Question about engines on hot days. Is this normal?

[72 Challenger]

Hi Guys,

this question is about the engine in my 1970 Duster and driving on hot days.

First the engine:
It's a date coded 70 block with a 416 stroker kit. 160 degree t-stat, factory 22 inch rad, aluminum intake, eddy carb, Challenger T/A heads and stock manifolds. The water pump is a stock style, only 1400 miles on it but 20 years old. Coolant is new from when I redid the heater core this past spring. No leaks. Rad was re-done when the car was restored 20 years ago but only driven 1400 miles since.

Today I drove 35 km's in hot weather 33C or 92F on the highway. The gauge normally sits at half. However it's abnormally hot here. I cruised the car at my normal 75mph and the temp started to climb to about three quarters. Slowing down to 60mph the temp stabilized. The drive on the highway was only 25 miles. Is there something wrong or is this normal as a result of the temperatures? or have I got something else causing me issues?

the car otherwise drives mint.

[dodj]

Well, that 160* t-stat isn't doing your engine any favours but not the source of your heat issue unless it isn't opening. Cruising at 75 usually is not when people report increasing temps. A shroud would make no difference at 75mph. I suspect you have a coolant flow problem. Either rad blockage or pump output is down.

[YellowThumper]

Well, that 160* t-stat isn't doing your engine any favours but not the source of your heat issue unless it isn't opening. Cruising at 75 usually is not when people report increasing temps. A shroud would make no difference at 75mph. I suspect you have a coolant flow problem. Either rad blockage or pump output is down.

Agree with Dodj on all accounts. Old means it was sitting. Was it sitting with coolant? Or dry?
Flow definitely sounds like the issue.
T stat is not issue except what was noted above. It is a simple change if desired. Go with 180 though. T stats regulate flow to control temps. They are not just open and shut valves. If open too early and flow exceeds radiators ability to cool water. Then overheating will happen. T stat info stated is more for lower speeds.  Overheating at speed you stated refer to Dodj statement.
Food for thought... improper timing can cause overheating as well. What is your all in advance?

[7212Mopar]

Was the coolant changed while the car sat for 20 years? Corrosion protection property of the coolant will be lost in a few years. When I took my 340 apart a few years ago after removing the freeze plugs, I found a lot of sediments inside the block. I vacuumed, scraped, flushed the block before it was hot tanked by my engine builder. Anything inside the block including scale build up will diminish heat transfer and reduce coolant flow. Also, get a temp gauge and you don't need to mount it permanently so you get a better temperature reading. You can't tell with the factory gauge.

[blown motor]

I'm no expert but a 22" rad sounds small for 412 ci engine. At 75 mph you're burning a lot of fuel and creating a lot of heat. I know that your forward motion will help force air through the rad but you need enough rad to handle the amount of heat being produced.

[usraptor]

Agree with everything said above.  Additionally, what rear end gears are you running?  To low a gear can also cause issues at that speed. I know it's not a Mopar, but I once owned a '64 Vette with a Hi-Po 350 transplant, close ratio M22 4-speed and 4:11 gears.  It would idle and drive around town all day at normal temp, even on 90 degree+ days.  However, once I got it to freeway speeds the temp gauge climbed up past 3/4s and sometimes even hotter.  This was in the days before electric fans, etc. so I never really solved the issue.  Just didn't drive it on the freeway on hot days.  Otherwise, if you're not running tall gears I'd look at everything mentioned above.  You either have an air or coolant flow issue.
For further comparison sake, my current '70 'Cuda has a 440 stroked to 512 CI with fairly hot hydraulic cam, aluminum heads, headers, etc with a hi flow aluminum water pump and stock fan, stock 26" original radiator, 4-speed and 4:10 dana and even on 90 degree+ days the temp never gets past half, idling, surface streets or freeway.  Good luck and let us know what you find. 

[bennydodge]

What's the compression ratio? Higher compression will need colder spark plugs for sustained high speed operation. My 416 has 11.25 to 1 comp. ratio and will run a little warmer if I sustain 75-80 mph down the interstate-it won't overheat but will get hotter. I'm running the factory 22" radiator and it has never overheated.

[IMNCARN82]

On the bottom of the water pump shaft housing is a weep hole.  Look and see if there's any drip or residue.  That can be a sign if the pump bearings are going. 

[72 Challenger]

Thanks guys! glad we didn't read "Duster" and go to a new topic!

Compression ration is 10.25:1
Rear gear is 3.55 Sure grip
The car sat with coolant in it and it was driven throughout the 20 years, just sub 100 miles per year.

I am not sure what the timing is but it starts hot, cold, has instant and great throttle response, zero backfires or pops in the exhaust.

I will check the weep hole.

As soon as I get off the highway it cools down and I can drive it in traffic all day. I am starting to think there is merrit to the 160 degree t-stat alowing water to move too quickly for the rad. Runs like a champ in stop and go traffic.

[HP2]

Old rule of thumb was overheating in stop and go is an airflow problem, overheating at speed is a coolant flow problem.

Does lower hose have a spring in it? This spring is to prevent hose collapse, which can occur at higher engine speeds due to the pump spinning faster, which cavitates it within  in the coolant, collapses the hose, and pinches off flow.  Do you have an ac or non-ac water pump? Non ac pumps have fewer blades on the impeller for moving coolant. Of course there is the possibility of plugged tubes in the radiator but if it was redone, I wouldn't consider this an issue. Finally, we have pulley ratios. Often times engines get assembled with whatever pulleys are lying around and line up without consideration of the ratio between them. The size of the crank and water pump pulleys directly impact coolant flow. If you're spinning them too fast,  you may be decreasing the heat exchange within the system.

[IMNCARN82]

Good call on the spring in the hose HP.  I was also thinking there's air in there.  You can't put the tstat in and fill up the rad. Gotta be full at the manifold. Then put it together.   

[bennydodge]

Good call on the spring in the hose HP.  I was also thinking there's air in there.  You can't put the tstat in and fill up the rad. Gotta be full at the manifold. Then put it together.

??? I've R&R'ed countless small block manifolds without ever filling the manifold first. No problems either. I'm betting the OP's car is just fine BUT if you want to run long, high speed highway operation a larger radiator and colder plugs will help stabilize the temperature.

[Oldschool]

All great info. I had issues with my Cuda on very hot days. Just like you, I was cool around town, but on the highway under a constant pull, the temps crept up. I was running a Meziere electric water pump with stock BB Mopar flow rate. 42-43 GPM IIRC.  I switched to the high output pump which is 55 GPM and now it stays cool no matter what. Coolant flow was my issue. 0.02.....

[HP2]

I usually drill a 1/16 hole in the large, flat, flange part of the t-stat to avoid the trapped air situation. But, typically an air pocket will work its way out the next time the t-stat opens.  That's one of the reasons why its important to use an overflow tank instead of just a drain hose.

[72 Challenger]

I have been working all day and haven't even looked at the Duster but I can assure you when I replaced the heater core I also addressed a very slow leak at the thermostat housing. Since the T-stat was out I filled the block from there. It's been driven over a hundred km's since. I am confident the issue is not air. Also 100% the t-stat is the right way. I checked and doubled checked.

I will squeeze the lower hose and check for a spring. I hope there is one in there. The previous owner put one in the top hose so I am hoping he did the same in the lower. He wasn't a slouch with cars so most everything on the car is quality parts. The engine was built by professional machine shop as well.


So far when I have a chance I have to check for a weep from the snout of the water pump, a spring in the lower hose, there is also a chance that the temp gauge is not reading accurately. I don't have another temp gauge to throw on it though.


the car is non A/C and has the original pump pulley. He owned the car from 1970- till I bought it in 2019. I know this means nothing but he was originality obsessed. I will make sure to try and measure it but I am not sure what the original size was?