I really need help. I have a 99 civic ex sohc and I am having trouble with it overheating I have replaced everything to do with the cooling system besides the head gasket but I am having an issue with my thermostat not opening while it's in the car. I tested it in boiling water and it opened fine but in the car it will not open. The car idles just fine but when I drive it it overheats almost instantly then when I'm sitting at a light or parked the temp goes down and I also have heat almost instantly upon starting the car.

 

Make sure the coolant is bled properly! Its pretty easy to do.

 

I have attempted to bleed the system multiple times as well as have had a local shop attempt it twice and they told me both times that the thermostat wasn't opening and I took it out and tested it in boiling water and it opened fine its also a brand new thermostat rated at 170° like my car calls for and I know coolant is getting to it but it won't open to allow the system to fully bleed or allow coolant to get to my temp switch to activate the fan

 

Did you buy a genuine Honda Thermostat ? If not, GET ONE and try again.

 

I did not but I will most definitely try and keep you updated

 

couple things here
Get an obd2 scanner and see what the coolant temp is when running. That will tell you if the thermostat is actually opening.
Sometimes the coolant hoses will collapse when you drive. Do any of them feel soft and mushy? Ifthey do, that could be the problem.. Start the engine, let it warm up. Then while watching the hoses. bring the engine to about 3000rpm , see if any collapse( suck in). Its usually the hose that leads from the radiator to the engine, but sometimes its the engine to radiator hose..
When you put the t-stat in, is the bleed hole at 12 oclock?
Are you bleeding the system by opening the radiator cap, or are you using the bleeder?
Did this just start happening? or has it been an ongoing problem?
How old is the timing belt? Reason: the water pump is supposed to be changed when the timing belt is replaced. If the water pump is old and corroded the impellor can cause cavitation in the cooling system. whick would cause overheating at high revs, but once you are at idle there is no more cavitation and the coolant flows as it should

 

Even brand-new thermos can be defective like JRcivic1 said try an OEM unit.

 

I just replaced the water pump and all timing components and the same thing was happening before since I got the vehicle in January of 2022 but for the first year it wasn't overheating cuz the previous owner installed a failsafe therm. When I discovered that the car was never reaching operating temp. When I replaced the thermostat for the first time nothing changed but no overheating then the rad. blew so I replaced it then shortly after one of the heater hoses blew so I changed it then another hose blew so I just bought a silicone hose kit from eBay and after I changed all of my hoses the overheating started (probably should have stated this from the beginning, my apologies) then I started going crazy by just replacing all cooling system components (except head gasket) now I'm kind of at a stand still scratching my head.

 

Almost seems like you have a blockage in the coolant system not allowing it to flow so the hot coolant is not making its way to the thermostat housing quick enough.
Did you make sure to check the hoses before installing them that they had nothing inside them and they were clear?
Personally, I would disconnect the rad hose from the head that leads to the top of the rad then disconnect the hose from the thermostat housing and hook up a (garden)hose to the upper rad hose then turn on the hose and see if it's flowing freely out the lower hose end that would mate to the thermostat housing.
If it's not, your problem is blockage in either the upper hose, inside the rad or the lower hose..

 

Thanks for the advice I'll try that and post an update I also just ordered a Honda genuine therm

 

if the temp gauge is moving down when your at idle then the thermostat is operating.
When you say OVERHEATING, how high is the temp gauge going?

 

The temp gauge shoots all the way up to the red...someone told me it might be my cluster but my confusion is why the lower rad hose is cool to the touch

 

you have a clog somewhere or a bad waterpump ( I think)

 

Water pump was just replaced as well as therm, all hoses, heater core, radiator, and all switches and sensors

 

There have been alot of great tips here and I'd like to add my two cents as well.
I'm not sure if the USDM rad caps differ but its imperative you run the correct bar (usually 1.1).

Also I'm hoping you didn't use rtv or Honda bond on the water pump assembly as it partially blocks the bypass port.
Personally i don't trust the silicone hoses but ill assume you've checked the quality before install.
I hope you figured it out, best of luck!!

 

When you replaced the hoses, did you replace all of them or just the easy ones?
If you replaced all of them, did you do one at a time or did you take them all off at once?
Get a service manual and check to make sure that the hoses are routed correctly

 

Did you use a funnel to bleed the coolant?