My ’55 Customline has been experiencing a similar problem in that over the last two years the temperature has been steadily been reading lower.  With the water pump bearing finally making some noise, I changed out the water pump a couple of weeks ago as well as replacing the existing 180°F thermostat with a new one.  The under dash ‘Autogage’ temperature gauge still reads 150°-155°F when fully warmed up and part of that can be attributed to the sending unit bulb being located at the rear of the engine at the original location.  While this can cause a gauge to read 10°-20° lower at this location than at the thermostat, the engine temperature still looks too low based on the gauge reading.  I haven’t bothered to check with an infrared gun yet but I suspect the gauge itself is reading on the low side.  Engine runs good with no evidence of being too cool when looking for moisture build up in the oil fill tube so it has not been a pressing problem to address.

Lorena, Texas (South of Waco)

Most of us wish we had that problem.

My concern is the sudden change. On the hyway at 70 mph it always ran 190. Around 200 in town with stop and go traffic.

Durham Missouri

Both my roadster and 56 Ford run around 160 on the highway. Possibly the aluminum radiators? I'm running a 180deg thermostat in both. First time I've had problems with underheating.



Y's guys rule!
Looking for McCullouch VS57 brackets and parts. Also looking for 28 Chrysler series 72 parts. And early Hemi parts.

MoonShadow, 292 w/McCulloch, 28 Chrysler Roadster, 354 Hemi)
Manchester, New Hampshire

I'm fairly confident the gauge and infrared thermometer are correct since they are both within 5° of each other. Also by the way the engine is running its not getting up to proper temp. I ordered another thermostat and am going to change out the coolant. Its 6 years old and probably wouldn't hurt to change it anyway. Hopefully this solves the problem.

Durham Missouri

I'm thinking that both the thermo gun and gauge happen to be just as incorrect.

Probably the easiest way to verify this is to remove the thermostat and boil it on the stove, both to verify that it functions correctly and at the right temperature.  If it does, then the temp is definitely higher because the engine will not stay cool enough to keep the thermostat closed.  If you still have the 190 around, start with that.
You can also do this to validate the thermo gun, and gauge since water boils at a given temp.  You can use a heat gun or hair drier for the thermostat but this doesn't give you the benefit of a known temperature, for validating the thermo gun or gauge.


Lawrenceville, GA

It could be a weak or defective thermostat, some brands are inconsistent. I most definitely enjoy a 190° thermostat in cold northern weather, heater output is noticeably better. And that's usually what people notice first when a T-stat fails though it's pretty gradual usually.

What can cause a y block to not reach operating temp? My car recently started to run colder than usual. With a 190° thermostat my car stayed in the 190-200° range even in 100 ° weather. It started to run in the 150-160 range so I suspected a stuck thermostat. I changed it out with a 180° thermostat and with the cooler temps it barely reaches 150 now. In warmer weather it would barely reach 160. I verified the gauge is correct with a thermo gun. With winter coming on I need to get it back up to normal temp for heat plus it doesn't run well with the cooler temp.

Durham Missouri