The car's got the 3.4 motor in it with about 136k miles. Radiator has been replaced, thermostat has been replaced, water pump has been replaced, temperature sensor has been replaced, I haven't seen any leaks, fans are turning on, and it heats up at random.

It's the weirdest thing I've ever seen. We'll drive it out for about 10 miles and it will overheat. Other times it won't overheat at all. When I say over heat, I mean in the red zone. Not sure what the degree is but it's no bueno.

What do I look for next?

 

Could be a faulty gauge, could be air in the system, could be a blockage in a line. I would bleed it first.

 

Those motors had issues with intake gasket/head gaskets failing - could cause this, and air to to get in the system continually during warm up and cool down cycle. If it is this issue, GM extended its' coverage for these parts. Just an idea...

 

I used to own a 2004 impala.

It's the head gasket.

To make sure go rent a pressure tester.

Sounds very close to what mine did.

 

But if it was a head gasket, wouldn't there be water in the oil? it isn't losing coolant. I guess there could be air in the system that I need to bleed.

What's the best way to bleed the coolant system? I don't really want to start a head gasket if I don't need to.

 

If you're getting real hot, but you don't have any coolant boiling out of the overflow reservoir....its almost certain that its NOT a bad head gasket. Boiling over of coolant is almost always associated with a bad head gasket because air is getting pushed HARD into the cooling system.

I'd say you either have a "bad" new t-stat that is not opening....or an air bubble on the engine/head side of the t-stat.

Go down to the Bleeding part....give it a try. Works every time when I do a coolant flush.

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Best/easiest way to flush and get every drop of old coolant out.

****My power steering fluid leaked into my block, so it was bad, but this flush process works for normal maintenance flushes too. You might just want to skip the degreaser stages.****
-Cold engine.
-Remove radiator fill cap.
-Remove the t-stat from the housing. ((Buy a new t-stat housing gasket, they're like $3.00)) Leave the housing attached to the rubber radiator hose, just remove the 2 housing bolts and pull it away from the water pump to get to the t-stat. (2-3 minute job).
-Put t-stat housing back on. (1 minute) Just put one bolt in, no need to put them both in, there’s no pressure in the system during the flush.
-Take the entire radiator drain valve (petcock) "off" and let it drain, don't just open the valve itself. It'll drain faster with it off and that’s what you want. ((Buy a new petcock valve before starting this flush process, sometimes they break when you remove them all the way just because they're cheap plastic and they get briddle over time, they're like $2.00))
-Take a hose and stick it in the radiator fill cap, running medium to high.
-Start the engine when the radiator looks like its full again.
-Turn heat on full blast
-Let it run for about 15-20 minutes or until the water is running out the drain CLEAR.
-((If you want to, you can wait till it runs clear, "close" the drain valve, add some degreaser (I use ½ gallon of Formula 88 to clean mine) and let it run for 15 minutes, then let it sit for 15 minutes, then run it for 5 minutes, then drain it all again. Then open the drain and put the hose in for about 5 more minutes and run it all out till its CLEAR. The degreaser will help break up the crap thats stuck DEEP in the BLOCK that sits and swirls and doesn't like to come out.)) ***NO…degreaser will not hurt anything.***
-When it runs clear your entire system is clean.
-Remove the overflow reservoir from the car and clean it out real good. (I had to use gasoline to clean mine out because the sludge and grime was so thick inside. The gas broke it all down and then it flushed right out. I filled it about 1/3 up with gasoline and shook the hell out of it real vigorously, the black stuff kept coming out. I did that like 4 separate times with gasoline till no more chunks of black crap came out. Make sure the lines that go to the reservoir are also cleaned out. Or just buy a new piece of 3/8” heater hose and replace that line, 3 feet will do, then cut to fit. My sludge came from my power steering fluid leaking into my coolant system.)
-Put the t-stat back in.
-Put the overflow reservoir back in.
-Put the drain valve back in. Use the new one, what the hell.
-Put half a jug of Dexcool in the radiator. (Or if you live in very cold places, 1 to 1 ½ jugs of Dexcool)
-Fill the rest with water.
***You do not need to use distilled water, clean hose water is just fine, just make sure your city water is clean and not total crap quality.

**Bleeding the system of air:
Take the radiator cap off when its COLD, start it up and let it idle, and let it warm up till the t-stat opens. I rapidly squeeze the upper and lower radiator hose like 20 times each while its warming up to help move any air bubbles through the system and by the t-stat on the engine side. When the t-stat opens you’ll see the level drop as you squeeze the hoses, its sucking the coolant through the system. You will also see the coolant start to flow in the radiator fill neck, once it starts to flow the level should drop down a lot, IMMEDIATELY top it off with coolant/water. Then the flow will stop when the t-stat closes. Wait one more time for the t-stat to open again and start to flow, if it drops down again top it off again. Do it a 3rd time if you want to make sure. When the level does NOT drop down when the t-stat opens and coolant is flowing....you're system is free of air bubbles. I always massage the upper hose during the whole process to keep any air bubbles moving through. Always works like a charm. Just keep checking your temp gauge until the t-stat opens for the first time to make sure it’s not sitting there overheating from a trapped bubble. May take 10-15 minutes for the t-stat to open the first time.
If you do start to get hot while sitting there and the t-stat will not open…..you have an air bubble on the engine side of the t-stat. Shut the engine off and rapidly squeeze the upper and lower radiator hoses again. Then start the engine again and see if the t-stat will open. Sometimes you just have to work those hoses to move the air through. Even after it seems topped off after a couple cycles…check it the next time you have a cold engine…top off if needed.


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Sure it's not loosing ANY coolant? At all?
If that is the case then you have a flow problem if it's overheating only @ cruise speeds.
Of course the thermostat. old new?
it would be a water pump < unlikely due to the 3.4 having a cast iron impeller.
(unless a cheapy was installed)
A clogged radiator, debris over the years has collected in the bottom of that single row radiator. Never said anything about the fans are they going when it says it's overheating?
Sure it's really getting that hot, seen many gauges that are off too.
Or a clogged heater core. Yes these can cause problems sometimes.
Many applications do not have a by-pass hose & the heater hoses/core serve that purpose. This being one of those applications.
I would look @ the radiator & thermostat 1st IMO. That would be the common problem.
Make sure it has good air flow thru the cores & that the system is clean.

 

It's not loosing any coolant and the oil looks fine too. My brother has been driving it around and he says it hasn't heated up in the past few days. The problem is that it's been doing it at random. Who knows..maybe the bubble is gone, we'll see in the next few days.