Weird Overheating Problem (Update now included)
 

Hi Guys. I just got a 79 280ZX coupe. I got a very clean one, all original as far as I can tell, only 60k miles. The car was very well cared for. I took a road trip this weekend and put 300 miles on it, so I have a better feel for the problems. Most are small things that I can pretty easily deal with, but there is one thing that is weird at least to me.

I kept a close eye on the coolant temperature gauge to make sure it wasn't overheating. It was fine, mostly indicting the middle of the gauge. But after I had driven for an hour on my way back home, I turned the car off at a gas station, filled up, and went in and got some food. When I came back out, the car was tough to start. It only went to 200 or 300 rpm on the tachometer (it usually idles at about 1000) and would stall out. I noticed that the coolant temperature gauge showed it was super hot, way over in the 250 range.

I got the car to start after a few minutes and as soon as I drove it a couple of blocks, the temperature returned to normal. Since then, I've been watching this and every time I turn the car off the coolant gets a lot hotter. I don't know much about cars but I am trying to learn (I've been learning a ton from going through this forum) but isn't that counterintuitive? Why would the car overheat only after it is turned off? I realize that the fan goes off (I don't have an auxiliary fan) but I wouldn't think it would actually get hotter at that point.

I dont know enough yet to deal with this myself, but since the car is new to me I'm going to take it to a mechanic I trust and have him give the whole car a careful inspection and a tune-up. I was told that all fluids were changed before I bought it, but who knows. I will tell him about the overheating issue, but I thought I would check here and see if any of you had dealt with this. It isn't realy hot here yet where I live, and if it is already doing that I will be in trouble this summer if I don't get it fixed.

I've searched the forums and didn's see any similar issues mentioned. If you have any thoughts on what is going on here and what to suggest to the mechanic, I would be greatful.

Welcome to ZDriver! :006:

Good plan.

Indeed... who knows. Usually the seller of the car will say anything if he thinks he can close the deal. :D

A good coolant flush and new OEM thermostat will usually do the trick.

Doesn't hurt to "burp" the system. Never had an S130 give me grief but my 300ZX Vert had an air bubble in it and when the heater was on/off (don't remember which now) it would overheat but with the heater the other way it ran bang on. Burped it (make sure the heater is on when you burp the system) and it's been just fine.

the reason behind why the car gets hotter after you shut off the engine is similar to the reason why a person would get hotter after they run a long distance and suddenly stop; while you are running you have wind and stuff to cool you down, but when your stop there is no wind to cool you down so you heat up.

Perhaps on an air-cooled VW engine :biggrin:

or an air cooled radiator lol

Follow Up
 

I think the problem is under control now. I took it in for service since I am not able to do the work myself. After two trips to the shop, the ZX has now had a thorough tune-up, changed thermostat housing gaskets (coolant was leaking out of two places), a new cooling fan clutch was put in, a belt that was slipping tightened up, and the radiator had a flush and fill. The car is driving very smoothly, the tune-up made a huge difference. The overheating I described at the top of this thread is still there but no longer is bad enough to affect starting up. After I turn the car off, the engine temp gauge still moves to the right quite a bit, but it isn't as bad as it was at first. Maybe that is normal for the coolant to heat up after the car is off and the fan is no longer running.

A disappointing thing from the service was that I asked them to test the compression. I got 120 on five cylinders and 115 on the other, so that is pretty consistent. But I'm worried at how low the numbers are. I'm happy for now, it really is driving beautifully.