Not sure about your other problems, but you have air in your coolant system. The much hotter air pocket is periodically hitting the temp sensor, that’s why you’re getting random spikes of overheating. The reservoir keeps puking for the same reason - air expands a lot more than water so it’s forcing whatever coolant is caught between the pocket and dump line into the reservoir.
You need something like this to properly burp your system: OEMTOOLS 87009 No-Spill Coolant Funnel Kit, Near Universal Fitment, Translucent, 15 Piece Set, Cooling System Funnel Allows Access To Hard-To-Reach Radiators https://smile.amazon.com/dp/B01A2CQSU6/ref=cm_sw_r_cp_api_glc_fabc_7wu6FbH9CFYS1
Depending on where the cap is vs the highest point in your system, you may also need to angle the car a certain way on a driveway while you’re doing it. Hope this helps
The radiator version with the stopper is really slick.
The thermostat is probably closed because there is air in your system. If you've changed it 3 times in 4 months I think it's very likely you may have a leak where combustion gasses from the engine get into the coolant lines, or you just have air in your system. Your best bet would be to get a lisle funnel, jack the front of the car up put the heat on high, and let it idle until it reaches operating temp. When the thermostat opens make sure the coolant in the lisle funnel doesn't run low. When the car is operating temp let rev it at a constant rpm (2-2.5k) until you feel the heat coming out the vents, and the bubbles stop from the funnel. After that turn the car off and leave the funnel connected with coolant while it cools down for maybe 45ish minutes. The reason for this is there could still be a bit of air trapped and these cars are a pain to bleed. My first car was also a G35, loved it but got rid of it because of a partially blown Head gasket. Regret getting rid of it but such is life. Good luck
Those motors are notoriously difficult to bleed the cooling systems on. Watch a YouTube vid on it for the exact way to do it and any tricks and tips. Definitely need to open bleeder, and having the correct funnel helps immensely.
The proper way to bleed the coolant system is to use a Coolant Funnel Kit. Doing what you described, leaving the radiator cap is going to cause coolant to overflow and spill everywhere.
It's very important to have zero air in your coolant. You water pump pumps water not air. Failure to bleed air from your coolant will result in overheating.
You need to circulate the coolant with the engine at operating temperature, when the thermostat is open. Do this by running the engine withe the radiator cap off.
Before the coolant gets hot, remove the radiator cap. With the cap off, the air will be able to escape when the coolant is circulating. You will make a huge mess if you don't have something like a radiator funnel. You need to keep the engine running while keeping the radiator topped off on coolant the whole time. You'll see air bubbles coming up from the radiator, which is the air escaping and coolant taking its place.
Or buy one of these funnels from Amazon!! Cheap and easy to use OEMTOOLS 87009 No-Spill Coolant Funnel Kit, Near Universal Fitment, Translucent, 15 Piece Set, Cooling System Funnel Allows Access To Hard-To-Reach Radiators https://www.amazon.com/dp/B01A2CQSU6/ref=cm_sw_r_cp_api_glt_i_RP704H57YQX3CKJ3Q475?_encoding=UTF8&psc=1 I have the same one, work like a charm
Changing the water pump was the easy part,changing the timing belt took a bit more time,"finessing",and research but if you have the day free (6 ish hours) it's a good $1,200 saved atleast.
OEMTOOLS 87009 No-Spill Coolant Funnel Kit, Near Universal Fitment, Translucent, 15 Piece Set, Cooling System Funnel Allows Access To Hard-To-Reach Radiators https://www.amazon.com/dp/B01A2CQSU6/ref=cm_sw_r_apan_glt_i_5ZZ5SAR8RT55VD7J7KDW?_encoding=UTF8&psc=1
I would also recommend this for burping the radiator,makes it a hell of a lot easier.
I had my entire cooling system torn apart to replace the radiator and water pump so it's not really the same for me to do a standard flush. I would check out the youtube videos out there to get an idea of how it's done. It's a bit messy but probably wont need to do it more than once every few years. If you decide to do a full flush, get yourself a burping kit - they're worth it big time. (https://www.amazon.com/dp/B01A2CQSU6)
Also, I suggest using a radiator funnel. It mounts to the cap opening & keeps a head of water to help prevent introducing air bubbles.
OEMTOOLS 87009 No-Spill Coolant Funnel Kit, Near Universal Fitment, Translucent, 15 Piece Set, Cooling System Funnel Allows Access To Hard-To-Reach Radiators https://www.amazon.com/dp/B01A2CQSU6/ref=cm_sw_r_apan_glt_fabc_0E4DRYMK7MNNP5B7G5N2?_encoding=UTF8&psc=1
It's possible for sure but bubbling coolant could also be a bad thermostat, radiator cap, or dreaded head gasket. However, there are other symptoms that go along with a head gasket.
That being said, it would be best to make sure the system if fully filled. Just to rule out an air bubble.
I always use one of these funnel kits when refilling coolant. Makes the job cleaner and much easier to burp out air bubbles.
https://www.amazon.com/OEMTOOLS-87009-No-Spill-Coolant-Filling/dp/B01A2CQSU6
And just a heads up, if you have a rear heater, the vehicle holds more coolant than the ones which do not have one.
It took me almost 30 minutes to full get out all of the air when I did a full drain and flush on my 98 with a rear heater.
Yes, add coolant to both. Fill the radiator to the maximum amount and add coolant to the overflow somewhere between the two max and min lines. Coolant moves from the radiator to the bottle when it gets hot and everything expands (think high school physics). When the car cools down, everything contracts and sucks coolant back in to the radiator. On this system, there is a pressure valve (the radiator cap) between the two so you can’t just add coolant to the bottle. On some cars the radiator cap is on the bottle, and that’s when you can just add coolant directly to the bottle and it will flow into the radiator without any restriction. GM seems to like the second design with the pressurized bottle. For ease of service, I like this system better. If you want to get fancy, you can get a coolant funnel that helps you burp the air from the cooling system. You put it in the radiator and over fill the system and let the car run till all the air comes out. Honestly though, you really shouldn’t need this unless you do a major service on the cooling system like a water pump r&r. A quick search on Amazon found it for under $20. OEMTOOLS 87009 No-Spill Coolant Funnel Kit, Near Universal Fitment, Translucent, 15 Piece Set, Cooling System Funnel Allows Access To Hard-To-Reach Radiators https://smile.amazon.com/dp/B01A2CQSU6/ref=cm_sw_r_cp_api_glt_fabc_ND5QBPB5VR1B45YSHJSG?_encoding=UTF8&psc=1
Could be air in the system. That was causing a civic I have to overheat. You need to burp it if that is the problem. Let the car run snd slowly add coolant the the radiator as the system while it circulates, and the air bubbles fart out. give the radiator hoses a squeeze every now and then. Run the heat too to circulate the coolant in the heater core. They sell kits that are pretty useful and are really cheap. That and the overflow tank was clogged with shit on mine. I took it out and blasted the inside with a garden hose and replaced the radiator-overflow tank hose. I replaced the radiator cap for good measure. Your radiator could be clogged too. If you take a lazer thermometer, you can just point it at the radiator. If its all hot and part of it is cool, its probably clogged somewhere.
Kit:
There is more air trapped in the system. Turn it off, let the car cool down for a few hours with the rad cap off, then squeeze both radiator hoses, turn the car back on with the heater blowing.
This tool is very helpful if you don’t already have one.
OEMTOOLS 87009 No-Spill Coolant Filling Plastic Funnel Kit https://www.amazon.com/dp/B01A2CQSU6/ref=cm_sw_r_cp_api_i_GsmBDb1PT730X
Also what is helpful is one of these funnel sets it helps you to bleed out the air
https://www.amazon.com/OEMTOOLS-87009-No-Spill-Coolant-Filling/dp/B01A2CQSU6
these are the workshop manual steps on bleeding the system and coolant ratios. from what it seems is you are running 70/30 coolant and that made be part of the problem. try a 50/50 mix.
https://www.amazon.com/OEMTOOLS-87009-No-Spill-Coolant-Filling/dp/B01A2CQSU6
try using something like this to fill the radiator and cooling system with as it will use the pressure of the coolant to push out the air pockets. if that doesn't help with the problem have the cooling system checked for combustion gases getting into the system.
I went OEM, more for my own piece of mind than anything else. I also used this funnel kit for topping off the coolant and getting rid of the air bubbles.
https://www.amazon.com/OEMTOOLS-87009-No-Spill-Coolant-Filling/dp/B01A2CQSU6
This guy did a great vid on the swap and coolant top off and air bubble bleed off.
Here ya go
OEMTOOLS COOLANT Filling KIT, Multi, One Size, 87009 https://www.amazon.ca/dp/B01A2CQSU6/ref=cm_sw_r_cp_api_glt_i_JQ86HBW76649KMYEMF80?_encoding=UTF8&psc=1
Yup, you could buy a 3/4” heater hose connector at autozone for $5, 2 gallons of coolant for $20 each, and one of these funnels for $20
With as often as the coolant needs to be fucked with I'd recommend this
Product name: OEMTOOLS 87009 No-Spill Coolant Funnel Kit, Near Universal Fitment, Translucent, 15 Piece Set, Cooling System Funnel Allows Access To Hard-To-Reach Radiators
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You can use either Honda Original Equipment Manufacturer (OEM) Blue Coolant or you can use the yellow fluorescent coolant. Never use the orange coolant with DEX. Also buy the full concentration and not the pre-mixed 50/50 solution. You'll save money because the 50/50 mix is half water and half coolant.
I also recommend getting a No-Spill Coolant Funnel. it makes refilling any car with coolant easier.
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When you replaced the radiator, did you bleed the air out of the cooling system? There is a plastic radiator bleeder valve at the back left, against the battery compartment. If you didn't bleed all of the air out, it will cause overheating.
https://www.infinitiscene.com/threads/coolant-change.27966/
Helps to also have this no-spill funnel to help with bleeding air: https://www.amazon.com/OEMTOOLS-87009-No-Spill-Coolant-Filling/dp/B01A2CQSU6/ref=sr_1_4?dchild=1&keywords=coolant+bleed&qid=1603412346&sr=8-4
Only needs a few turns with a Philips head screw driver to open (does not need to be removed). Make sure engine is dead cold. Leave radiator cap off. Put no-spill funnel on if you have one, fill half way with coolant. Start engine, let engine idle or give it a few revs to get the temperature to rise. You might notice some coolant coming out the bleeder valve behind the engine, or bubbles coming up the no-spill funnel. that's a tell-tale sign there was air trapped. however if your temps keep rising past the middle point, then likely thermostat. double check and make sure both radiator fans are running when engine is hot. both should turn on before engine overheats. if only one or none turn on, then you need new cooling fan.
Also it is possible they didn't burp the system all the way, go to your local Wal-Mart or auto parts store and buy a gallon of DEX-COOL, or go on Amazon and buy it with a spill-free funnel
To re-burp it yourself and search for "how to bleed a cooling system" on youtube.
Before you do this make sure they used the correct coolant or you will have issues down the road i.e. the vehicle calls for DexCool and they used some universal all vehicle stuff.