Bought 1970 Olds 442 w/455 engine and factory M21 4-speed trans about 3 years ago. Car was freshly painted, running but needed finishing/updating. I started with completely redoing interior, updating gauges and new A/C unit. Then when got it road ready or so I thought had repeated over heating problems. After much time, work and money to try and correct I discovered the engine had been bored .060 over and decided I just wasn't gonna make it run cool so opted for engine swap. Bought a rebuilt long block deck honed with heads and my aluminum intake ported and polished, also has mild lift cam. After install with new stock water pump, 160 degree thermostat, new aluminum cross flow radiator, and new dual 2-speed electric fans with shroud the engine holds right at 180 degrees, which I like, but for some reason after driving 10-15 miles the temp suddenly raps the gauge then settles at 200 then within a very short time returns to 180, Later it will suddenly jump to around 250 then ramp back to normal again. I noticed that the 2nd jump happened when I was decelerating quickly from highway speed entering a town with a much lower speed limit. Only have about 250 miles on the engine and every time I take it out it does this, Have 2 temp gauges one mechanical and one electric and they track together. I suspected air in system but would have thought by now it would have worked it out as I have ran heater too to eliminate any pockets of air that could contribute to this event. If anyone has any ideas I would appreciate your input. Be Blessed and have a great day.

 

It is most likely an air pocket. With the engine running and the radiator cap off, I usually jam a water hose with a slow flow into the radiator filler neck when the thermostat opens. I have relatively soft water here in my area.

 

Is the factory fan shroud and fan still there along with the electrics? 60 over bore shouldn't effect the cooling system efficiency...at least not to that extent. Try a 190* T-stat if the trouble persists. Also a cheap or well worn(soft)lower rad hose could be collapsing on you? Step up to a good thick OEM brand name.

 

Really sounds like air trapped- sometimes no amount of driving will remove it-try filling radiator with front of car 15 inches higher than rear and open the highest coolant hose or port in the engine as you fill the radiator.

 

Clear the air from the heater hose once warmed up. Installing a radiator flushing tee in the hose makes this a lot safer to do.