Hi everyone,

I did a coolant flush a few weeks back. I noticed that the coolant temperature gauge fluctuates between in the middle to slightly pass it. I checked underneath the car and no leaks. Fan clutch has no issues either. The Z doesn’t seem to be running badly.

 

i was wondering if i didn’t burp all the air out. It seem to sit right in the middle before I did the coolant flush. I relaxed the coolant with predilluted Peak coolant. What could be wrong?

That could be an air pocket, to be sure all the air is out open the rad cap when the engine is cool and jack up the car front of the car as high as the jack will go, start the engine and turn the heater on full.  Leave the cap off as long as you can before it overflows then cap the rad and bring the engine up to normal temp, turn the engine off, leave it the car on the jack and let it cool over night. Next morning open the cap and top off the coolant if needed, that should do it or at least it has always worked for me.

If that doesn't do the trick you could replace the thermostat or at a minimum pull it out and test it in a pan of water on the stove with a thermometer.

Are you certain the new coolant was a 50 50 mix?   Antifreeze does not dissipate heat as well as water / antifreeze.  Heater temp control at max hot when you circulated the new coolant?

5 hours ago, S30Driver said:

Are you certain the new coolant was a 50 50 mix?   Antifreeze does not dissipate heat as well as water / antifreeze.  Heater temp control at max hot when you circulated the new coolant?

Yes, the coolant was definitely a 50/50 mix. The heater temperature was at max hot but only cold air came out from the vents.

5 hours ago, grannyknot said:

That could be an air pocket, to be sure all the air is out open the rad cap when the engine is cool and jack up the car front of the car as high as the jack will go, start the engine and turn the heater on full.  Leave the cap off as long as you can before it overflows then cap the rad and bring the engine up to normal temp, turn the engine off, leave it the car on the jack and let it cool over night. Next morning open the cap and top off the coolant if needed, that should do it or at least it has always worked for me.

If that doesn't do the trick you could replace the thermostat or at a minimum pull it out and test it in a pan of water on the stove with a thermometer.

Thank you for this! I followed your instructions this morning. I am waiting for the engine to cool down so I can add more coolant. The heater blows cold air for some reason.

 It the coolant temp is hot but the heater is cold the problem is likely in the heater control valve.

If you are running a 180 degree thermostat, your temp gauge pic is spot on - not indicating overheating.  The gauge needle should swing a bit every so often as the thermostat cycles to admit cooler water and then closes.

5 hours ago, BayAreaZ650 said:

Thank you for this! I followed your instructions this morning. I am waiting for the engine to cool down so I can add more coolant. The heater blows cold air for some reason.

As Mark said, heater control valve or quite often I find the push cable that actuates the valve can bend and the valve never opens.

I'm going to guess it's a 280Z?

The other reason for cold air is a loss of vacuum at the AC control bottle.  Pretty common.  Split hoses.  The vacuum woter cack (water ****) is normally closed.

 

18 hours ago, BayAreaZ650 said:

The heater temperature was at max hot but only cold air came out from the vents.

The heated air will NOT come out of the vents. That is not how the system is designed. @Captain Obvious and I have had lengthy discussions on possible improvements.

The three positions of the control panel will be Bi-level, heat and def in order to get heated air from the system.

The two places that the heated air will exit is at the heater flaps or the defrost tubes.

Did you check the operation of the heat with the mode AS heat or defrost?

Attached are two pictures - One is showing the outside of the heater case that shows the SMALL flap the heat will come out of, the defroster tube, and about midway up, is the mode door flap that controls if the air from the fan passes through the vents or the heater door.

Second picture shows the inside of the case. The mode door could be lacking insulation allowing air to leak to the vents, or as @Mark Maras, @grannyknot and @Zed Head have eluded to, the heater control valve, cables or vacuum system (if factory AC equipped) is malfunctioning.

Thank you guys for all your help! The temperature still goes past halfway but drop back to halfway within 30 seconds. 

6 hours ago, BayAreaZ650 said:

Thank you guys for all your help! The temperature still goes past halfway but drop back to halfway within 30 seconds. 

That's thermostat opening, not sure you have a problem. Get or borrow an infrared temp guns and see what the actual temp is.