My car heats up at 70 mph
I have a 1966 Thunderbird with the 428 cid engine. The engine received a major overhaul about 25,000 miles ago and it has run very well with no temperature issues...until recently. Lately, if I drive below 70 mph it behaves normally, but as soon as I get above 70mph it begins to slowly increase in temperature and eventually will reach the overheating stage. As soon as I get below 70 mph it drops back to normal, even on a hot day. It even remains normal when driving in city traffic on a hot day.
The radiator has been cleaned, the engine thoroughly flushed, it has a new water pump, new thermostat, new fan clutch, new hoses, etc. Several knowledgeable cooling system people and I are baffled by this situation. I hope you may know of something we have not thought of.
A baffling situation, indeed! One possible cause is a collapsing radiator hose. Sometimes a hose will be fine at lower engine speeds but will collapse at higher speeds, leading to overheating only at those higher speeds. This can happen even with new hoses, which is why OEM radiator hoses sometimes came with coils of steel wire inside. This is more likely to happen with the lower radiator hose, which may collapse at higher engine speeds when the water pump is spinning faster. The faster speed of the water pump can create suction that is strong enough to collapse the lower hose.