I recently needed a new engine on my 68 442 with stock 400 engine.

I put in a freshly rebuilt 455 and didn't change anything with the cooling system.

This engine is overheating (I didn't change anything with the 400 cooling setup).

It only occurs at idle. If I'm moving above 30 mph it will be right where it's supposed to be. If I idle too long it will go all the way to the top of the temperature gauge and start running bad.

It cools down quite quickly once the car is again moving at proper speed.

Any suggestions? Is the radiator too small? Can I try to increase airflow some other way?

Thanks!

 

They should have the same cooling capacity. Is your radiator a 2, 3, or 4 core? What thermostat did you put in the 455? What type of fan are you using?

 

The radiator is probably ok, you need some additional flow at idle!!! I have the same problem! DO you have a shroud and a good clutch fan??

Also make sure your timing is correct.

 

It has a 6-blade fan, direct driven (no clutch). It has a tight shroud.

I was mistaken about it being the original radiator when I went to look at it just now. It's a new aftermarket unit (plastic sides).

Not sure how to assess cores, I didn't install it and can't find out what kind it is now.

Also not sure on the thermostat but when the car is moving it hovers between 1/4 and 1/2 on the temp gauge which was the same with the 400.

Thanks for the tips I'll explore them.

 

Sounds like you have a good radiator, fan and shroud.
One thing to check, is your idle speed correct?, if it is too slow the pump might be struggling to move the coolant through the radiator fast enough, and the fan not moving enough air either. It may be the impeller blades on the water pump have eroded slightly which would have the same effect.

Roger.

 

Is the cooling system getting pressurized check your upper hose it should be hot and firm. Have you checked the weep hole in the bottom of the water pump sometimes you can have a small leak just enough to reduce the system pressure. You might also check all of your freeze plugs for leaks and if someone installed the rubber freeze plugs which can restrict the cooling system especially if used on a head.

 

I'd back-off the timing a couple degrees, might be a compression/fuel kinda thing.
FYI - a fan clutch can be good for up to 50 HP, according to the magazines!

 

before you get too carried away you need to take your radiator cap off when the engine is cold and see how many rows of tubes your radiator has. the points that have been raised are all valid but to me are not as likely to be a factor if the radiator is properly sized for the application.

 

I have the very same setup as you, except no shroud yet. I have and always have had an idle cooling issue! Now with the new motor, I've been exploring different options, electric fans, bigger radiator, etc... for my situation! Our radiator is big enough as it cools at speed, so its an issue of airflow @ idle!

 

Thanks a lot for all the responses, guys!

Best thing is, I don't see anything really expensive in any of those ideas ;-)

 

A friend had same problem (68 with 455) and it was the lower hose collapsing. Swapped to one with spring in it and fixed the problem.

 

Need some more info:
Does it ever BOIL?
Was the new engine bored oversize? Too tight of piston/bore clearance can cause overheating(although usually at all rpms)

Check the ignition timing, both initial, at idle, and rev it to make sure your advance is working, both vacuum and mechanical.A light will show you the advancement.

Are you using factory pulleys, or aftermarket underdrive ones?

Is it the same pump that was on the 400, or a new one? I've seen new pumps with loose impellers slip, thus lowering the flow, esp at idle.

Using a 50/50 mix of coolant?

Try turning the heater onto high at idle to see if that helps drop the temp, if it does, probably means your flow is ok, but verify by looking in the cap hole just after the t-stat opens to make sure coolant is flowing thru ALL the tubes.
May have to go with a different fan for more air flow at idle speed.
Hope this helps

Greg