Odd temperature activities

[BKCowGod]

Hi all -

Patient is my '65 Dart slant. I have verified using both an IR thermometer and a resistance box that my gauge is accurate enough to be trusted. Ambient temps have been between 60 and 95. The gauge usually rises to (or lowers to if it's been parked hot) the first line and then stays centered on that line, with the expected minor variations as the thermostat opens and closes.

But every once in a while it rises to the second line and then the variations become more pronounced, making it 2/3 of the way to the third line and then back down to 1/2 way between the 1st and the 2nd before settling again in the middle. I have not seen it hit the third line, but if it makes it to the second line it will not settle back at the first one.

My working theory is a partially stuck thermostat, but it just seems so danged precise about when and how it happens that I thought I would consult the brain trust. I am also aware that none of this is overheating, but I have learned to watch any deviation from the norm, and normally it is pretty rock steady at line 1, even in 100+ temps, idling with the AC blasting.

For visual reference, it is between what I am calling lines 1 and 2 in this pic.

 

[RustyRatRod]

What kind of radiator and what condition?

 

[BKCowGod]

What kind of radiator and what condition?

Forgot to mention. Radiator is new, 3 row aluminum. Fan is new, mechanical. Most recent change before this behavior started was switching from no thermostat to having a 180 degree thermostat, but that change was at least 500 miles before the behavior started.

 

[RustyRatRod]

Maybe you reckon you can give some details? Like what engine and mods if any you know.....all the stuff we need to know?

 

[1WildRT]

Have you verified with the IR thermometer that it's hotter when the gauge says it's hotter? Or just when the temp is normal?

 

[BKCowGod]

Maybe you reckon you can give some details? Like what engine and mods if any you know.....all the stuff we need to know?

Sorry - thought I mentioned that in the initial comment. 225, completely stock other than an electric ignition. No mechanical changes at all in the 500 miles leading up to the first occurrence of the described symptoms.

Have you verified with the IR thermometer that it's hotter when the gauge says it's hotter? Or just when the temp is normal?

Tracked it with IR all the way up to the hottest I had seen, 205 on the IR.

 

[RustyRatRod]

Sorry - thought I mentioned that in the initial comment. 225, completely stockI nirmally other than an electric ignition. No mechanical changes at all in the 500 miles leading up to the first occurrence of the described symptoms.



Tracked it with IR all the way up to the hottest I had seen, 205 on the IR.

I tell you man, the dead best thing I did to mine was put a 160 high flow thermostat in it. I doubt you need a 160 but a high flow 180 won't hurt a thing. I normally recommend a Stewart Components thermostat. But the slant 6 head's thermostat port is not quite big enough for one, so here's what I recommend:
https://www.oreillyauto.com/detail/...-car-1968-plymouth-valiant?q=thermostat&pos=1

 

[BKCowGod]

I tell you man, the dead best thing I did to mine was put a 160 high flow thermostat in it. I doubt you need a 160 but a high flow 180 won't hurt a thing. I normally recommend a Stewart Components thermostat. But the slant 6 head's thermostat port is not quite big enough for one, so here's what I recommend:
https://www.oreillyauto.com/detail/...-car-1968-plymouth-valiant?q=thermostat&pos=1

I think I am running that same thermostat in my stroker motor in my '77 GMC, and that thing never turns on either of its fans. I think I just stuck in the cheapest thermostat I could find, so probably worth experimenting with a new one. That's cheap enough for me.

 

[RustyRatRod]

I think I am running that same thermostat in my stroker motor in my '77 GMC, and that thing never turns on either of its fans. I think I just stuck in the cheapest thermostat I could find, so probably worth experimenting with a new one. That's cheap enough for me.

I would also flush the ENGINE as well. If that's not been done, it's worth doing. And if the engine is really dirty, go ahead and flush the radiator as well, since it is now contaminated. Get it all as clean as can be.

 

[BKCowGod]

I would also flush the ENGINE as well. If that's not been done, it's worth doing. And if the engine is really dirty, go ahead and flush the radiator as well, since it is now contaminated. Get it all as clean as can be.

First thing I did - the entire cooling system resembles Hi-C Ecto Cooler, just the way it should, but I guess you can never flush enough so I may do another run with a reverse flush. That first flush was how I discovered the previous owner blocked off the heater lines at the engine instead of just running a loop.

 

[RustyRatRod]

First thing I did - the entire cooling system resembles Hi-C Ecto Cooler, just the way it should, but I guess you can never flush enough so I may do another run with a reverse flush. That first flush was how I discovered the previous owner blocked off the heater lines at the engine instead of just running a loop.

If it looks clean, then great. But you never know. Some of them have crap built up in nooks and crannies and it can take more than once to get it all out. I've never really had confidence in many flush products, but I've found one that works. Blue Devil radiator flush. You can even drive it around a day or three. Now's the time since there's no chance of freezing, since you have to run it with plain water.

 

[Dana67Dart]

I have noticed a similar behavior with my 67 dart 273.

All stock 2bbl, HP stock copper /brass radiator.

What I notice is on the interstate at about 70mph, the temp will rise above "its" normal place about a needle width above low normal (67 guage has an offset high normal that was corrected in later gauges)

It will get to about 3 to 4 needles width above its normal. Then it will drop with no change in driving or road conditions.

Caveat...

I toasted the temp gauge about a year ago and it sits 1/4 inch below the normal key off position. But it still sweeps and I know where its normal is.

So in my case it could be a bad gauge.


But I got to thinking, what if the lower hose was collapsing then opening back up?

The water pump is unknown age/miles, the rad was rodded out in the last 2 years, thermostat is 4 years old.

I never noticed this behavior before BUT I also never had a reason to look, now with toasted gauge I'm a bit over cautious and am checking it often.

I did do some driveway testing.

At idle I had a 44deg inlet to outlet Delta, the temp rose to 215 and stayed there for 30 minutes on a 90deg day. And the gauge was at the line I now call high but not scary.

I have a GoPro camera and am thinking of putting it under the car to watch the hose as I drive.

 

[jos51700]

If it was me, i would pull the thermostat sand change nothing else, and see what happens.

Then, when you replace it, find one that's not made in China. See if the issue persists

 

[toolmanmike]

If it was me, i would pull the thermostat sand change nothing else, and see what happens.

Then, when you replace it, find one that's not made in China. See if the issue persists

Test it (new one or old one) in a pan of water on the stove with a thermometer.

 

[RustyRatRod]

If it was me, i would pull the thermostat sand change nothing else, and see what happens............find one that's not made in China.

Gosh, good luck with THAT!

I THINK Stewart Components thermostats are made here, I'm not sure.

 

[BKCowGod]

If it was me, i would pull the thermostat sand change nothing else, and see what happens.

Then, when you replace it, find one that's not made in China. See if the issue persists

Gosh, good luck with THAT!

I THINK Stewart Components thermostats are made here, I'm not sure.

MotoRad is made in Israel, Edelbrock is USA. Stant is international and luck of the draw what you end up with. MotoRad makes thermostats for most of the other brands, which is why you end up with Israel for a lot of the house brands and OE labels.

 

[RustyRatRod]

MotoRad is made in Israel, Edelbrock is USA. Stant is international and luck of the draw what you end up with. MotoRad makes thermostats for most of the other brands, which is why you end up with Israel for a lot of the house brands and OE labels.

Really? I had not knowed dat.