Austin Trevor White

Just got my 72 cutlass w/ a 455 running for the first time. Engine wants to overheat. (230 degrees) Took out the thermostat and that works fine as long as I drive 55 or slower, but when I mash it and run 80 it climbs back up to 210+ and won't go down. It's got the thermal fan clutch on it with no shroud, and a new radiator. I'm thinking of just putting a solid (no clutch) fan on it. Am I working in the right direction? It was only 50 degrees outside while driving. Thanks

allyolds68

The first thing I'd do is put a shroud on it


Was it overheating before you put the new radiator in?

oldcutlass

You have a radiator airflow or undersize issue if it's getting that hot while moving. The fan is more for idle temp control. What you may need is a shroud in stead of a fixed fan.


Also incorrect timing can cause an engine to run warm. You need to put a thermostat back in it.

Austin Trevor White

No idea car was in pieces/unfinished project when I got it.

Qwik71442

I would put the correct shroud on then see what it does....

47 Convertible

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Whatever you do don't just start throwing new parts at it hoping to fix. This almost never works. The best thing is a correct diagnosis. That is the right process. Some of these points have been made by others.

Re. Fans:
At speed you shouldn't need a fan. The ram effect of your car moving through the air should provide sufficient air flow on its own. Fans are for moving air when stuck in slow traffic or idling on hot day etc.
To be efficient fans should move the most air at low speed/low rpms when air flow is most needed. Engine-driven fans do just the opposite and move the least air at idle and slow speed and move the most when its least needed at high speeds.

For decades there wasn't any better answer but now electric radiator fans do the right thing and move the most air at low speeds and will shut off when not needed i.e., when coolant temps drop which is usually at high speed. Clutch fans and so-called flex fans were helpful in a limited way but not nearly as good an answer as electric cooling fans. For now though don't focus on the fan or a shroud. You can come back to that after checking out more obvious and easier to check possible causes

Radiator:
Assuming you have the stock radiator in a car originally equipped for a 455
If it climbs back up to 210+ at 80mph without a thermostat, the most likely cause is you are either not flowing enough water or air or both. It could also be timing and carburetion but we will get to that next.

An easy way to check coolant flow is to drain radiator, take off the bottom hose and try to refill. The radiator should drain as fast as a normal residential hose can flow. If it fills up and takes more than a few seconds to empty you have clogged tubes and coolant can't move through the car fast enough. The fix is a radiator shop to clean the tanks and tubes.

If radiator drains as fast as the hose tries to fill it then you may have blocked air flow. You should be able to look through a radiator and easily see what is on the other side. Given enough time, dust, bugs etc. a radiator will lose its ability to let air flow through efficiently. Try an air hose from the back side of the radiator to see if debris comes out. If so you have to clean it up. A radiator shop can advise you but a combination of air from the rear and a water hose with a nozzle to build pressure, also applied from the rear of the radiator, should clean it out. Just be careful not to do it at an angle and bend fins. If a lot of the fins are bent over they will also restrict air flow and you need a 'fin comb' to straighten them.

Engine mis-tuning:
As others have said running the fuel mix too lean and/or retarded ignition timing can make an engine run hot. be sure to check those or have them checked if you don't have the tools or skill set. Not everyone can adjust a carburetor fuel mixture particularly on the high speed circuit. A timing light will check timing and looking at spark plugs will tell if fuel mix is too lean. Pure white insulators with a powdery deposit indicate lean fuel mix but they should be checked right after a period of normal operation.

Austin Trevor White

thanks guys for the help so far. Second question, at an idle I can put a stick in the fan and stop it, is that how it's supposed to be. My only experience with fan clutches is on older semi trucks where if the clutch is on you can't turn the fan without trying to turn the engine.

Fun71

That fan clutch doesn't sound right to me.
I agree with the others that a shroud is essential for proper cooling.

As for your new radiator, what kind is it?
Copper / brass or aluminum?
Two row, three row, or four row?

My experience has been that many direct-replacement radiators don't have the same cooling capacity as the original. They many times have a lesser number of tubes and fewer fins-per-inch than the originals. I have learned to compare specs instead of relying on the listing in the catalog.

Austin Trevor White

It's a four row copper core, bought it at NAPA, they told me it was the HD cooling model.

455man

4 row should be plenty of cooling if it's clean inside and out. Let us know what you figure out.

joe_padavano

First, if the overheating problem is at speed, but not at idle, it is NOT a fan or shroud problem, it's a blockage problem in the radiator. I had exactly this problem on my truck one time. The truck would run fine at low speed, but at highway speeds it overheated. It turned out that the lower third of the radiator was full of sediment. With the truck overheating, I could hold my hand on the lower part of the radiator and it was barely hot. Have the radiator flushed or replace it.

On the clutch fan vs. electric, a good mechanical thermal clutch fan will move more air than most electric fans, plus it's totally passive with one less failure mode to worry about. The only reasons most new cars use electric is 1) it's pretty difficult to put a mechanical fan on a transverse FWD engine, and 2) the ability to control the fan on/off time with the computer buys a small fraction of an MPG on the EPA test cycle.

In any case, an overheating problem at speed is NOT fan-related.

69442C

Are the upper and lower radiator hoses new? And does the lower hose have a spring in it? If the hoses are soft, you could be collapsing the lower hose at speed which will cause a significant flow issue.