Hi, i have an over heating issue with my '68 XR7, it’s a 302 auto.
Here’s were i am
Rad is recent aluminium unit and temps are uniform across it
Hoses are newish and don’t collapse
Fan is standard mechanical with shroud and correct spacer
Engine is rebuilt 2 years and 2 thousand miles ago.
Thermostat opens and closes
coolant is just above rad fins and fresh and clean.
I can drive the car all day and the temp stays 190-195, However if i hit traffic and have to stops the temp creeps up to 220+ but comes back down quickly when moving again. I’ve been having a fiddle today and i’ve discovered that with the hood open the temp levels at 195 and will stay there, close the hood and it goes up quite rapidly.
These cars were always on the edge of overheating while sitting still on a hot day. Two things that helped mine were high flow water pump and thermostat controlled 7 blade fan. But mine can still creep up to 220 sitting still on a 100 deg day. 50/50 antifreeze at 12 psi doesn’t boil until above 250 deg.
I had the same exact issue with my 68. It was frustrating! I had a high flow aluminum water pump. An aluminum 3 core radiator with a flex fan and it made no difference. I simply dumped the mechanical fan. I got an electrical high flow Flex-a-Lite puller fan from Summit. $169.99 Part # FLX-105390 I got the fan control module to go with it. $48.99 Part # FLX-121281 Since I had to yank the radiator out, I replaced the anti-freeze with Zerex 50/50. Part # ZRX-ZXG05RU1 $15.99 each for 2 gallons. One afternoon later, problem solved! It even looks way better without the old fan.
If your timing is slightly retarded it will run hot at idle and low speeds. See if you can set the timing to 10 to 12 BTDC. Also running the idle at about 725 in drive can help speed up the fan.
Good point about timing, and also worth noting that later model years had the dual diaphragm vacuum advance. which retards the timing when the manifold vacuum is high (including at idle). This is an emissions hack in order to burn exhaust gasses more completely, but also causes the engine to run hotter. Mine is disabled.
What are you running for a distributor? If you’re using stock timing specs, don’t. Those are for emissions compliance and may not work well with your current setup. I ran my 302s at 12° ~ 15° base timing, with the total mechanical limited to 35°.
The easiest way to gain more airflow at low speeds is a thermal clutch fan. There was a clutch fan option for '68 302s, it was a 17 1/2" diameter 7 blade unit - C4OZ-8600-D. They’re pricey to buy used, but you can use the Derale 17117 17" unit as a replacement with a Hayden 2710 clutch.
If you have it isntalled, remove the weather seal from the hood and/or cowl. Hot air is not escaping from undee the hood and this seal can hold in the hot air.