I recently replaced the radiator and added an overflow bottle in my '75 280z. After refilling the system with coolant, I took her for a test run to check for any leaks. The temp gauge was reading good - about 5/8ths...then the gauge dropped to the bottom of its range. I checked the connection on the temp sender and topped off the coolant and took her for another spin. Again the gauge was reading good then stopped working again. I let the system cool again and took off the radiator cap to add more coolant, but the coolant was at the top of the radiator. Would an air bubble in the thermostat housing cause the temp gauge to not register?

Edited September 27, 201016 yr by twsutt

Check all connections.Based on your description,the sender would be the next logical culprit.

Could also be a bad gauge. I've had that happen. Inside a typical gauge there are coils. As the resistance from the sending unit varies with the coolant temperature, the current at the points along the coils will also vary. This is in turn will cause the strength of the magnetic field generated in the coils to increase or decrease which will force the needle to move to the proper position. If there is a 'dead spot' in one of those coils, the needle will drop when it reaches that area.

A little more information...

I'm sorry that I didn't mention this before...prior to the replacement of the radiator all of the gauges were operating correctly. I did change out the thermostat housing with a used housing complete with all sensors (one of the thermostat cover bolts snapped off and the old housing cracked when the machinist tried to drill it out.) I'll attempt to swap out the new (used) sender with the original sender.

So, an air bubble isn't the cause of my dead reading? I figured that even of it weren't in contact with water it should still be reading more than the 120 degree F. start point on the gauge.

...snip...prior to the replacement of the radiator all of the gauges were operating correctly. I did change out the thermostat housing with a used housing complete with all sensors...snip...

Well, there's your problem right there!. I doubt if you have/had an air bubble. Never experienced that on a Datsun. Other cars, like my GTO, or the Lincoln LS I once had, yes, but never on a Datsun.

Well, there's your problem right there!.

As always, Stephen, I thank you for your candor!