The perils of a high mileage 20 year old sports car

The last few months of 2025 were pretty rough on my Z due to the large increase in the amount I drive per month.

October had an unsuccessful convertible top repair, again.

In November, the car began having problems with idling and backfiring, leaving me nearly stranded in rush hour traffic. Thankfully I was able to limp it home, and it was simply the cam and crank sensors (a common problem.)
Unfortunately, the fun didn't stop there.

Later that week, I got in my car and attempted to start it, only to be greeted with dead battery like symptoms, despite the voltage gauge showing 12v. All it was were heavily corroded battery terminals I cleaned on the spot.Thankfully this happened in my school's parking lot and not during the ~100 mile journey I completed in it the day before!

I also found that one of the aluminum wheels had a hairline fracture, so I got it replaced.

However, here was the final nail in the coffin.

In early December, I began to notice that the heat wouldn't work unless the car was actively driving or being revved. I suspected it was a bad thermostat that was stuck open. Unfortunately, I should have seen this as a warning sign that I shouldn't be driving the car, and a few days later it had a pretty major overheating event.

Oops.

I was able to limp it home, and ended up replacing the thermostat, coolant radiator hoses, radiator (which had a hole blown in it,) and bleeder valve.

I bled the system, which was a PITA but doable, and began driving the car around again. During this time it was running and driving perfectly fine, with no head gasket like symptoms (smoke out of the exhaust, coolant being used, etc.)

Unfortunately, the heat went away again, and it overheated (but this time it was less extreme.) I thought that maybe air was still in the system, and after bleeding the system in a parking lot the heat was back and I drove it home just fine.

...but of course the heat went away again! This time I thought the water pump was bad and therefore causing low coolant circulation. So I replaced the water pump, timing belt tensioner, and had a professional shop bleed the system. I also did the combustion gasses head gasket test multiple times during the 2 hour bleeding process, and the fluid did not change color. The professional shop also didn't believe the head gasket was blown.

This whole overheating and heat ordeal really burnt me out and honestly I needed something more reliable, so now I have a new daily driver. But I can't bring myself to selling my first car and I want to fix it.
So for now, until I get more money and time, my Z is in hibernation.

Now I ask,
Is it a blocked heater core? I believe it is, since the lower radiator coolant hose was pretty cold even after everything warmed up. Obviously this is a huge undertaking to replace since OEMs love to bury them under the dashboard.
When/if I go ahead and replace the heater core, (probably in a "few" months,) what other things should I do while my dash is out? Any tips and tricks?

Of course, it could still be a microscopic head gasket leak. All aluminum engines hate overheating.

Also, all of the parts I got besides the radiator (which is a CSF aluminum one) are all new OEM.

TLDR: Man is shocked that old sports car does old sports car things, replaced with new daily driver, has continued no heat and overheating problem despite replaced thermostat, water pump, coolant hoses, radiator, bleeder valve, negative head gasket test, and a professional shop's help.

 

I don't think your heater core is blocked, that would have no impact on the temps of your lower rad hose. A stuck thermostat, finless water pump, large air pockets (not enough coolant), or some kind of major coolant blockage inside the engine block would cause this. I think it's more likely your heater core is leaking and thus introducing air into the system. Alternatively, you could have a small coolant leak somewhere else on the engine introducing air. There's a lot of hoses routing coolant around the engine, mostly in areas where it gets hot, so if the leak is small, the coolant could burn off before it would ever show drips or pooling. Top areas to check would be the coolant hard pipe connections at the back of the cylinder heads, the bypass hose that runs from the thermostat over to the passenger side hard-pipe. The small coolant hoses that run to & from the oil cooler (oil filter sandwich plate), also the small coolant lines that run to and from the throttle body. Lastly, the heater core hoses and heater core itself.

To rule out the heater core, you can simply disconnect the hoses at the firewall. Cap off the heater core and then couple the hoses together. Drive it for a while like that and monitor coolant level + temps. If the engine stops overheating and your coolant level remains constant, then you have your answer for the heater core. Needless to say, this is best done come spring when you won't be freezing.

Regarding the CSF aluminum radiator, pay very close attention to it. I have seen some bad reviews and failure reports on their products. In short, they don't have a stellar reputation. This is why I went for a Koyorad, they're pricey, but are considered top notch in terms of quality & reliability. Case in point on CSF, here's a video from the "The Aussie Shed" on a CSF radiator he purchased for his FI G37 project. It did not last very long before it developed serious leaks.


One last thing that could help explain the loss of heat in the cabin would be broken blend door actuators or debris clogging the veins/fins of the heater & evap cores housed inside the HVAC box. But if you have good airflow through the vents in all modes (front vents, defrost, and floor), then this would rule that out.

Check back in when you're ready to pick this up again. Fingers crossed it's not a headgasket issue!
-Icer

 

Looking at your engine photo, it's hard to tell, but is your bleeder port on the heater hose present? It looks like there's some kind of coupler there, but I don't see the bleed port.
-Icer

 

Good info @icer5160 Always providing good input