Hi there, this is now my second 85' 300zx i'm pretty familiar with the engine by this point, but the issue i've been having lately has really boggled my mind, looking here to see if anyone has had this issue, or any issues related to this.

So when I bought the Z it had an over heating problem, so initially I went down the chain of part replacement and here I am now.

Parts and service done within the last month.

Radiator Cap
All radiator hoses (Upper Hose, Upper front hose, Lower hose, all resivoir hoses and return hoses)
Thermostat
Water Pump (Timing belt done with this as well)

The only thing I have yet to change is the radiator. Now the issue i'm having is a bit odd. I have done a chemical test and shown that there is not a blown head gasket so that rules that out. I am experiencing a loss of coolant now, no visible signs of leaks, no water in the oil, but the overflow end up becoming 100% full way above the Max fill line. Water will spew from the reservoir only after driving for roughly about 35 minutes, The water tempature will jump between Normal and almost 270 (Obviously once enough water is dispensed from the resivoir)

Now i'm just not sure if this could be a clogged up radiator? Maybe it is not circulating properly?

if anyone can shed some light on this for me it would be greatly appreciated. Thanks!

 

check the hose from the bottom of the reservoir tank to the system. if it is kinked it will do what you say. there will be enough pressure when hot to push fluid into the tank but when if cools off it can't suck the fluid back into the system. Happened on my 85T. Get new hose. be sure it isn't kinked when you reinstall the tank. be sure the system is full of fluid. on flat level ground if you take the top off the heater inlet valve on the firewall and pour fluid into the "radiator cap" it will come out of the heater valve. you then know the system is full with no air pockets.

 

I will definitely try this today and let you know what is going on, I didn't even think of that lol. I really hate over heating issues, enough to make you pull your hair out!

 

Have you pulled the thermostat and ran it with out it to see what changes . I would pull the radiator if that does not change it , the rad. May be junk or need a rod out (if you can find a shop that will even do that anymore)
I wish I could drive my 86 on I 95 in S Fla.

 

This issue has been solved, seemed to be a clogged up radiator seems to be the issue.

 

Replaced the Radiator, and all seems to be well! Thanks guys!

 

Thanks for posting your answer, mine is doing the exact same thing and I went through nearly the same steps in replacements. Though I had replaced all hoses too. When I replaced the fan clutch, the overheating went way down to only on large hills. I'm pretty sure mine is the head gasket because I can visibly see a leak, but haven't gotten it tested for that yet. I've been guessing it has to be the radiator or head gasket now. I may sell it soon if where I'm moving has no parking, so I'm not looking into it until I move and know for sure. Will update if I find the problem. Thanks for the post though, really helpful!!!