Question - I have a 1972 240Z.  Recently replaced stock radiator with a Motorsport Auto stock 3 row one because I didn't like the temp gauge reading.  Also, replaced radiator cap and installed a 160 thermostat.  My problem is that when on the highway at 60-70 MPH the temp needle goes to the "M" in "Temp" on the gauge.  As soon as I get off the highway it comes right down to middle......I can idle for hours and it will stay in the middle.....it's only on the highway at highway speeds.  It may be OK but it worries me when it starts to climb like that.  I have another '72 Z car and it NEVER moves off the middle point on the gauge.

Suggestions on what to try next??  Maybe replace Temp sensor unit?    I would appreciate anyone's thoughts on this.

Jeff Mader

 

 I think I'd try a cooling system flush.  At higher RPM's the engine logically produces more heat. Members who have pulled the freeze plugs in the block have reported vast quantities of grunge in the coolant passages.

OK thanks I'll try that!

 

If you can source an IR thermometer you could figure out what "M" really means. These gauges are inexact at best. Once it got to "M" you could pull over and check temps on the hoses, thermostat housing and radiator to figure out what is really going on...

what makes you think something is wrong? dont get me wrong, you can waste all the money you want chasing a problem that doesnt exist

11 hours ago, Jeffrey Mader said:

 My problem is that when on the highway at 60-70 MPH the temp needle goes to the "M" in "Temp" on the gauge.  As soon as I get off the highway it comes right down to middle......I can idle for hours and it will stay in the middle.....it's only on the highway at highway speeds

If it doesn't keep rising, past the M, on th highway, you don't really have a problem.  Ranges have a high and a low.

Mine stays on the right leg of the "M" all the time.  I've measured the coolant temp with a digital thermometer, 175F at idle with no air flow other than the fan.

 

Mine used to sit dead center on the gauge and never move once it was warmed up.  Then it started climbing to the edge of the M if I went over 65 on the highway.  It could idle all day in traffic and stay at the center.   

Years ago a company truck where I worked would overheat only on the freeway.  They decided it was a blown head gasket and bought a reman engine for it; while it was apart I convinced them they should get he radiator serviced.  The guy at the radiator place said it had 2 tiny pinholes in it that once the heat and pressure started to climb would likely bleed off enough pressure to make it overheat like it was doing.  But not leak enough to show.

For my Z I got a pressure tester and I found I had a radiator hose that wasn't sealed perfectly.  At full pressure only when I'd move the hose would it leak.  I tightened all the hoses and now it climbs a bit above the middle on the freeway when it is 75 or hotter out but not as high and I have to get on it pretty good to get it to do that.  I may still have a tiny leak somewhere but I'm not a huge fan of pressurizing the 45 year old heater core 

Edited September 27, 20177 yr by ta240

That heater core scares the crap out of me to. 

As long as it sits on the same spot on the gauge I'll leave it alone. Those old 240 gauges are finicky, my 280 is pretty accurate, moving with the cars situation. Higher  in traffic, lower on the freeway.

Have you tried removing the sensor from the thermostat housing and sanding off the calcium/crud, clean it up the connection to?

I replaced my sensor and that improved the accuracy of the temp gauge.  I also installed a shroud around the original radiator and that has helped lower the operating temp.  

11 hours ago, Burl said:

I replaced my sensor and that improved the accuracy of the temp gauge.

How do you know?   

Edited October 2, 20177 yr by Captain Obvious

Great question. How would I know what the temperature of the coolant in different parts of the engine? Is the an ideal temperature? Is there a critical maximum?


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