Hello everyone new here and new to the slant six world I just got my car it has a 225 in it all stock runs great the only issue is it over heats when I get on the freeway it has a brand new radiator has new thermostat fresh coolant and the water pump seems to work fine it also has a 16psi radiator cap I have a new water pump but haven't put it on yet and I have read that the timing can cause it to over heat so I was wondering what the timing should be set to its a 72 plymouth valiant 225 stock with 124000 and what should the air fuel screw be set at any help would be great I want to drive my car home and not ride my bike every day

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Old but hopefully a goodie

It can be also caused by a lean mixture.

Did you flush the coolant when you changed it with a gardening hose?
There could be gunk blocking the cooling passageways.

Does the bottom radiator hose have the spring inside to prevent it from collapsing?

Have you checked if the water pump works?

Open the radiator cap when the car is cold and start her up.
Rev the engine and check if the coolant level goes down.

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I would make sure that the thermostat opens fully at its rated temperature and that you installed it correctly.

Cooling System

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1965 Plymouth Barracuda,
225 engine, Quadrajet, HEI, Dutra Duals, 904 Torqueflite, 2.76:1 axle, Addco front bar
Rods & Relics - Fort Erie, ON / Collector Car Tech

To test the thermostat, get the car hot and then squeeze the upper radiator hose.. If it feels stiff, the thermostat is open. But if it feels mushy and squishy, then it's empty of coolant and the thermostat is shut closed.

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I flushed system with simple green and a garden hose until the water ran clear hoses get hard bottom hose does not have a spring but it doesn't collapse how do I set the air fuel mixture and do you know what initial timing is

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Old but hopefully a goodie

It will also sit and idle for ever and never over heat only when I get on the freeway or start driving it

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Old but hopefully a goodie

Timing should be set to 5-10* BTDC normally. Start at 5 and work your way up. Don't go too far or else it'll start to ping. Check TDC and make sure the timing mark is actually at TDC and not off a bit.

Verify that the thermostat is in working order. I usually place mine in hot water on the stove before putting it in the car to make sure it opens. What temperature stat did you buy?

I got one from oriely didn't ask what temp or anything like that but is a 16psi radiator cap to much the old radiator I took out had a 7psi should I go back to that one

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Old but hopefully a goodie

What temp thermostat should I run and should I use the 16psi radiator cap or the stock 7psi

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Old but hopefully a goodie

Use a 195 degree thermostat and the stock radiator cap. Overheating at highway speeds suggests either a coolant flow or air flow problem. Make sure the water pump/fan belt is not slipping and the fan is installed correctly (less than 1 inch of space between fan and radiator and fan blades oriented so the fan is pulling air through the radiator).

So I should use the 7psi radiator cap and the fan is in the right orientation but it's almost 2 inches off the radiator but it's in its stock location, the belt doesn't slip and it seems to work just fine at idle or street driving only on the freeway is it an issue

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Old but hopefully a goodie

I would be second-guessing my own work. I'd start by removing the thermostat and checking that I didn't get it installed backward. There are instructions stamped into it. Also check that I didn't get too much sealant on stuff and have some sealant break off and jam the thermostat. After that.... follow all these other folks' good advice.

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"Louise", a 1976 Dart Custom project, (now sadly reverted to being just an "organ donor" to our other project Darts.)

16 psi cap is standard. Higher pressurization of cooling system will elevate boiling temperature of coolant, also 50/50 mix antifreeze will elevate boiling point of system.

So if you are using straight water and a low pressure cap it is very likely you have local areas on surface of water jacket boiling. Also corroded water jacket surface will insulate or prevent efficient heat transfer between metal surfaces to coolant preventing coolant from being able to carry the waste heat to radiator where it is dumped to atmosphere.

How do you know engine is over heating? Have you checked coolant temperature with hand held laser temperature reader?

Spring side of thermostat sets into head, if installed backwards it won't open because its heat activated spring is not in coolant, and engine will over heat.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

Thermastat is installed correctly and I'm using a 50/50 mix

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Old but hopefully a goodie

If you have verified that the thermostat is correct temp and oriented correctly, I would look at volume of coolant flow (water pump functionality/efficiency) and the radiator (functionality/efficiency).

Are there any clogged water jackets in the radiator?

The fan location was an excellent suggestion. There are a few spacers of various thicknesses that when matched with the appropriate length bolts will move the fan closer to the radiator.

Is there a heater core installed in the system? You can use the heater to marginally boost the cooling as you drive. I have to assume you are watching the factory installed dash gauge to measure temp.

Please post what you find.