I own a '93 Chevy K1500 pickup with a 5.7L engine. Recently I had the
intake manifold gasket replaced to stop an oil leak. 3 or 4 days after
the work was done I found there was no coolant in the overflow
reservoir but the radiator was still full, I guessed there may have
been air trapped in the system. I called the shop and they wanted to
do a pressure test to be sure there were no leaks. After the test was
done no leaks were found and the mechanic told me that he had pressure
tested the cooling system to 18 psi. This is 3 psi. higher than the
radiator cap and now, 250 miles later, I can smell antifreeze outside
the truck after the engine has warmed up. I can't see a source or hear
a leak but from looking underneath the truck I can see a few drops of
coolant that have collected at the back end of the block,  between it
and the torque converter cover. There is no sign of oil in the coolant
or vice versa. Is 18 psi too high a pressure to use to test the
cooling system and if so, could it have unseated a freeze plug on the
back of the engine block? Does it seem more reasonable that the new
intake manifold gasket is leaking?