began creeping up at idle and only idle. Truck has never gone above 210F.
Has never gone above 215F to 230F when it’s acting up.
Happens regardless of outside temperature and conditions and with A/C on and off.
Upon accelerating, temps immediately drop to 210F until stopping again.
It doesn’t happen every time but, I would say it’s about 85% of stops for a red light cause the issue.
Noticed coolant was low, about a half gallon. Figured I didn’t get all of the air out when I did the heater hose.
Filled coolant, all good.
My considerable experiences with LT1 B- & D- sedans & wagons lead me to wonder if you've ever had the ECM reprogrammed?
What you described is reminiscent of how LT1 B- & D-cars with electric fans behave until the fan-on coolant temp thresholds get reprogrammed.
The mildest headwind - forward motion - keeps your radiator cooler than GM's programming for your fans.
That gets fixed with a tune.

Can't tell you GM's fan-on coolant temp thresholds for your truck.
Bet that they're set just high enough to replicate this behavior.

On my LT1 cars, I typically had to burp the cooling system several times over a week or two to be SURE I'd burped it completely (air likes to find places to hide in the LT1 cooling system).

And yeah, the baffle curtains' job is to make air do its job - to go THROUGH the radiator instead of around it.
So, replace / properly install those.
 
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ShakinNBakin663

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You need to flush the radiator and possibly change water pump. the LS engines are known to eat water pumps....
Radiator was flushed when the hoses were replaced and the water pump is less than a week old. Can confirm it is operating as well. Radiator flush came out much cleaner than expected.
 
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ShakinNBakin663

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My considerable experiences with LT1 B- & D- sedans & wagons lead me to wonder if you've ever had the ECM reprogrammed?
What you described is reminiscent of how LT1 B- & D-cars with electric fans behave until the fan-on coolant temp thresholds get reprogrammed.
The mildest headwind - forward motion - keeps your radiator cooler than GM's programming for your fans.
That gets fixed with a tune.

Can't tell you GM's fan-on coolant temp thresholds for your truck.
Bet that they're set just high enough to replicate this behavior.

On my LT1 cars, I typically had to burp the cooling system several times over a week or two to be SURE I'd burped it completely (air likes to find places to hide in the LT1 cooling system).

And yeah, the baffle curtains' job is to make air do its job - to go THROUGH the radiator instead of around it.
So, replace / properly install those.
Tune is stock as far as I know. I’ve owned it for 50k+ miles. Any tricks for burping it?
 

salisburyv

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i would say make sure both fans come on. IMHO, sounds like the radiator is clogged, and doesn't have enough flow at idle. once you spin the water pump faster it'll force more coolant thru the radiator. Before tearing it out thoroughly inspect both sides, make sure there's no surface obstruction. But i'm sub'd for results
 

j91z28d1

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what are the outside temps? and you're sure both fans are coming on, even with the ac on which will turn the fans on no matter the coolent temp and you're only overheating at idle. I mean the baffles won't hurt you do. but kinda surprised they are needed at idle.

honestly unless the temp sensor is bad,nor the wiring to it, 5v sensor power, ground or signal I believe all come from the ecm on gm stuff. if not, sounds like air in the system to me.

you'll have to look up radiator fill, I don't remember but seem to remember it was eazy, like just leave the cap off and idle it till the stat is cycling and head on front and rear.

I'm guessing you used a oem stat. I'm not a fan of the motorad stats. but even the oem ones can open at a little be different Temps. ls engines are weird about their thermostat Temps, they are on the inlet side of the engine, so the cooled water hits the stat first, but the fans run off the in head temp on the hot side. so they have to put a 187deg to run 210 in the head. I've seen some weird temp cyl till everything gets up to stabilized Temps. there's also 2 different size stats and what year the split was is different for different models. like 08 was the split year for the corvette. below had a smaller stat, newer had the bigger. but you could mistakingly put a smaller in a bigger one. messes with the bypass and stuff too.
 

Geotrash

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My vote goes to fan motors not putting out full power - either because the motors are weak or the wiring is. Classic symptom.

The baffles don’t do much at idle - they’re there to keep ram air from going around the radiator at higher speeds such as when climbing a mountain with a heavy trailer.
 

Marky Dissod

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Tune is stock as far as I know.
Vast majority of those who got tuned, wish they'd gotten tuned sooner.
Seriously consider it, not just to disable V4 mode, or to add longevity to the 6L80E, or to keep it cooler ...
Any tricks for burping it?
Burp it more than once. The day you're sure you burped it enough, maybe burp it once more a few days later.
 

Doubeleive

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my experience with the EXACT same issue has always been the radiator, at 150k you are on borrowed time anyway.
as others have noted check out the fans as well, pull the harness off each fan and see if the connector has melted at all, if it has replace both the harness connector and fan motor
the 103285 fits all gmt900's
1800radiator.JPG
 

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