Hello All,

This one has me stumped:

- 2007 GMC Acadia, 198,000 miles, been in the family its whole life
- Had timing chains done as part of the recall at 80,000, while they were in there the water pump was changed as the original has started to leak. Both of these were long ago at a GM dealership
- About 10,000 miles ago, coolant temperature started to get hot easily, but could be calmed down with light-throttle no-hill freeway speeds, idling, and/or cold weather. We've nursed it along, knowing this and keeping an eye on the temperature gauge, for a while.
- Now if ambient temperature is above ~95 degrees, coolant temperature can't be kept below 230 (OBD2 reading) without running both heaters full blast.

Along the way, I have replaced the thermostat, coolant temp sensor, and now the radiator and cap. The new radiator is the GM Genuine extra-thick core option for "heavy cooling"(it turns out the original was the same thickness). Same behavior.

- No leaks
- No noises
- No collapsed hoses
- Runs fine otherwise
- No sign of blown head gasket (no disappearing coolant or steam/bubbles etc. in radiator, no overflow, no limping cylinders, no discoloration of anything, no extreme pressure)
- Cools down predictably with heaters on full blast
- Old radiator looks as new on the inside as the new one (looking inside from fill neck)
- Coolant came out with zero junk in it
- My IR thermometer says there is consistently about 30 degrees temperature drop between upper hose at neck and lower hose at thermostat housing once the thermostat opens, generally showing ~195 in and ~165 out.

Next on my list is replacing the water pump. Maybe the impeller came off the shaft or something?(?!?!?)

Since that'* a huge pain, and doesn't make sense with these symptoms, I wanted to see if y'all had any ideas of that else could be causing this.

Weirdest thing is, when this behavior started, it was sudden. We lived in an area with large hills nearby, like 7% grades for miles at highway speeds. Aside from warming up, we had never seen the temperature needle move, no matter what we did. Then this behavior started one day driving up one of those hills, and has gotten progressively worse over time.

Thanks all!

Any pinging going on? Could be carbon buildup. Fully functional fan?
Have you checked for electrolysis? That can eat away the fins on the pump and cause over heating.

No pinging. Front three plugs look really clean for ~84,000 miles that they've been installed, no carbon buildup. No sign of anything in the exhaust at WOT shift points like many cars that are carboned up. It also doesn't see much cold-running slow driving. 98% of it'* life has been warmed up and 45+MPH, 10+ miles per trip. Much of that 98% has been freeway and highway driving.

Fans both work well at low and high speeds, and do what they are supposed to with the air conditioner running.

No signs of electrolysis on anything else. I guess I'll know for sure on the impeller when I swap out the water pump. I was thinking along these lines too, just everything else was so clean that I'm betting I'll be swapping out a perfectly-fine water pump. I'm okay with that as the first water pump lasted 80,000 miles so I wanted to replace this one just to be safe. I just don't want to replace it and not have evaluated other possibilities.

Run vehicle until normal hot. Take the rad cap off and use your multimeter. Put one probe into the coolant and one on ground and measure the volts. Anything more than a few tenths can eat away at things. Especially aluminum parts.

Cool, thanks! I've never heard of this test before. I'll do it and report back.

It slowly counted its way up from about 0.135VDC then leveled off at 0.143VDC.

Interesting thing that I noticed: I let it warm up, then took the cap off. It gushed out a little bit of coolant and then that was it. Like maybe half a cup in like half a second. Usually, just about any car will gush like a quart or more out in the same circumstances, and will keep wanting to for a while. Not this one. It just did it'* little gush, then I finished unscrewing the cap, then watched it sit there with the coolant about an inch below the top of the neck and not do anything at all, for over five minutes. It'* almost 105 out there right now. I would expect the level to be going up or down or something.

Now I'm thinking maybe the water pump really isn't doing much, electrolysis or not, like something is plugged up in there or something.

Thoughts?

See what it reads cold. Take a look at the impeller when you take it out. See if it'* pitted at all.

It is down to about 98 degrees coolant temperature, which is only a couple of degrees warmer than ambient. It started at 0.132 and worked its way down to 0.079 . It slowed its progress the further it got, and might have continued forever but I didn't want to stay out all night. Does this mean anything exciting?

That'* a slight amount. I'd look at the pump. Depending how long it'* been doing this you may have impeller problems.

Okay cool. New Gates unit showed up today. Will report back when I swap it.