Hello!

I have a 2010 Cobalt SS turbocharged. 70k miles. I was driving it around town the other day when I started to hear a weird noise. Soon after my car began to overheat. It turns out that the noise was that coolant boiling into the overflow tank. The coolant was also a bit low. I used to work on the car myself but I have been working long hours at my job and do not have as much time anymore. So I took it to a local trusted mechanic who did some tests and told me the water pump needed to be replaced. The coolant was low because it was over pressurized and bled out of the system. He says he knows it's a water pump because the radiator hoses are not getting hot nor is the cabin heat, so therefore no coolant is flowing. I would very much like to verify this story before I spend $800 on a new water pump. He says it can't be the thermostat because whether the thermostat is open or closed coolant still flows to the heater core. I have not been able to verify this, is it true? It looks like some cars the coolant always flows through t heater core the and some cars it doesn't.

I am hoping to find a diagram of the engine's coolant flow that will either show that the thermostat controls the flow through the heater core or does not. Any other advice on how to verify this story would be very helpful.

Thank you so much!

 

The coolant does flow through the heater core no matter the tstat position.

If it's an independent shop you might point them towards ZZP for the water pump, as they're likely significantly cheaper than his current parts source.

 

Thank you so much. So you agree with the assessment then? If car is overheating, the radiator lines are cool, car heater blows cool, and the coolant overflow tank is boiling, that would indicate a bad water pump? Is there anything else I can try/test? I am thinking of doing an exhaust gas test in the coolant overflow tank to rule out a bad head gasket.

I really appreciate the help.

 

Water pumps aren't a common failure point on these cars, but it can certainly happen. Unfortunately, because of their location and connection to the chain system, replacing them can be pricey because of the labor involved. I would definitely have the timing chains, guides, and tensioner replaced at the same time.

Bad head gasket, while possible, also isn't a common issue. But you'd also still have heat and flow through the radiator in that case. Unless your thermostat is stuck closed and your heater core is clogged.

 

As you typed that, it seems to me to be about as flawed logic as you could possibly have.

You did not mention if the coolant system was refilled, and bled out.

If the coolant is low enough how would coolant circulate?
If the coolant isn't circulating, how would the thermostat operate?

To suggest that a cold heater core, points to a water pump would be a good assumption if the system was full.
To suggest that cold hoses point to a water pump would be a fair assumption if the system was full.

If the system was full the temperature on the hot and cold side of the thermostat can be read with a laser thermometer diagnose a stuck thermostat.

If I remember correctly the lower rad hose goes up to the reservoir;
The reservoir is supposed to be the highest point in the cooling system - this aids in bleeding out air, and prevents excess coolant loss should the reservoir have a defective cap.

If the system over pressurizes, that would in my mind suggest a defective component which prevented venting.
The first item that can cause issues is the thermostat;
Over time the thermostat can stick in either the open or closed position;
There is supposed to be a bypass built into the thermostat, but if it sticks in the closed position, not enough coolant will be able to pass it, and coolant in the engine can over heat.
The next item to cause pressure issues is a cap.

I like you, have been often too busy to take care of all our vehicle maintenance;
The last time I took our SRT4 to a "trusted" shop, I knew he was giving me a load of bullcrap - I just didn't say it to his face, and I learned that for mechanical issues, that this shop* is no longer trustworthy.
He told me the kind of test his tech performed, and what needed to be done and why - It was obvious to me that the diagnosis was done with a scanner, and no eyes on it, otherwise the pricey job quoted would not have been quoted.
* = The ownership of the shop has changed in the last handful of years - the old guy I knew would never BS me.

If this is as you say a trusted mechanic, you really shouldn't be asking our opinions - It is very tough to diagnose nearly anything without seeing it first hand.
If this isn't really a mechanic you can trust, take it somewhere else.

Good luck!

 

I just noticed post #3;
A compression test can point to a bad head gasket;
If the head gasket is pushing pressure into the cooling system, that can create bubbling in the reservoir.

Alternately pressure can be introduced to the cylinder via the spark plug, and if that bore is unable to hold pressure, that points to bad rings/guides or a bad head gasket;
Bad rings or guides would vent pressure to the crankcase, while a bad head gasket... well, you can figure this out...