Car: 99 EX (stock d16y8 with 244k miles)
Recent Maintenance: Honda OEM Water pump, timing belt, and other misc done at around 210k, Honda OEM thermostat done a month ago.

Problem:
The problem just started today FYI. The car seems to overheat only after an extended period of driving and the engine has a chance to get really really warm. The needle will go up about 80% of the way and hold, then slowly come back down to normal temps, and do it back and forth. I tried turning the heat on full blast and after a minute, the temp gauge came back down to normal temps and stayed there for a few minutes. After that, it started overheating again but not nearly as much as when the heat was off. When I'm idling, the temps stay normal. When I take off, it starts to climb again. Turning the heat on keeps the overheating down a bit.

All fluid levels are fine. Upon closer inspection of the radiator cap (which is original AFAIK), the seals/gaskets underneath are pretty shot. I burped the system about 10 minutes ago and the temp gauge was at normal temps the entire time. I let the fan come on twice and they seemed to stay on longer than I remembered. Dinner was ready inside so I put the cap back on and when I shut the car off, I saw that the needle rose ever so slightly.

Thoughts? Thanks in advance.

 

<p>Get a new cap and pressure test the system.&nbsp;</p>

 

Unfortunately, I have to wait until Monday after work to be able to buy one, but it is on my to-do list.

 

Well I replaced the radiator cap today and went for about a 5 mile drive. Once the temp needle got to normal, within a minute, it sky rocketed. I instantly turned around and by the time I was pulling into my driveway, the needle was almost at the very top. Holy ****... Full heat helped way less this time.

I opened my hood, felt both radiator hoses, and the top one was boiling hot and the bottom one was still cold and hard. Brand new thermostat and it isn't even opening. Alrighty then. Time to switch for ANOTHER new one? This will be going into my 3rd one within the last 40k miles.

edit - scratch that. I just went out to reburp the system and the lower hose got hot and everything worked like normal. I just can't get the engine to overheat when sitting.

 

Properly and thoroughly bleed the cooling system.

https://honda-tech.com/honda-civic-d.../#post39830585

 

I've done this a few times already but I think I messed it up the 2nd time. After letting the fan come on several times, I didn't top off the fluid as much as I should've because today, I added a ton of fluid lol. Woops.

My friend mentioned something called "airlock" which makes sense since I didn't properly bleed the system.

I went for a drive again and no matter how hard I tried, I couldn't get the needle to budge past normal. All seems fine for now!