So my problem is a bit unique. I have a 1996 Honda Civic DX with a D16Y7 Engine. I'm fairly new to the automotive industry and this is my first car. This is also my first post on any forum so I apologize if I'm not following any guidelines. I also apologize ahead of time if this topic has been discussed. I looked and couldn't find a similar situation.

Two months ago, my car was overheating when idle and coolant would be pushed out of the radiator so I did a leak-down test which showed that my head gasket was blown (coolant spilling out of the radiator with pressure). I replaced the head gasket and everything went well. There were no leaks. Fast forward to a couple of weeks ago and my dashboard temp indicator was showing that my engine was overheating. I checked for codes and found nothing. I took it driving with the device connected so I could read the Engine Coolant Temp Sensor (next to the thermostat) while on load/idle and the temp was consistently between 194 F and 204.8 F which is within specifications for this engine. The odd thing is that this isn't reflected on the Dashboard despite the sensors being next to each other. In my car, mostly when idle, the needle will shoot up to the maximum H and drop and then shoot up again. It'll bounce in between the middle and the H and then settle in the middle. I feel like something is wrong because that bouncing of the needle is shown with the engine when it's idle. The engine RPM will fluctuate (VERY slightly) in sync with the needle as it's bouncing up and down (like needle goes up, rpm goes up, needle settles, engine rpm goes down). Today, there was dry coolant beneath the Distributor Block (this is new). As of right now, the only thing I could think of as the problem is the Temp Sending Unit since it's the OEM sensor and the Engine Coolant Temp Sensor was replaced in February. This is just an assumption though and I'm really lost here. If anybody has had similar issues and can shed some light or some recommendations I'd really appreciate it. I will replace this sensor, burp my car, and test to see if this solves my issue.

(If anybody could answer, the Temp Sending Unit is only in charge of what is displayed on the dash right? It doesn't interfere with any of the ECU processes?)

It's worth noting that within the last two months, I've replaced the Thermostat with one that's Honda OEM, replaced the air filter, replaced the Coils, replaced the Fan Switch sensor, replaced the ECT sensor, replaced the spark plugs, replaced the Head Gasket, Intake Gasket (cleaned too), and Exhaust Gasket, resurfaced the Head, rebuilt the Cylinder Head, replaced the belts, replaced the coolant hoses, and last year I replaced the Radiator. The only things left for me to replace (or rebuild) are the engine (low but ~= compression in all cylinders), the temp-sending unit, and the water pump. It's also worth noting that essentially everything in the car is stock with around 164k miles. If more information is required I will provide it.

 

change the sending unit

 

So I got to change it and now it's worse than before. The gauge reads "hotter" than it did with the other sensor... To be fair, the sensor I bought is not OEM, it's aftermarket Autozone. I'm going to buy one off HondaPartsNow that's the same model number as OEM Honda. For anybody that finds this thread in the future, the OEM part number is [37750-PH2-014] and it's official name is [Thermo Unit].

 

When you add coolant, are you burping the system? Air pockets can cause all sorts of problems

 

I did, I even used one of the kits below to burp it. I let it run until the thermostat opened and revved it a bit until no more bubbles came out and all of the coolant tubes were hot. The whole process was about an hour long. I'm not sure if there is a more specific process that I'm not following, but the temp was climbing pretty fast on my dashboard while my Engine Coolant Temp sat between 194 and 203/204.8 as usual. I would think over time that pocket of air would eventually escape. With that said, when I removed the OEM sensor to replace it, about half a cup of coolant came out, and then nothing else... I'm not sure if that's a cause for concern but if it is because there is no coolant in that pocket, that makes sense. My only problem then would be, how do I burp it properly haha? Before I replaced the part, my radiator and reservoir were always filled.
Dash Temp: https://imgur.com/a/EulR8O4
Coolant Kit: https://imgur.com/a/kpp8B5L

 

So for anybody reading this in the future, I replaced the sending unit again and I believe this solved the issue. The one I originally bought was from AutoZone, this one is OEM.

 

which sensor are you changing? the one on the thermostat housing is not for the coolant temp on your cluster (the 2 wire sensor) the sensor for the cluster is the single wire one that is on the cylinder head, u can test that by disconnecting the single wire from it then grounding that wire straight to ground and you gauge will read full hot