I flushed the coolant once before, and the coolant started looking rusty again pretty soon. This was after replacing the thermostat, water pump, radiator, radiator hoses, and heater core hoses. However the engine was running at a good temperature still. It never went past the middle on the gauge. So anyway, I finally got around to flushing it again. This time I used a Kool It cleaner after draining it, then water, then new coolant. Now it runs hot.

While I was running the Kool cleaner, it seemed to run very cool. I drove it about 5 minutes, then 5 minutes, then idled for about 5 minutes. After that, I put in a new coolant overflow tank (a generic one from O'Reilly's) and put in the new coolant.

Now it runs hot, especially when idling. On the freeway it's okay, but still runs hotter than before. On the streets the gauge goes past the middle. I have to turn the heater on to keep it cool, after only about 5 minutes of stop and go driving on the streets with a warmed up engine.

I'm not sure why it would run hot, when I have a brand new water pump, thermostat, radiator, hoses, etc.. All we did was the coolant change. I can't imagine the reservoir tank might be causing this problem, even though it is a generic one that doesn't fit perfectly.

I read that it's probably caused by air in the system, but we ran the engine with the cap off and it didn't really bubble or burp any air out.

Anyone have any suggestions?

One thing that might suggest that it's air in the system is that when I cold start the car now, it takes even longer to start. I usually have to give it gas for a good 3 seconds when I crank it, but this time it wouldn't even start the first time. I had to do it twice for longer than usual.

 

Helps if you state year and engine when asking questions ; not all cars are the same

Sometimes have to " burp " the system.
Get engine up to temp with the cap off so thermostat is open ,
rev the engine and see if the coolant level in the radiator drops ,
if so add more coolant to full and put the cap back on while holding the revs up

 

Yes, almost certainly there's still air in it. The air will burp out easier if you jack up the front right side of the car to put the rad cap opening as high as possible. Like vetteoz said, revving up the engine a bit with the cap off can sometimes force the air pocket out the top.