Thermostat not working properly

2002 FORD FOCUS
118,000 MILES • 1.6L • 4 CYL • 2WD • MANUAL
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JORDAN GATES
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Good morning, I have the car listed above 1.6 Zetec. had issues with the temperature gauge shooting up under load and jumping back down very quickly when coasting. I have replaced the thermostat, bled the system the best I can. Now the temperature gauge just sits at 90 but there is now bubbling water coming through the return line from the back of the engine to the header tank. It's not boiling water as after 20 minutes of driving you can take the tank cap off and there's only a small amount of pressure and only warm as expected. The larger hose from the thermostat housing is cold in comparison to the other after coming up to temperature. I'm tempted to remove the thermostat and try running it, beyond that would I check the water pump next? There's no milky oil present and no loss of coolant, the engine runs smooth in idle and when driving.
People are saying head gasket but I can't help but feel it may be something else.
Any advice will be really appreciated.
Many thanks.
Jordan
Sep 18, 2021 at 1:13 AM
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SQM
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Hello,

It is possible to have a faulty thermostat. As when you have a stuck closed thermostat the temperature between the two radiator hoses will be noticeably different.

Also if the temperature on the gauge fluctuates so much, it can be due to a faulty temperature sensor. Because regardless of overheating or not it should not fluctuates that rapidly.

https://www.2carpros.com/articles/how-a-coolant-temperature-sensor-works
https://www.2carpros.com/articles/symptoms-of-a-bad-coolant-temperature-sensor

It might be a good idea to run without the thermostat to see if you still experience similar issue. That way you can rule out the thermostat.

https://www.2carpros.com/articles/how-an-engine-thermostat-works

If the water pump does not circulate the coolant it could also make engine run hot also you might have temperature difference within the cooling system.

https://www.2carpros.com/articles/symptoms-of-a-bad-water-pump

Please let me know what you find.
Thank you.
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Sep 18, 2021 at 11:05 AM
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JORDAN GATES
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Thank you for your response. I'm going to remove the thermostat this afternoon and see how I go. Would water returning the the header tank be a symptom of a stuck thermostat? That would kind of make sense I guess, but I wonder why it seems to bubble out not just flow through.
I will keep you updated.
Many thanks,
Jordan
Sep 19, 2021 at 3:14 AM
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SQM
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If the thermostat is stuck closed then heated coolant can return back to the overflow tank.
Are you using water or coolant? Remember that water has a lower boiling point then coolant.
Sep 19, 2021 at 11:41 AM
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JORDAN GATES
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So I've taken the thermostat out and have no issues with the flow now, no returning water and the level in the expansion tank stays at max perfectly.
It is only water in the system.
I tested the thermostat by putting it in a mug and pouring water immediately once the kettle had boiled and It did open the first time, I did it again 20 minutes later in exactly the same way and it didn't open more than about 1mm.
Being that without the thermostat it runs smooth with no smoke, no loss of coolant and does come up to temperature after 10 to 15 minutes and drops to about 60c if I'm going downhill on a fast road, plus the heater works well.... am I safe to say it's definitely not a head gasket issue as everyone was telling me?
I'm going to get a new thermostat today and hope that fixes everything.

I really appreciate your help with this, thank you.
Sep 21, 2021 at 11:46 PM
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SQM
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It does not seem like a head gasket. Keep in mind that typically with a blown head gasket you will get white smoke out of the tailpipe and your oil will be milky in color.

I am happy to help. Let me know how it turns out.
Sep 22, 2021 at 8:09 AM
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JORDAN GATES
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So the headache continues.
I replaced the thermostat, bled the system, same issue occurs, its a 74c thermostat this time not an 82c but still get bubbling water this time hotter and cold bottom hose. No indication of overheating on the gauge but the water in the expansion tank was red hot and at one point boiled over without the cap whilst stationary after a few revs. Definitely seems to be a flow issue. Of some sort when the thermostat is in.

Sep 23, 2021 at 3:02 AM
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SQM
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So just to clarify, it worked fine when you did not have the thermostat installed?
You did not have any bubbling?
But after a new thermostat you get the same bubbling and different temperature on the radiator hoses?
Sep 23, 2021 at 8:24 AM
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JORDAN GATES
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Yes, all correct. With any thermostat installed there's the returning water and the return line is red hot, warmish top hose and cold bottom hose. Without a thermostat it takes 10 to 15 minutes to come up to temp but then stays near 90 degrees unless I'm rolling in top gear down a hill then it drops slightly and without one all hoses are warm as expected but not hot.
I'm totally lost with it now. My number is 07432460949 if you ever want to call about it. I'm glad it's not the head though.
Sep 24, 2021 at 2:37 AM
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SQM
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This a strange behavior. As it only overheats with the thermostat. Did you try with a new thermostat this time?
Also keep in mind that the part with the springs of the thermostat goes towards the engine.
Sep 24, 2021 at 8:50 PM
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JORDAN GATES
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Yes, replaced with a new one this time and spring side towards the engine. I'm completely at a dead end.
Is it possible that if one of the fins in the water pump had broken that with a thermostat installed it's causing the water pump to be inefficient?
Sep 25, 2021 at 2:14 AM
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SQM
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It is possible to have a weak water pump. As with a damaged/broken fin the water/coolant flow will not be as strong.
You could also have some other restriction in the system. Have you tried flushing the cooling system?
Sep 25, 2021 at 3:38 PM
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JORDAN GATES
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I'll try and investigate the water pump if I have time.
I've not flushed the cooling system, what's the best way of going about that? I'm lucky I know how to change the stat and do basic bits but some things I'm clueless, ha.
Sep 26, 2021 at 11:15 AM
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SQM
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One of the simplest ways to flush the cooling system is just to take off one of the hoses and attach a water hose and run water thru it. You can have the water hose (garden hose)
hooked up to a faucet and slowly increase the water flow to flush out any contamination.
Sep 26, 2021 at 11:35 AM
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JORDAN GATES
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Awesome, I can do that. I'll let you know how I go. Thank's again.
Sep 27, 2021 at 12:17 AM
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SQM
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You are welcome.
Sep 27, 2021 at 6:07 AM