WHY? Engine temp runs cool in traffic, but overheats on highways?

azmusclecar

azmusclecar

Master Mechanic
Feb 13, 2018
425
209
43
Arizona
#1
I always thought this was the opposite where if the radiator cannot get enough air across it then the coolant temp climbs.
Faster rpms will spin the fan and keep the temp down.
Highway should provide plenty of air across the radiator and engine.

I have an nice 180 degrees in traffic, driving 40 in town.

I take the car out on a road of 55 and the temp climbs to over 200 to 210.

Now I DID replace the water pump with a brand new one and there are two belts driving the water pump.

Any ideas as to why this is happening? Thanks AZ
 
Last edited:
azmusclecar

azmusclecar

Master Mechanic
Feb 13, 2018
425
209
43
Arizona
#3
olds307 and 403

olds307 and 403

Comic Book Super Hero
Oct 14, 2008
4,410
2,177
113
Melville,Saskatchewan
#4
That sounds like adequate air flow and a working fan clutch. Are all the factory deflectors in place? It could be a partially plugged rad or the new, probably stamped impeller water pump isn't moving as much water as the old closed impeller pump. I had overheating on the highway due to a bad Mr Gasket no flow, I mean high flow thermostat. It basically closed off at high speed. Second the rad was too small, especially for towing. After dealing with sometimes 250+ with my 403, 210 was was the hot weather normal cruising temp.
 
azmusclecar

azmusclecar

Master Mechanic
Feb 13, 2018
425
209
43
Arizona
#6
All the factory deflectors are in place.

So if you had to choose between a 40 year old radiator and a bad fan clutch............which one do I choose?:unsure::unsure::unsure:
 
MrSony

MrSony

Comic Book Super Hero
Nov 15, 2014
4,958
3,004
113
Des Moines, Iowa
#7
...both? It'd be under $400 if you do it right. Nice thicccc aluminum rad and a new $30 clutch, if yours is bad.
 
Canon_Mutant

Canon_Mutant

Royal Smart Person
Aug 15, 2015
1,011
1,017
113
#8
If the ONLY thing that has changed since you had good temps at highway speeds is the water pump, that is where the Dick Tracy has to begin.

If not [it appears you've been doing a number of things lately], then we can go from there . . . like new carb, running too lean at low rpm highway speed = excessive heat? etc etc.
 
Huen79H/O

Huen79H/O

Apprentice
Jun 5, 2018
64
138
33
#9
When my fan clutch was bad it only over heated in traffic I would say cooling system not flowing or radiator to small does it have a 3 core radiator or a 2 core
 
jlcustomz

jlcustomz

G-Body Guru
Nov 22, 2011
626
313
63
#10
Had the highway speed only overheating issue on a 10,000 lb gmc panel truck powered by a 350 &th400 years ago. Gradually got worse & higher speeds really pushed it.
Fan clutch fan can make loud whirring fan noises when locked up on acceleration from air movement and can barely be spun by hand.
I drove the LS el camino home 90 miles with electric fan relays burnt, was fine on highway. Not needed.

Swapped the original 4 core for a crappy new plastic aluminum radiator & it made an absolute world of difference. Ran cooler than it did 8 years earlier.
Random examples.
 
Last edited:

Similar threads


Please support GBodyForum Sponsors

Classic Truck Consoles Dixie Restoration Depot Mike's Montes P-S-T Southside Machine Performance UMI Performance

contact [email protected] for info on becoming a sponsor

copyright ©2018 Ripp Technical Services, All Rights Reserved.