My 68 Dart is running cool on the back roads, and boiling over on the freeway. the entire cooling system is new. The only thing I have done to it recently is to lean the fuel mixture (backed off 1/2 turn from sputtering) is there a connection?

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My 68 Dart GT 225

If occuring at only high speed would suspect bottom radiator hose being pinched closed from sunction, all bottom hoses should have a spring installed inside hose to prevent this. Last hose I had to replace was missing it. Incorrect timming will also cause warm running but then you would know due to degraded performance and increased fuel consumption. From a personal standpoint have never had incorrect carb adjustment cause a hot running condition, poor running or no start yes.

You have changed the water pump??
If not you will probably find out it is not too good!!

The pump, and everything else in the system is new. I removed and tested the heater core. The lower hose is new, but does not have a spring.
An interesting note: when the engine was hot. the AF boiled out from the overflow tube, on the reservoir tank, but there was no other leakage.

When I cooled the Radiator Fins with cold water, the upper hose collapsed from the pressure change, and did not seem to be drawing fluid back into the system. is this normal?

It does not reach the "normal" range on the guage when cruising at 35-45, but will over heat on the freeway!?!?!? Ive dealt with many overheating cars, and this one has me puzzled!!

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My 68 Dart GT 225

The awnser is no, when you run rich the cylinder temps will be lower, it's when you run LEAN that everything heats up (not good).

I'd check out the cooling system. Overheating at high speeds is totally weird, your rad hoses should not collapse and that'd be my first suspect, even if they "look" good it doesnt mean they are!

Is the antifreeze boiling? Test the temp of the antifreeze when if starts to boil (it should be like 230*F or more before it boils). I suggest just going through everything, maybe flushing out the cooling system and replacing the hoses/thermostat (you can go with an "all season" 180* T-state for extra protection.

Justin

A leaking head gasket will cause it too. You might want to re-torque the head.

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Chuck Rivers, Webmaster

At least on a Holley 1920, the only thing that adjusts the mixture with a screw would be the idle mixture screw. This has practically no effect once you open the throttle. The hose colapsing does not sound normal - either the hose is not strong enough, or I think it could be the valve in the radiator cap sticking, as it's supposed to draw air in through the hose when the thing cools.

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"Mad Scientist" Matt Cramer
'66 Dart - turbocharged 225
My blog - Mad Scientist Matt's Lair

Is your distributor and timing ok. Late ignition causes heating too.

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Sorry, my english is not very good

I am going to vote for a leaking head gskt., too. My '66 Cuda began to run a little hot, so I swapped on a fresh rebuilt head and new head gskt. Overheating disappeared.

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'66 Cuda 225/4spd
'66 Dart GT convertible 225/auto
'64 Dart GT 340/4spd

I can relate to the missing spring in the lower hose, been there done that.
Another thing to look at REAL GOOD is your radiator. My brother's Jeep
did the same thing, esp with warm weather, ran hotter as you went faster. Replaced the rad 2 years ago and has not had trouble since. My dealer buddy had as a side job at home, a caravan that did the same thing, this one had a brand new (within 1 mo.) rad in it that was bought at Garbage Zone, he went thru head gaskets, t-stats, etc, still could not figure it out. Put in another new rad (this time Modine) and all was well again, so watch out for cheap rads.