dart overheating
 

My 67 /6 225 was running a little hot the other day when my wife drove it. Did not want it to leave her on the side of the road when she drove it so I replaced water pump, t-stat, flushed system, and bypassed leaking heater core, and installed new belt. Has flex fan (no clutch) and no shroud thanks to the p.o. after doing the above work took the car for a drive five minutes on surface streets was ok. Turned it off to fill it with gas left there drove five more minutes on surface street was still ok. Got on highway drove three miles at 65 and temp went almost to hot. Got off turned around and headed home on surface streets, was right at the hot mark when I got home, used towel on my hand to feel the upper hose, and could feel the water boiling inside. Need advice on where to go from here.

Is the radiator core clean? Can you see crud around the openings through the radiator mouth? Also, do you have an auto tranny? Check the fluid to see if it's discolored. Sometimes a faulty auto trans can cause a radiator to boil over, as the radiator has a auto trans cooler built into the bottom.

Flex fans are crap, sorry to say. If you can find a clutch fan in a junkyard or such, you would be better off using it. For those who recommend flex fans, keep in mind that Chrysler used clutch fans on all hi-po engines for a reason. In tests I read several years ago, an engine with a clutch fan made more power than an engine using a flex fan, and an engine using a clutch fan actually made more power than engines which used an electric fan.

A shroud is important, especially for stop-and-go driving.

A good cooling system flush uses compressed air and water to blow out crud not only from the radiator, but from the coolant passages from the engine block and head. You should find a shop which can perform a good flush for you, you would be surprised at the amount of gunk which comes out.

Lastly, the small, 2-core radiators which came on the old slant 6 cars is not very effective. Luckily, most of the radiators Chrysler made in this period were the same size. The old cars are getting hard to find in wrecking yards, but a radiator out of a big block car, or a truck, or better yet, and old police car, would be best. Police cars come with an accessory tranny fluid cooler, which helps keep things even cooler. Just make sure when shopping around for old radiators that you find one with the inlet and outlet on the correct sides.

just curious, what temp thermostat did you put in it?

metal -

Assuming this is a NEW occurrence: A small correction to the above post. Flex fans are not "crap" and typically have higher flow rates than most, if not all clutch fans, at the expense of A FEW HP, literally A FEW. Nowhere near enough to even notice on a street car. For race applications where you're trying to gain every last ounce of power, then an electric fan is in order.

What you're describing is the exact opposite of the norm, where the car would heat up in street traffic and cool down at higher RPM / MPH. Almost sounds more like an engine heat problem than a cooling problem. Just curious, have you looked at your spark plugs lately. Is it possible that the car is running to LEAN or that there might be a restriction in the exhaust system?

Also, I read what you changed, but the T-stat can still be flaky and restrictions to the water flow in the radiator and engine might still be present.

Just guessing, but when the data doesn't match the theory, time to look elsewhere, so you might be able to increase the cooling efficiency when there's a heating problem that's not being addressed. The 2 row stock radiators aren't great, but were talking about a stock slant 6, right?

Archer

You are using coolant right?

same thing happened with my dart. if it over heats on the highway you can eliminate the fan / shroud right?

anyway i replaced my radiator with one from oreliey's and i was good to go.

Shouldn't be pure coolant, should be a 50/50 mixture at -34 degrees. (give or take 10 degrees)

Are you using a 180 degree T-stat?

And did you check it after you had it up to temp to make sure it didn't need to have more water/coolant added?