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Discussion Starter #1
Recently after replacing a bad thermostat that was stuck open my cobalt has been over heating to 220-240 degrees it’s has done this is the past but was fixed. When I drive the car it runs at normal temps (185-210) then keeps going up after I’ve been driving for about 10 minutes I end up having to stop and open the purge tank cap to release pressure letting the tank fill then closing it, then I go to squeeze the cold side hose of the radiator and it forces antifreeze in the tank and bubbles start flowing out, then I move to the hot hose coming off the engine and that one it blistering hot and very pressurized the same with the cold side I keep doing that for a few minutes and wait for the bubbling to somewhat stop and I open it again and it would suck antifreeze back into the system then I’d squeeze the hoses again and open the cap to release any more pressure that was being held back and I go to drive again a and it’s fine for a while then the whole problem repeats.

sorry for such a long post but I need to get the details down as best I can for some kind of solution to the problem. If anyone knows about some good info on this, would be greatly appreciated please and thank you.
 

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Have you considered the thermostat is bad? I know you said it had been replaced. But it’s is not uncommon for aftermarket thermostats to not work properly. Especially the economy type. Either use a premium version name brand or get an oem ac delco part. I have run into thermostats that work for a while then fail a few months later over the years. Also as mentioned above, the cooling system needs all the air bled out of it, because it can cause similar problems.
 

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Discussion Starter #4
What’s the best method to get the air out? I just changed the coolant and sole hoses in my car over the weekend and it still does the same thing but it doesn’t get so hot. Whenever I’m driving causally at the speed limit it will go past 220 but when I floor the gas pedal the temp drops significantly to 190s’. I’m beginning to think my water pump has gone bad.
 

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Hmm, the water pump is a possibility. Not sure how many miles your car has gone. It’s possible the impeller has worn out. Especially if the coolant has never been changed or has more water than antifreeze. I have seen older coolant have sediment in it, which could cause abrasion on the impeller.

Last year after I purchased my ‘06 Cobalt, I did a chemical flush on the cooling system. I used Prestone flush. I had already purchased the flush kit they also make that connects to the existing hoses on the engine. It allows you to connect a regular garden hose and expel all of the coolant from the engine, radiator, heater core, and expansion tank. Also did this on my ‘91 Town Car. The Cobalt actually had pretty clean coolant though. But I got rid of the Dexcool and replaced it with the yellow Prestone coolant. I have seen some nasty cooling systems that use Dexcool over the years. Usually happens because the coolant wasn’t changed or has gotten low and caused it to oxidize. Although changing it about every 5-6 years can keep that from happening with whatever coolant you chose to use.

Another thing to consider is the radiator being restricted. Again lack of maintenance can cause sediment or sludge to collect in the radiator and reduce cooling efficiency. The chemical flush can sometimes clean it out, sometimes not if it’s pretty bad.

Anyways, what I do to purge the air out of the system is start the car from a cold start with the coolant cap off. Run the heater at this time on high. It may not be totally necessary since I don’t believe there’s a heater valve, but I do it anyways to make sure the coolant is going through the heater core. As the engine gets close to operating temperature, I’ll run the engine for about 30 seconds at about 1200-1500 rpm. This helps circulation through the thermostat if it’s staying closed due to an air pocket. If the temperature won’t rise and is staying too low, I’ll turn the engine off for a minute and restart it. This may cause the expansion tank to overflow momentarily until restarted, but this method can help open the thermostat without having to put the cap back on and drive the car. Usually takes about 20-30 minutes to do this process if it’s being stubborn with an air pocket. If it starts to overflow the coolant, go ahead and put the cap on if it’s safe. Sometimes I’ll use a towel to put the cap on if it’s overflowing at this point. But usually don’t have to. Now let the engine cool off. Top off the coolant, and then drive the car for 7-10 minutes. Like around the block. Usually these cars will have all of the air out of the system at this point. Let its cool once again, then recheck the coolant level. If it had air in it, it should have dropped the level in the expansion tank. Other than that, these cars should not be very hard to purge air out of the coolant. I’ll have to look and see if theres any other place to purge the air out. Some cars do have a bleeder valve. Although I don’t remember having to do it on this car. I believe the GM V6s do, like the Saturn Aura with the 3.5 liter. I believe I remember having had to open a bleeder valve on one not too long ago.
 

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Discussion Starter #6
The car has 183k on it and the water pump was changed (Water pump 9/30/18 142K) I keep notes what I do things to my car, other than that the water pump should be good it’s got just over 43k on it I’m not sure about the rad I used the prestone flush as well and filled it with water and drained it twice I was thinking about replacing the water pump and getting the one with metal blades I believe acdelco sells one of them but it’s upward of $200 I have seen small pieces of clear plastic here and there before and after flushing the system. When I drive it, it seems to run fine at around 194° for a while but every now and then it ramps up to 220°s then after a bit it goes back down to normal temps. I’m just about ready to get another car a newer one with little mileage. If you or anyone else has other suggestions. I
 

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I would think the water pump should be find then. Highly doubt that it’s bad in that amount of time. I have seen the seals go bad in that amount of time but not too many impellers.

I would personally try another thermostat. I really have seen it many times where a brand new one is a dud and opens at the wrong temperature. And as easy as it is to get to and also fairly cheap, I would try that first.

My ‘06 would run in the 190-217 degree range when I got the car last year. I thought it wasn’t too normal but a lot of people were telling me it’s just fine. I sure didn’t think so. After I changed the thermostat the engines runs between 187-200 degrees. Really never gets much hotter. And it’s regularly been around 95 degrees outside lately. I noticed my ac also works way better now as well. The radiator had been replaced in my car due to a collision at some point so the cooling system was fairly clean. But it did make a huge difference.

Another overlooked part I see cause overheating problems is the cooling system pressure cap. One that’s faulty can allow the boiling point to lower, which causes the engine to run hotter. So that’s worth a shot also. I have fixed quite a few slightly overly hot running engines with just a new cap.

It has nothing to do with a Cobalt. But I have a 2013 VW Tiguan 2.0T at the shop. Has 114,000 miles. Hasn’t ever had the cooling system serviced. Water pump seals went bad, has a plastic water pump. The coolant has a little bit of sludge in it. The water pump probably would still have to be changed around this mileage. But I can see that draining the coolant twice in that mileage would have greatly reduced how dirty the system became.

I hope you can get your car to run cooler. Really I see Cobalts come in with a lot more mileage than yours. They are one of the cheapest cars to do repairs on. One reason why I couldn’t pass up buying mine and why I probably will hold onto it for a while.
 

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Discussion Starter #8
Alright I’ll try doing that and see what happens I’m hoping to not drop a lot of money trying to go it running like it should. Over the past couple of days it’s been getting up to 230° which is what I used to do before I had it fixed the first time I also unscrewed the oil filler cap and saw a small amount of white sludge which is a sign of coolant leaking in the head gasket it has happened to my car in the past and my dad had used steel seal to fix that problem which held for a while but I’m not quite sure how long ago that was nor do I know how long that stuff lasts for in engines. But I think I’ll start from the cheaper end of repairs.
 

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I recommend the OEM ac delco thermostat. I replaced mine on my ‘08 two years ago and the temperature only climbs over 200 if standing still without the ac on (cooling fan not running). Normal summer temps for my car is 183F-194F. Winter time on the highway the temp will sit at 180F-185F.

I also recommend the Ac Delco expansion tank cap. I bought an aftermarket one that I guess didn’t seal well because the temperature at idle my temp would climb a little, replaced it with an ac delco and the temps are now stable at idle.
 

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Discussion Starter #10
I recommend the OEM ac delco thermostat. I replaced mine on my ‘08 two years ago and the temperature only climbs over 200 if standing still without the ac on (cooling fan not running). Normal summer temps for my car is 183F-194F. Winter time on the highway the temp will sit at 180F-185F.

I also recommend the Ac Delco expansion tank cap. I bought an aftermarket one that I guess didn’t seal well because the temperature at idle my temp would climb a little, replaced it with an ac delco and the temps are now stable at idle.
That’s what I have in my car actually both the cap and thermostat the cap I know is fine thermostat not the same case. I drilled four holes into the thermostat to increase constant flow to keep it cooler and it works car runs like it should.
 

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Did you buy the new style thermostat from Ac Delco? GM changed the style of these thermostats since the original ones had a rubber seal that failed often.

It may be possible you have some air in the system. Try this and see if it helps. Try parking on a flat surface or a surface that is slightly inclined so the front end is slightly higher than the back end of the car. With the engine cool, remove the pressure tank cap, start the engine and monitor the coolant temp. When the coolant temp reaches 180F (thermostat opening temp), turn the heat all the way to hot. I always keep the fan speed on low. Now monitor the temp gauge and get out of the car and watch for overflow from the expansion tank. Give the engine some brief revs (2k rpm is fine, no need to go higher). Just rev it for a second or two then let off the gas. Repeat this a few times but make sure to monitor the coolant temp and the expansion tank for overflow while doing this.

You should eventually see coolant coming from the overflow hose that runs along in front of the valve cover to the expansion tank. At this point the expansion tank coolant level may be pretty high so this is when you need to put the cap back on. You can repeat this whole process a second time from a cool engine if you feel there is still air in the system.

This is a good method to purge air from the cobalt’s cooling system. The cobalt has what I call a “self purging system” because there is a hose that comes from the cylinder head on the passenger side near the coolant hose. This purge hose essentially is the highest coolant point on the engine, so any air in the system is likely to make its way there and then make its way to the highest point in the cooling system, which is the expansion tank.

When I replaced my thermostat 2 years ago this is the method that I used. When I was idling the engine with the coolant cap off and the heat on high, I eventually heard a surge or a stumble while the engine was idling. That was essentially the air pocket that was trapped in the system after replacing the thermostat.The cooling system is pretty much idiot proof on a normal daily basis since it’s always purging itself but after opening up the system for a thermostat change or other repair, it’s always recommended to purge it. GM actually has a procedure to filling the cooling system after draining for these cars that involves and air compressor and special adapter to push air while adding coolant. It’s time consuming, but the method I described to you will work just fine.
 

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I see Cody and I have the same process for removing air from the system.

I just re-read your comments and saw the part when you see white sludge under your oil cap. That might be a sign the head gasket is bad. If you’re seeing white sludge under the oil cap during warm ambient temps, then I would highly suspect the head gasket is bad. So no amount of air purging will help with that.
 

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When I first bought my car, the oil cap had white residue inside. I was hoping it wasn’t a head gasket, and didn’t really think it was since the engine ran pretty great. I came to the conclusion that it was because the previous owner drove it infrequently and for less than 10 minute intervals. In 2-3 years he’d only done around 1300 miles on the same oil. Conventional oil at that. So this leads to moisture build up inside the crank case. I see this in my customers cars that are usually older people who drive extremely short distances, not much different from my grandma. I have to change grandma’s Honda’s oil about once a year, and use synthetic so to avoid acidic oil.

I regularly check the oil cap to see if it’s getting the moisture build up on my Cobalt and so far it has not returned. But I drive about 250 miles on average per week. About 100 miles of it on 25 minute drive to and from town. In town I do only drive short distances to and from work. But the weekends the car is usually all highway mileage.

Anyways, I was going to ask if you have been losing coolant? If so, what condition is the coolant expansion tank in. I had a GM vehicle come in with a similar tank like the Cobalt has. But it has a pinhole in the plastic. It was only after I pressure tested the system that I was able to find the leak. It never seemed to leak a lot, but was one reason the coolant temperature was out of range. They refused the replacement part and repairs unfortunately.

I looked at the one on my car. And while it’s not leaking, it is degrading externally. The finish is looking powdery and flakey. It’s original to the car so not really a surprise.
 

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Your engine cooling fan needs to work.
It should come on when you turn on the Air Conditioning. If you’re air conditioning does not work you can connect the cooling fan to a 12 volt source such as the cigarette lighter.
 
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