I'll try to keep this as brief as possible. I recently had my 351W rebuilt by a reputable builder. He has been very helpful with this problem, but we haven't solved it yet. Since the rebuild, the engine consistently overheats suddenly, from the normal 180 degrees to around 230, then back down. No obvious correlation to operating speed, etc. After each overtemp, the top radiator hose is soft (no pressure) until the system re-builds pressure and the cycle repeats. No external leaks, no overflowing radiator, no water in
oil, and I believe the gauge (Stewart Warner mechanical, if the gauge was bad it could not cause the pressure loss). I have checked and changed the t-stat and radiator cap with no change. Also did a NAPA combustion test, where the blue fluid would change to yellow if there was a compression leak into the coolant, and it passed, no leak. The engine was rebuilt to the same specs, about 400 HP, iron heads, new rotating parts. One difference is the water pump, now a new Edelbrock hi flow vs the OEM pump removed. All other cooling components are the same, and the car has always run cool in the 7 years I have had it. I believe the 230 degree overtemps are pertinent because that is the boiling point of 50/50 coolant mix at ambient pressure. It seems like the system is suddenly losing pressure, which starts local boiling in the combustion chamber area, then circulates to the temp sensor. We suspected air pockets in the cooling system, but the builder added two bleed ports at the rear corners of the intake manifold, and I have bled some air out of them as I fill the block with coolant, and also after running the engine. I have also run the engine with radiator cap off to purge all air. Did a static pressure test with a rented tester, and the system holds 20 psi very well, losing only 1 psi over 20 minutes(as it cooled). My pulleys are the same aftermarket parts, but as I research them I find that they are not sized for optimum cooling. The crank pulley should be larger than the water pump pulley, so the pump is overdriven, to provide good flow in traffic at idle. My crank pulley is 5.5 in. and pump pulley is 6.25 in., (opposite of optimum). But, the engine cooled just fine with these pulleys and the stock OEM pump before. My next move is to try a different pump, because this pump is the only common denominator to the issue. I have had the pump off twice, to check for impeller/shaft slippage(press fit, but feels very tight). Then I'll try a larger crank pulley, changing only one thing at a time. All the OEM pumps seem to be made in China. Any advice welcome, sorry for the long post.