OK, here's the lowdown. 97 civic sedan, Canadian spec EX (which is USDM DX) with the y7 engine.

A few months ago I was having overheating symptoms (even though the dash temp gauge never went above its normal spot). I replaced the ETC (fan control) switch, swapped in a new 180degree thermostat, refilled with fresh coolant, and bled the system. All was well.

Now, I'm having issues again. I have discovered that the thermostat is NOT opening when it's in the engine block.With the engine running and up to temp, I can remove the rad hose from the thermostat housing and no coolant will escape from there. However, if I take the thermostat out, toss it in a pot of boiling water, it opens just fine.

I checked the inside of the thermostat area (housing side and engine side) for anything that could be restricting, or obstructing the thermostat from working properly. Everything appears to be fine, nothing seems to be in the way there.

With the engine off, I removed the upper rad hose and thermostat, and ran water (using a garden hose) through the engine, and flow seems to be the same coming out of the thermostat area, so it appears there is nothing restricting flow within the engine itself.

When it is at operating temp, the engine side of the thermostat is wickedly hot, almost too hot to touch, and so is the upper rad hose. However the lower rad hose is cool to the touch, meaning nothing is circulating.

I double checked the engine temp with an OBD-II scanner. According to that, the engine temp can hit 200+ without the thermostat opening.

Yes, I've double, and triple checked that the thermostat is installed correctly, (copper sensor bulb to the engine side, the little jiggler thing at the top) and that there is no air in the system. However, since the thermostat is not opening, the ETC/fan switch never sees a hot enough signal to turn on the rad fan.

For now, I have removed the thermostat completely, to allow the coolant to completely circulate through the entire cooling system. However, this is a temporary solution until I have a diagnosis. I'm stumped!

This is where you fine folk come in. RonJ and MK, you were super helpful last time - hope you've got some insight.

PS any chance I've cooked the head gasket from running so hot?

 

I have seen NEW Thermostats that are BAD. I would install another thermostat & see what happens. From what you described I'm wondering if the little vent hole that goes to the top on my 90 Civic is correct. The reason I say this is I had an overheating issue with my Civic the other day. It worked great until I found myself in downtown Las Vegas in mid afternoon with temps around 110 degrees. I babied it back over the hill to a friends shop. We pulled the thermostat & installed a new one. But what was interesting was the pin in the vent holes were different. The one that was in there opened oppose the new one. Anyway, after installing the new thermostat & bled the system I have not had any trouble. I have driven if back over 300 miles from Las Vegas & drove around 300 miles yesterday in both high altitude 8000 ft 100+ degree heat.
PS Just for info I bought the new thermostat from AutoZone. I have had little problems with their parts over the years.

 

OK, here's the update.

Today I took the "jiggler pin" off the thermostat, leaving just the hole. I flushed out the whole system with running water from a garden hose and drained it. Used low pressure compressed air to blow out as much remaining water as I could.

To re-fill the system, this is what I did. It was suggested by a mechanic friend and seems to be the ticket to re-filling the cooling system with minimal chance of airlock.

I jacked up the front of the car as high as I could, to make sure the top of the rad was the highest point of the cooling system. Turned the heater on, fan on high. Then I removed the UPPER rad hose from the rad and filled as much coolant into the engine as I could that way. Once the coolant was coming out the upper rad hose where I was filling I re-connected it to the rad and continued to fill the rad normally. Once it was full I put the rad cap on loose and ran the engine until it was pretty hot. I carefully removed the rad cap and continued to let the engine idle, topping up the coolant as air escaped. Once the rad fan came on the first time, I put the rad cap back on tight, and let it run some more, and the can cycled afew more times. I removed the rad cap once more, double checked the coolant level in the rad and overflow bottle, and closed it all up.

Left the car running while I took the car off the jackstands, and checked for leaks. Rad fan was cycling normally, coming on at approx 205F (according to the OBD II scanner I borrowed)

Just to be sure, once I was done I disconnected the negative battery cable to reset the computer, even though I had no CEL or codes.

 

I really wouldn't recommend any procedure that involves closing the radiator then again removing the radiator cap while the engine is hot. It is very easy for the coolant to erupt out and cause severe burns. Weeks in the hospital, skin grafts, the whole works. Maybe you'll meet some cute nurses but it's still a very high cost for DIY repairs.

 

No eruptions this time! (I stood well back, and had the rad cap well wrapped in a rag) but you're right MK, it wasn't the smartest thing in hindsight!

 

On mine we did just like Honda recommends. We added anti-freeze until there was a steady stream out the bleed hole. Then finished topping the radiator until full then let the engine run with heater on until the fan cycled on & off three times. (Honda says twice). I haven't had another issue with that.
So I'm assuming that fixed the problem?

 

90 - yes I did solve my problem. However (and I'm not sure why) it appears that the 97 models did not have a bleeder screw/bolt... well my car doesn't anyway!

If what I posted above had NOT resolved my issues, the next thing I had planned was to drill and tap a hole to make my own bleeder. Thankfully I didn't have to.