Getting hot on the highway with the AC on.

-
Clelan

Clelan

Inferno Red Duster
FABO Gold Member
Joined
May 11, 2006
Messages
2,549
Reaction score
3,593
Location
Martensville Sask. Can.
The patient,
74 Duster
Stock 78 318 with a tiny cam and a 670 Holley on a performer 318/360 intake.
Stock ignition, timing is 20 initial and 32 total.
A-500 OD auto and 4.10 gears.
2500rpm @ 70 mph.
Stock 26" rad that got recored to a 3 core about 3000 miles ago. It's nice and clean!
Underhood AC stuff is from an 80s Dodge truck with an aftermarket cross flow condenser about the same size as the rad.
In car AC stuff is Stock 74 Duster.
It holds steady on 180 which is the thermostat rating around town and on the highway with no AC on.
Yesterday it was 90°f outside and I was taking my son and his buddy to football practice about 30 miles away. By the time I got into city traffic (about 20 mile drive with AC on high) I was tickling the low side of 200°f on the gauge.
I got nervous and shut the AC off after a bit of stop and go in the city with the temp going up to just a hair over 200 on the gauge.
My old 360 (R.I.P.) never got over 180 with all else being the same. Should I even worry about it? I am curious about your opinions on what may be causing this.

Cley
20210601_111215.jpg
 
89on35s said:
The patient,
74 Duster
Stock 78 318 with a tiny cam and a 670 Holley on a performer 318/360 intake.
Stock ignition, timing is 20 initial and 32 total.
A-500 OD auto and 4.10 gears.
2500rpm @ 70 mph.
Stock 26" rad that got recored to a 3 core about 3000 miles ago. It's nice and clean!
Underhood AC stuff is from an 80s Dodge truck with an aftermarket cross flow condenser about the same size as the rad.
In car AC stuff is Stock 74 Duster.
It holds steady on 180 which is the thermostat rating around town and on the highway with no AC on.
Yesterday it was 90°f outside and I was taking my son and his buddy to football practice about 30 miles away. By the time I got into city traffic (about 20 mile drive with AC on high) I was tickling the low side of 200°f on the gauge.
I got nervous and shut the AC off after a bit of stop and go in the city with the temp going up to just a hair over 200 on the gauge.
My old 360 (R.I.P.) never got over 180 with all else being the same. Should I even worry about it? I am curious about your opinions on what may be causing this.

Cley
View attachment 1715746405
Click to expand...

Holy smokes, that's perdy.
 
89on35s said:
My old 360 (R.I.P.) never got over 180 with all else being the same. Should I even worry about it?
Click to expand...
Typically heat at freeway speeds is an airflow problem.

But 20 deg rise on a 90 deg day with AC on might not be a problem.

In the Mojave desert there are stretches of road with a sign that reads "turn off AC next 10 miles" or the like. Sometimes the difference between good and overheated is the AC.

Tell us about your fan and/or shroud
Tell us about your old engine build
 
The newer car engines run 200 to 205 nowdays so around 200 would not scare me. I am not sure on your model year but the early cars will get hotter in traffic if the rubber strip is not there to seal the hood with the core support. With that missing it allows the fan to pull hot air through the radiator from underhood.
 
I worked for GM in the 90's and 2000's and most cooling fans came on at 217* F. Others most Cadillac's came on at 227* F and that is where they like to run. 200* F makes most engines happy.
 
Factory Duster fan shroud with a 5 blade clutch fan from the same mid 80s Era Dodge truck that the AC components came from. The rubber seal between the hood and core support is not there but I do have one I could install. I guess I better check my fan clutch is good too. The fact that it's almost new don't mean much these days!

Cley
 
Doesn’t sound like an issue if it doesn’t keep creeping up. You might consider a high flow thermostat if you don’t have one.
 
Fan clutch is only useful at lower than 45 mph. After that more air is pushed thru the radiator by the vehicle speed than the fan blade can draw.
 
Your radiator might also be plugged up with calcium and lime deposits. I have cleaned out many rads with just white vinegar. Take the rad out drain it good lay it flat on the driveway with the hose ends up, put the cap on and fill it with white vinegar, usually 1 gal will do it. leave it for 2 - 3 hrs drain and flush with garden hose for a while to get everything out. you wont be leave what comes out.
 
1. It might be annoying to you in the cabin but 200 to even 210 is not a problem for the motor.
2. 410 gears, 3000 rpms, unleaded fuel and a cammed up motor is just asking a lot of that motor on a 90 degree day with a 3 row radiator. Adding volume to your system would definitely bring down temps and likely make you feel more comfortable.
3. The Condenser is hurting your temps on the hwy by blocking most of that 60 mph air flow. If you can get some baffles in there to channel more air into the radiator, that might help.
4. Someone else asked about a shroud. That could be an issue but maybe not for the reason you think. A shroud at speed isn't going to help cool the car down at all. No fan or shroud can pull air through your radiator as fast as hwy speeds. So once you are at speed, the fans and shroud are not doing anything....EXCEPT if it is an electric fan shroud setup it could actually be blocking the air flow from escaping the radiator area and increasing temps.
 
I just reread the post and It says you got a new 26" 3 core ?
You should have no problem cooling.
 
One other thing I dont see posted is there are 2 different water pumps the one for A/C has xtra fins. Make sure you have a water pump on it for A/C cars.
 
chryslerfat said:
One other thing I dont see posted is there are 2 different water pumps the one for A/C has xtra fins. Make sure you have a water pump on it for A/C cars.
Click to expand...


The A/C water pump is the smaller less blade pump. this is so the water moves slower through the rad and gives it more time to cool the water. That is why high flow thermostats never help cool. The water goes through the rad to fast and can't be cooled down. Here is a page out of the factory service manual. Read it. Copy it and save it.

Water Pump Info All Mopars.jpg
 
Last edited:
The more holes that you plug between the radiator support and the engine compartment the more you'll get through the radiator at higher speeds!! If you don't have a radiator support to hood seal it doesn't help at all. There are other places that the air can escape through the radiator support. You can also easily build an air dam below the radiator to force more air through the radiator.
I have also found that the closer that the condenser gets to the radiator the more air flow you get. Best case scenario is to have the condenser dead up against the radiator to cut down on turbulence.
 
brian6pac said:
The A/C water pump is the smaller less blade pump. this is so the water moves slower through the rad and gives it more time to cool the water. That is why high flow thermostats never help cool. The water goes through the rad to fast and can't be cooled down. Here is a page out of the factory service manual. Read it.

View attachment 1715746476
Click to expand...
Thanks for the info and correcting me. I was always told opposite in here and was relaying that.
 
Just an obvious FYI. A properly working "cooling system" Like the cars A/C does NOT cool the engine , it removes the heat. it "radiates/removes" the heat from the coolant. That is one of the main reasons that radiators normally have a Black paint applied. Since black absorbs heat it also radiates heat. Think SR-71 Black bird.
A marginally working cooling system will be overwhelmed by the condensers heat transfer while the a/c is operating.
 
89on35s said:
I guess I better check my fan clutch is good too
Click to expand...
If it was heating at idel I would agree but at road speeds the fan is not providing any cooling.

Does the fan diameter fit in the shroud well? Approx 1/2 way in the opening and the tips of the fan close to the shroud?
 
Last edited:
if its any additional reassureance from the '75 Valiant owner's manual.
The times to be concerned is when the termperature keeps rising.

upload_2020-10-16_14-24-41-png-png.png
 
The blades should be deeper into the shroud. The shroud should be containing the blades. Not saying that it is his problem, just an installation issue.
Use a coolant recovery system and test your cap to make sure it holds pressure.
A very common cause of overheating is a faulty radiator cap. 16lbs. Test even if new.
 
Last edited:
Get a good high flow thermostat. The EMP/Robertshaw 180 cured my high speed cooling issues. It is very similar to a factory 1967 thermostat. My theory is that ALL std aftermarket thermostats are undersized. Opening diameter should be about 1 7/16".
 
MoparLeo said:
The blades should be deeper into the shroud. The shroud should be containing the blades.
Click to expand...
It wouldn't make any difference at higher speeds. His problem is not in town.....his problem is on the highway.
 
What I was trying to see is, is the shroud blocking air flow at speed, does not look like it.

I think you are over thinking the temp rise.

Only way you will know is to get out on the open road at speed for an extended time (no stop and go) with ac on them with ac off. And note:

if the temp keeps rising with ac on.
What temp levels out to with ac on and off.
 
You must log in or register to reply here.
-

Back
Top