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No Thermostat

good day. 6 months ago, when my honda civic 1997 overheats, I took it to a mechanic. he diagnosed ther is no problem with the fan, no coolant leaks, radiator is not clogged. he suspects the thermostat needs to be replaced. he suggested we just remove the thermostat for now and I observe if the car will still overheat. technically, I live in a tropical country. climate is always hot here, and city traffic is terrible. in my simple mind, it makes sense to get rid of the thermostat. the engine, while running on traffic (everyday) on a hot day (or night) will always need to be cooled.

Is this ok? Or should I still need to replace the thermostat?

6 months later since that trip to the mechanic, my car doesn’t overheat anymore even without a thermostat, EXCEPT when the AC has been running for 20-30mins straight while in the middle of traffic on a hot day.

Thanks in advance for any comment or tips.



The fact that you car overheats in traffic with the a.c on on hot days tells me that your cooling system is marginal. Some cars come from the factory like that, Jag XK120 to 150 models for example . When you see them at car shows, you can tell which ones are used as drivers and which ones are for show. The show cars have the lovely curved and tapered original radiators and the drivers have a modern upright aluminum multi-row rad.

Ant car over time can lose cooling capacity, deposits form in the radiator and engine impeding heat transfer even if they don’t block flow. It is possible that the slight restriction of actually having a thermostat in place can make your overheating worse. Are your cooling fans running when it overheats?

It is possible that replacing your radiator with a larger capacity would fix it but not certain.

Running without a thermostat is not good for your engine is bad for you car but overheating is worse.

A 24 y.o. thermostat failing is no surprise.
I change them every 10 years.
I hope the coolant wasn’t in there for 24 years too.
With the engine dead cold remove the radiator cap and look in there for corrosion.

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Thanks for your comments @oldtimer_11. My car’s radiator is relatively new compared to my car’s model. I had the original (worn-out) radiator replaced 2 years ago. It is the multi-row type.

Yes, the fan is working when the engine overheats (needle in gauge goes up past middle of scale) when the AC is running after 30mins. needle rises earlier (15-28mins) when in traffic with AC on. As I switch off the AC, the needle drops within 10-15secs.

were engine temps ok after you had radiator replaced? and now there is an issue?

Thanks for your comments @circuitsmith. It’s funny that you thought the thermostat I took out has been there for 24 yrs (as old as my car). The thermostat was 5 yrs.old.

I change coolant at least every 12 months. Last coolant change was 6 months ago. I did looked in the radiator for corrosion - - I didn’t find any.

@Cavell , yeah, engine temp after radiator replacement was OK. it was replaced in 2017. I started to experience overheating this year (2019).


with that level of frequency, I would double check to make sure you have purged all the air out of the cooling system.

@eddo, why not?

Yes, air purged out. Is this directly related to why my engine overheats when AC is on, but doesn’t overheat when AC is turned off?

there is more load on your engine when you a/c is on, as opposed to when the a/c is off. More load makes the engine work harder, which increases the engines temp. A/c being needed also means it is hotter ambient temp, which also increases engine temp over cooler ambient temps.
an inefficient cooling system will be less effective during heavy load and hot days- as you are finding. to answer your question: Yes, air in the cooling system will add to the inefficiency of the cooling system, and make your car run hotter.

you didn’t answer my question- why are you changing your coolant every year? Is this preventative, or is there some other reason for this? This is excessive, and if there is another reason, it may lead to why you are having the issue you are having.
Cooling systems are often recommended to be serviced at about 5 years. Your owners manual would describe the actual interval that Honda wants, but I guarantee you that it is not every year.

That is a waste of money . As Eddo says 5 years is plenty and more then most people do.
And really would avoid a mechanic that says drive without a thermostat.

Did I understand you? Does the fan only run when the engine gets into the hot zone? The fan should run anytime the AC is on, even if the engine is still cold from just being started. Actually you should have two fans, one that runs all the time when the AC is on and the other the other that comes on when ever the engine overheats.

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If he is using the old green coolant, the change interval is 2 years.

Have you checked the impeller on the water pump? It can become dislodged from the shaft and essentially freewheel. When it does that, there’s marginal circulation, which can cause the problem you’re seeing. Add it to the list of things to check.

Years ago some mechanics recommended removing the thermostat to workaround overheating conditions. It was a (perhaps ill advised) bandaid then. As noted earlier in this thread, it’s definitely ill advised now.

The thermostat proportions the coolant flow between the radiator and the engine. With no thermostat that proportioning doesn’t happen, and that can cause a variety of problems including overheating.

Keith, on a lot of newer cars the a/c fan is set to come on only when the high side of the a/c system reaches a certain pressure.

I’ve never heard of removing the thermostat causing overheating. How does that work?

I’ve never tried it, but removing the thermostat could cause too much of the coolant to flow through the engine and not enough through the radiator.

Dumb question from me, maybe. I thought the coolant flowed from the rad to the engine and back with no way to bypass, and the thermostat just acted as a valve to open / close and regulate the temp. I only see overcooling as an option with no thermostat.

Doesn’t matter, just curious. I may learn something.

The only time I’ve removed a thermostat was if I was in a bind, the truck was overheating, and the thermostat was suspected of being stuck closed. Overcooling beats overheating if you’re in a pinch.