Hi,

I drive a 1975 Plymouth Fury sedan with an original 318 / 904 transmission, 43,000 miles on it. I am wondering about what should be my coolant / water normal operation temperature should be under most conditions, or if changes are normal.

I just wonder if old cars are that much more sensitive to hot weather.

Here's what's happening:
Under mild and warm California weather, driving or idling my temp will look like this:


Under the same weather, if I turn on the A/C driving or idling, it look like this:


Now, if outside temp rises above 90ºF, highway driving without A/C will look like this: (It was 100-something degrees today outside when this happened.)


Highway driving with A/C + Idling with no A/C under same 90+ degrees outside:


And idling with A/C under same hot weather will make the temp gauge rise slowly, it seems to move slowly toward the Hot, tends to stick around this area, I don't want to experiment with overheating, so I always shut it off when the needle reaches that area:

After shutting off the A/C, the needle has a hard time going back toward the cold side, unless I get up to 50+ MPH it crawls slowly back to it, but never reaches the cold section, usually stays in the middle.

Now the reason I'm asking all this is because this car is a daily driver and I am used to see my temp gauge always pointing around the far left of the dial, not matter what, A/C on or off.

This was a bit unexpected, as I do know that the compressor running will heat up the water a bit more, but I wasn't aware it wouldn't cool down quickly when I shut off the A/C off.


Thanks for your help and Happy 4th!

 

This actually looks great to me. This is a 38/39 year old car and the guages still work! Holy cow!
Regarding the issue of the guage showing high after a shutdown, I'd say that is also normal. Once you shut down the engine, the coolant stops moving, the fan stops moving...But the engine takes awhile to cool down on its own, expecially in hot weather. If you have a good radiator and fan with a shroud and all the unserhood seals are in place, AND nothing is boiling over....You are doing fine.

 

Miles

Those readings look pretty normal. I have both the factory electric gauges with the same dials as you and after market mechanical gauges in my car, so I can calibrate the factory one.

first picture about 140*
second/third picture about 160 - 170*
forth picture 180 - 190*
fifth picture 200 - 210*

Unfortunately, those numbers may or mat not work for you.

If you want numbers, you can temporarily or permanently attach a mechanical gauge and calibrate the factory one or just use an IR thermometer (not expensive) aimed at a consistent reference point on the block, preferably near an exiting water passage.

Archer

 

I've never seen one run as cool as you are describing. Typically anywhere in that "safe" area of the guage is fine. My mopars always ran towards the upper end of the safe zone. I am curious if you replaced the thermostat anytime recently? If so, maybe your old thermostat was not functioning and stuck in the open position so the car never reached operating temp?

 

Kern Dog: I'm sorry I should have mentioned when I meant shutting off, I meant shutting off the A/C only but still driving the car around. The temperature keeps climbing up when using the A/C, and when turning the A/C off, it doesn't seems to go back down that easy.

It feels as if I keep it idling like that, it will at some point boil over.

Archer: Thank you for the numbers, that's awesome! So it's okay to have a temp of 210ºF and have the needle pass the half mark. (I'm used to newer cars having the temp always sticking in the middle).

schizm12487: I replaced the thermostat a few weeks ago when I did a tune up and other work on it as a preventive measure, and both old and new run like that.

 

The other thing to keep in mind is that the (now very old) gauge SYSTEM may no longer be accurate, due to several reasons

Wiring harness connections.

The sender may be inaccurate due to age

The voltage limiter which supplies the temp and fuel gauge may not be accurate

The gauge unit itself may not be accurate.

 

Miles -

Older car temps aren't controlled as well as the modern stuff.
If the highest you get is 210* and it stays there, you're probably fine, but the odds are good that if 210 is the norm, it will rise as the driving scenario worsens.

In my case with a 160* t-stat, my cruising temp is about 170*, if it's really hot and I'm literally not moving (somewhat common with traffic around here), it starts climbing. 230* is my shut down temp, but I usually reach my destination well before it gets that hot.

Archer

 

Check the thermostat
Check that the lower hose is not collapsing
Check the fan clutch
Have the radiator cleaned

 

Thanks you guys for the answers, much appreciated!

Were these cars behaving like this even when new? Genuinely curious.

 

of course you know that the higher temps helps keep water out of the oil. why do i talk about water in the oil? all or most acids are water based. and you make all most a gal of water for each gal of gas you burn. as long as it doesnt go past the rings your ok.

 

Miles -

Pretty much!

Still have fond memories of having warm up an engine before driving on a cold day, and getting caught in traffic on a hot day. No computers to sense and control things - just eyes and feet.

Great days for hot rodding; for plain old driving - not so much LOL

Archer