Could flushing cooling system break thermostat?

Title. When I flushed my radiator, I did a full flush (not just a drain and fill) and ran a bottle of radiator flush with gallons of distilled water through the cooling system. When I drained the water to refill with coolant, I saw a lot of dirt and debris out along with the water, so the radiator was definitely clogged. I refilled and bled the system properly.

I had just put in a new thermostat about a week ago, but only after flushing and bleeding the radiator did I notice the car is running a little less warm than it always has, now the temp gauge is a bit less than half and it will fluctuate from slightly less than half to exactly in the middle. Before it would be right in the middle and not move at all once it reached normal operating temperature. Also the heat stopped blowing as warm as before, after flushing the cooling system. I am certain I properly bled the system so air in the system cannot be a cause of this.

Did I damage my thermostat from flushing the radiator? Engine is running colder than usual and heat doesn’t feel very warm

This says you still have air in the system.


Where did you get the thermostat, and what brand?
Did it look similar to the original?
Some aftermarket 'stats are junk.
Work poorly right out of the box.

I used one of those funnels with a radiator cap adapter, added coolant so i could see it in the funnel and let the engine run for an hour and the fans cycled on 10 times, adding coolant when the level dropped.

Also if there were air in the system the car would run hotter than normal not colder than normal

How big was the debris? Did you repeat until it came out clean? If big enough I guess some debris could bind up the thermostat.

How old it this Focus, years/miles? Was the old coolant rusty looking?

Not too big, but coolant had a brownish tint to it so it hadn’t been changed in a while and saw small specs of dirt come out

It has 160k miles and is 2002

There’s still air trapped in the cooling system.


Forgot to mention, I did put a lower 160 degree thermostat on which is lower than the OEM one so would that explain the car running colder than normal?

Now why did you do that?


By accident. I didn’t even notice until yday I looked up the one by part number i found on the box i had ordered online to see if it had bad reviews and read the specs and it said 160 degrees

Of course. You need the stock temp thermostat in there, the one specified for your car, name brand, bought from a trusted source (not ebay).

It is very hot right now anyway where I am so could i leave this one in for a few more months? Or should i change it asap? Can using a lower degree thermostat damage the engine?

Yes, it can.
Your oil will need to be changed more often because the engine is not running “hot enough”. Additiionally, your gas mileage will plummet.
Whatever the oil change intervals are supposed to be for your car, you should shorten them considerably if you plan to defer the replacement of the thermostat.

But… think about this…
When you wind up spending more for gas and having to change the oil more often, are you REALLY saving money by deferring the thermostat replacement?

That’s true. Would you say the car running slightly cold is more than likely caused by this thermostat? Or is there any other issue at all that could cause the car to run cold?

If there is air in the system wouldn’t it cause the temperature to run hotter rather than colder?

Would have been helpful if you’d answered the questions in my previous post.

@Mrnicefordguy - I am having trouble why you are so reluctant to have this problem solved properly. It is possible that your radiator needs replaced ( not really that expensive ) , You could have just gone to a local parts store and they would have sold you the correct thermostat . Don’t you think that doing what is needed is better than possibly destroying your engine ?

If you have enough air in the system it can read cold because the temp sensor needs to read liquid, not air.

Don’t borrow trouble, put in the right thermostat and see how it acts before looking for anything else wrong, I don’t know why they even make a 160 degree thermostat. I don’t know of any car in the last 50 years that needs one,

Those were my thoughts also.
IIRC, Ford changed-over to 180 degree thermostats in 1965 or 1966, at the same time that they extended their oil change intervals. That was a long time ago…