Temp gauge readings 78 manual

Hi all. My car runs with the temp gauge at the second white line while under way and creeps up towards the red line when at idle for a period of time (traffic). However the temp readings at the water bridge are always within range. 

I have a fairly new radiator and fan clutch. The temp sensor on top of the water bridge measures correct resistance. I’ve recently installed the fan on when ac on wiring kit I got from Rich and it works great. Seeing the needle go up that high does give some anxiety even though I’m pretty sure the temp is fine. Would this be a problem with the gauge itself?  I have serviced the cluster and it has a new circuit foil. 

thoughts on this?

I think we too often forget that the primary cooling fan is mechanical drive, via a temperature-controlled fan clutch.  Folks who work on the cars for projects like timing belt/water pump replacement fail to follow the strict factory guidance for always keeping the fan "upright" when it's off the car.  The silicone fluid inside leaks through and often out, and the fan clutch no longer spins fast enough to cool the engine.  There have been suggestions in the past regarding a couple Toyota silicone fluid replacement options, and I've heard mixed reviews of the results.  Mark Anderson at 928 International used to carry an aftermarket version that was adapted to fit the 928, and wasn't nearly as sensitive to being stored in the wrong position.  Certainly worth asking him for info on that product.

Before going too deep into that, do verify that the fan drive belt is installed correctly and is tight.  The factory US cars share the fan drive with the air pump, and that pump is adjustable to manage proper belt tension.  As those pumps aged, many replaced it with the Euro idler pulley to be able to adjust the belt tension.  Others decided to try just a shorter belt directly between the crank pulley and the fan drive.  A balt that's snug initially stretches and wears though, often to the point where it no longer drives the fan.  Regardless, make sure there's proper tension on the belt.

For some reason, there's a trend toward adding electric fans to completely replace the combo mechanical-plus-electric factory setup.  You get to install a new fan console and fans, plus a controller of some sort to turn them on and off when needed.  Most of these setups fail miserably when it comes to actually moving a bunch of air sufficient to keep the engine cool.  I remember a few successful projects that adapted factory fans from a Ford Contour/ Mercury Mystique.  The factory fans moved plenty of air, much better than third-party replacements for the same cars.  But still demanded a separate controller.  A fringe benefit of the electric fans conversions was the need to upgrade the alternator and some wiring to manage the added load from the fans.  Total of all that fun was more than replacing the bearings and fan clutch in the very reliable and sufficient factory setup.

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While it's possible that your temp gauge reading is bonkers, it's not that likely.  I regularly preach the benefits of doing regular electrical system PM, including cleaning ground points.  The engine grounds via an open copper cable between the block and the front frame rail.  Look hard at that and replace it it's looking tired or corroded.  Resist (sorry...) the urge to substitute an insulated battery cable for that duty.  Deterioration of that cable isn't something that changes with engine temperature, so very unlikely it will cause your symptom.  Might be worth just sliding the instrument cluster connectors off and back on a few times to 'wipe' the connection points.  The sender itself and its wire connector are point most vulnerable to damage.  For grins, verify the sensor resistance from inside the car while hot, at least from the CE panel connector for the sensor.

HTH! 

Edit-add:
One other thing on may later car that haunted me for a while was a stuck thermostat.  I was in denial about thermostat issues, since everything seemed fine until higher-load (read: faster) driving in summer L.A. heat caused the gauge symptom you describe.  I cleaned/flushed the radiator even though it was incredibly clean inside per my borescope camera.  When I finally replaced the thermostat, all was suddenly well in the universe again.  The heater worked sooner in the morning, something less important in SoCal than here in central Oregon where we are now.  The thermostat was a relatively easy and inexpensive thing to test.  In hindsight, it should have topped my list.

Thanks yall. Since posting I tuned up the K Jet and it’s running better AND cooler. I also added Redline Water Wetter and decreased the ratio of antifreeze to water. Stays below the second white line and creeps up to just over when it sits. Cranks right up any temp and feels SMOOTH!

I did discover that when hot my temp sensor measures too low of a resistance even though it reads fine when cold, so that might be causing a high reading on the gauge. Gonna replace that and the fan clutch as it doesn’t have much of any change in physical resistance when turning by hand hot or cold. 

Parts arrive beginning of this week and I’ll report back once in and tested out. 

After replacing my temp sensor and fan clutch here are my findings…runs about the same! Just like Rich said actually. With the new temp sensor the needle on the gauge rose to its usual spot, but with an added bonus of the coolant temp warning light coming on prematurely. I put the old one back in and will return the new one. 

Probably didn’t need a new fan clutch either but at least now I know. The new one seems to have about the same resistance when turning the fan by hand as the old one. At this point I think I’ve covered all the bases and I’ll try to stop obsessing over the gauge reading and enjoy the driving!