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Discussion Starter #1
Hi, Tried to search this with no luck. Probably me. At mile 400 of 425 today, I noticed the trans came out of OD and I was running 2500 rpm. Also noticed the temp was about 235 on gauge. I was thinking that maybe if the temp is too high the trans won't go into OD to help cool the engine. More revs, lighter load more cooling. If this could be true, then a stuck thermostat or other cooling issue could cause problem. Drove 425 on Friday and 425 back today. only acted up on last 25 miles today. Any thoughts?
 

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Discussion Starter #2
On a follow up question, is the thermostat the only device that controls temperature of the engine? Because I saw the temp gauge go well above normal I suspected the thermostat. However with today's computer everything, I wonder. Local dealer wanted over $500 to change the t'stat. They wanted to change the housing, t'stat and labor. Looks like I can do it myself with youtube help. Doing a 1K mile trip on Thanksgiving and don't want to worry too much. This is a 2011 Enclave CXL. I drove around town yesterday and the temp held at normal.
 

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Not sure if the trans will automatically lock out OD in an overheat situation but it wouldn't surprise me. And yes, the thermostat is all that controls the cooling. I assume once it was fully cooled off you actually checked the radiator to make sure it was full, in additional to checking the overflow tank? You have to pull that plastic cover with all the plastic push lock connectors to access the radiator cap.
 

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Also make sure your rad fan is working. Even with an open t-stat situation, at low speeds it can overheat if the fan is not pulling air through the rad to cool the coolant. As JayS mentions, check your levels first, don't rely on the overflow bottle for an accurate level reading.
 

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Discussion Starter #5
So, thinking along those lines I did look at the overflow tank and it was empty. Mixed antifreeze and H2O and added quart and 1/2. Drove till well heated and went home. Cooled and checked again. Empty. Added another 1 1/2 quarts. Drove till normal operating temp reached and went home. Overflow was now full when hot. Let it cool and overflow was at 1/4 full. Do I still need to check radiator ? Can do, but necessary?

kevcol, I was at 70 mph for 2 hours when overheating occurred. Would the fan still be in play?

Thanks guys.
 

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Discussion Starter #6
OK, beautiful day so I pulled the cover and opened the radiator cap. Could see coolant down the tube. Still I filled till it was up in the tube. Less than a pint to get there. We will see how it goes. This has only happened twice, and both times at sustained 70 mph. Hard to believe a thermostat would stick partially closed when running good for hours. But then this is modern engineering.
 

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At 70 mph, not likely a fan issue. However, the loss of coolant is the concern now, where is it going? Any leaks under the car? Or noticeable white smoke coming out the tailpipes? If so, bit of a "sweet" smell to the exhaust? I believe the watermpump leaking is not uncommon with these. But also look everywhere under the vehicle, these have coolant lines running to the rear HVAC unit as well. Good luck
 

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I doubt it has anything to do with fans or the thermostat. It overheated because you're were way low on coolant. I'd be really concerned that much coolant was missing because it's a closed system so it didn't just disappear.

Step 1, check for white smoke on acceleration.
Step 2, pull the oil dipstick and oil fill cap and look for any evidence of coolant in the oil (milky colored oil, white foam in oil cap).

Assuming you're ok on 1 and 2 breath a big sigh of relief that it's probably not the head gasket leaking coolant into the oil or exhaust.

Step 3, start looking around for coolant leaks. All around the radiator, hoses, get under the car and look for wetness. If that comes up with nothing I'd get it pressure tested. Autozone will lend it to you for free with a $100 refundable deposit.
When the waterpump developed a slight leak on my 05 GTO the pressure test identified it immediately.

My money is on water pump weep hole.
 

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Discussion Starter #9
Thank you so much for all the replies. No white smoke, no milky oil. Big sigh of relief!
No drips from car. I have a new concrete driveway, and I have been looking for any drips from both my wife's and my car. Nothing so far.

I will monitor the coolant level going forward. Water pump would only leak when running, right? I will carry premixed coolant with me.
You guys are great.
 

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My biggest concern for you at this time is "Where did the coolant go?" As JayS mentioned, its a closed system, so you shouldn't lose any coolant under normal circumstances. Here is where you can lose coolant:
Head gasket/cracked head directly to combustion chamber- Burns coolant-white smoke
Head Gasket/cracked head/block to oil channel - Leaks into crankcase- milky brown/foamy oil
Internal Radiator Leak to Tranny cooler - Leaks into transmission - milky red/foamy tranny oil
External leak from hoses/waterpump/radiator - puddle under car somewhere over time
External Leak from heater core(s) - leaking into the cabin somewhere (front under dash/footwells but may pool in battery well eventually; rear in the passenger rear quarter, but should drain out a drain tube, or pooling somewhere)
Good luck in tracking the issue down. As mentioned, a pressure test would help to locate it.
 

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OP- check for a slow leak. It can be so slow- that it trickles down the side of the timing chain cover-- and it being hot, the coolant evaporates before it hits the ground. But it leaves traces of white stuff.
Heres a HOW TO spot a water pump leak...AND
HOW to change your thermostat (and radiator hoses)

 
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