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Discussion Starter #1
Don’t mean to post another one of these overheating topics but I’m at my wits end so here goes...

After chasing a stuck in traffic overheating issue and finally finding a fix, I now have one when driving at highway speeds.

For the past few years my stock 65 with a 289 has not done very well in LA traffic. After countless attempts to remedy this (taken and tried just about every suggestion under the sun from this forum) I bit the bullet and got an electric fan, and problem solved! Well no not exactly... while the car does great in stop and go traffic now with the AC on, it over heats on the highway at speeds of 65+. I don’t understand why, I thought at that speed airflow isn’t an issue? Hoses are about a year old and the lower has a spring. I thought maybe the radiator was clogged so I removed flushed, did the thermostat flush, and added a filter. Still over heats on the freeway. so I thought maybe it has a clog that can’t be dislodged, so I get a new copper radiator with 13lb cap and install that today. Runs the same on the streets, super cool, but overheats on freeway after 15 min or so. Only three more things come to mind... thermostat, water pump or deep clog in the block itself.

What do you guys think?
 

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Is your fan part of a shroud? Is it always on? Is it a pusher or puller fan. A fan that is always running can block the air flow throgh the radiator at high speed especially if it's a pusher fan.
 

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Additional possibilities:

1. Not enough spark advance.
2. Trapped air pocket.
3. Electric fan is restricting airflow at high speed.
4. Exhaust restriction.
5. Excessively lean fuel mixture.
6. Too much valve preload.
 

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I have had that symptom before. And it had all new parts except water pump. Replaced the non leaking pump with a high flow model from edelbrock and the car runs cool in all conditions. Increased water flow was the change that cured highway speed overheating for me.
 

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Discussion Starter #5
Thanks for the quick replies:

I got the electric fan from champion radiator for the American eagle rad I had on before today. It’s a puller that goes on from the inside of the engine bay. I was able to easily mount it to the new copper rad i put in today and like I mentioned, basically the same performance.. runs great in stop and go traffic but over heats on the highway.

The relay is set to come on at 185F and off at 175... basically it’s always on once it’s starts.

Just to clarify, before switching over to the electric fan, the car ran awesome on the high way and while moving. It just couldn’t idle for more than 20 min in traffic before overheating. One of the things I did before switching to the electric fan about 8 months ago, is take it to a specialty mustang place to check the timing and the carb with my over heating symptoms in mind. Didn’t make much of a difference because i ended up making the switch anyway.
 

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I'd change the settings to have the fan come on at 220* and off at 210*. There's no need to try and cool an engine that isn't running hot and 185* ain't hot.
 

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Stock 289 would have come with a 2-row radiator. A 3-row would work wonders. Also, I don't believe the 65's had a shroud... most I've seen either don't have one or if they do it's been added.
If your radiator has open spaces on either side, there are plates available (or you can make some) that prevent air from going around the radiator rather than through it.
Anything you can do to push air through the radiator instead of around it will help... and 3 rows help cool the coolant faster than 2 rows.
 

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Stock 289 would have come with a 2-row radiator. A 3-row would work wonders. Also, I don't believe the 65's had a shroud... most I've seen either don't have one or if they do it's been added.
If your radiator has open spaces on either side, there are plates available (or you can make some) that prevent air from going around the radiator rather than through it.
Anything you can do to push air through the radiator instead of around it will help... and 3 rows help cool the coolant faster than 2 rows.
Non-A/C cars had a 2-row with 16 or 17 fins per inch.
A/C cars had a 3-row with 11 fins per inch, a shroud, and a fan clutch. They also had a "Hot Idle Compensator" to increase idle speed when underhood temperatures were hot.
 

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My 68 would overheat at highway speeds. Turned out to timing was too advanced.
 

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It seems that from the OPs description that the fan is the only changed denominator. My guess is that since it never turns off at speed its creating an air dam. As mentioned before. Set it to come on at a higher temperature. Mine comes on around 205 and goes off around 200. That way when I drive it shuts off and free spin.
 

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I have a very similar setup, with exactly the same experience. I'm fairly sure that the electric fan is causing my issues. It's just not able to move enough air at speed. I also have a shroud, and I've thought about modifying it so the corners weren't blocked when at speed.
 

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Spring missing in bottom radiator hose? It stops the hose from compressing under the suction from the water pump at highway speeds.
He states he has hoses on there that are about a year old and the lower hose has the spring. Still wouldn't be a bad idea to give the hose a squeeze to make sure it's still solid.
 

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I have a very similar setup, with exactly the same experience. I'm fairly sure that the electric fan is causing my issues. It's just not able to move enough air at speed. I also have a shroud, and I've thought about modifying it so the corners weren't blocked when at speed.
The Fan shouldn’t have to move air at speed. The airflow from driving is more than the fan could ever move. At speed, the fan can just get in the way an slow the air flow.

Victor, good call on the spring in the lower hose. That’s an obvious potential issue.
 
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I would try burping the cooling system. With the engine cold remove the radiator cap and run the car with the heater on until you don't see any air coming out of the radiator.
 

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OP states that before the electric fan he issues with overheating at idle or in traffic but no issues at speed. That is an airflow issue. The only thing he changed was going from mechanical to electric fan. Why is everyone making it harder than it is? If the thermostat and hoses were fine before they are still fine now. He also stated that his fan turns on at 185 and off at 175 and that essentially his fan never turns of. That means that his electric fan instead free spinning at speed is actually creating an air dam. Blocking effective air flow at speed. Why not start off at the most recent change in his setup, the electric fan, instead of going down other pathways. @xela.zue I would either adjust the on off temperature of the fan or if you can't do that but have a place that you can drive your car at speed for a little bit without having to stop in traffic you can disconnect from the power source and drive the car to see that if at speed and the electric fan just free spinning your car still over heats. Just watch your temps and be prepared to stop and connect the fan back up. If the temperature still rises at speed then I would look at other causes.
 

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My buddy had a problem on his 66 with an electric fan that was wired backwards. Instead of pulling air as intended, it was pushing air from the engine bay back through the radiator out of the grille. After trying to diagnose the issue of running hot at speed he noticed this standing in front of the car in the garage while getting blasted with hot air.
 
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