1959 Cadillac 390 Gauge Shows Running Hot? But Maybe Not?

[JayRudig390]

This is my first post so forgive me if it's in the wrong place.

Recently acquired a 59 Caddy 2 door Hardtop w/ A/C. Runs good when warmed up, but temperature gauge reads 3/4 to hot, which had me concerned, so here's what I've done so far, and it still appears to be running hot:

1. Replaced Temperature Sending Unit
2. Replaced Water Pump - flushed the cooling passages in block and head
3. Re-cored original radiator
4. To eliminate a possible short or ground, I ran fresh jumper wires from a) temp sending unit to the back of the gauge b) ground wire directly to ground of battery to speedo cluster c) 12v hot wire from pos to back of temp gauge
5.  Hooked up a temp gauge that I know to be good from a 60 Chevy, temp still reads just under hot.
6. Replaced Thermostat w/ new 185 degree stat (boiled it first, verified it works)
7. Bought an IR digital temp gauge reader, when warm: top rad hose reads about 90 degrees, lower rad hose reads about 60 degrees. Radiator near filler cap reads about 175 degrees. Top of engine gets of to about 185 degrees.

It doesn't boil over or start steaming, but it bothers the crap out of me that both gauges I've tried read hot!
Don't' know what to do next, get an after market temp gauge?

[INTMD8]

Does the known good gauge you tried read in degrees or just hot and cold? 

Does the same with the original sending unit?

Engine to firewall grounds good?

Ignition timing?

[35-709]

The IR digital temp gauge should tell the story, sounds like a gauge or sender problem.

[60eldo]

  So the engine temp is fine, your just concerned about the gauge. When the car is runningyour temp gauge shows 3/4, so why not just let that be the norm. Mine when hot sits just above the 1st line, I know it should be around the quarter mark. But I just know that if it rises above that mark I could be in trouble.

[J. Gomez]

This is my first post so forgive me if it's in the wrong place.

Recently acquired a 59 Caddy 2 door Hardtop w/ A/C. Runs good when warmed up, but temperature gauge reads 3/4 to hot, which had me concerned, so here's what I've done so far, and it still appears to be running hot:

1. Replaced Temperature Sending Unit

James,

If you do a search i.e. “sender” you will find a few articles related to your issue here is one for additional info.

http://forums.cadillaclasalleclub.org/index.php?topic=150835.msg400195#msg400195

You did not mention if the sender is an OEM or an alternate equivalent replacement.

If you have the equivalent replacement those do NOT have the correct internal resistance for the OEM temp gauge, they are close but the gauge will show in the hotter area. The OEM sender would have about 200 ohm at around 212 degrees while the replacement will be around 155 ohms. This differences would cause the gauge to peg into the hot area.

One fix is to add an in-line resistor about 45 ohms to offset the sender low resistance, this will bring the gauge back into the correct setting.

HTH

[Jeff Rosansky CLC #28373]

I know you say this is your 1st post but I gotta say I am really impressed with your troubleshooting. Well done.
Something tells me you are going to fit in well here!!
Welcome
Jeff

[Dan LeBlanc]

Lectric Limited sells the sending unit with the correct resistance for GM cars of the era.

[JayRudig390]

THANK YOU ALL for the replies and guidance! This forum is sweet.

UPDATE / REPLY: I now have peace of mind knowing she is not overheating.  I installed a generic MECHANICAL temperature gauge that shows exact temperature readings; it threads into the existing hole, and guess what?? Warmed up this beast runs smoothly at the EXACT factory specified temp of 189 degrees.  I now have piece of mind knowing she is not overheating.

I suspect I wasted all that time and money on those parts, when I should have tried this first.  On the bright side I learned a lot.

However, my goal is to have the OEM gauge working properly. Next I will buy an OEM spec sender unit as recommended from Lectric Limited, with the correct resistance - -- if no luck there, I will perhaps try an inline resistor, 

PEACE! JRudig390

[76eldo]

For 20 bucks you can buy an IR thermometer and scan various parts of the cooling system and engine to read the actual temps.  It's a very good investment.

Brian

[bill_boos]

Does the known good gauge you tried read in degrees or just hot and cold? 

Does the same with the original sending unit?

Engine to firewall grounds good?

Ignition timing?

James, you mention ignition timing in this post. Can timing have an effect on engine overheating? My '47 started overheating after I had a Pertronix ignitor and coil installed. Have not checked timing.

[35-709]

Improperly set ignition timing can most definitely affect engine operating temperature.  Any time you do work on the distributor, checking the ignition timing should follow as a matter of course, like breathing in and breathing out.

[JayRudig390]

Prob solved:  incorrect temp sending unit had improper resistance!