I just replaced a set of stock Mercruiser stock manifolds with a set of CMI headers. I also installed a pressure gauge in the top the thermostat housing. At idel I bounce between 0-1 psi and about 3000 I have 2-3 psi. I changed the impeller with a stock Mercruiser impeller and housing about 10 hours hours ago. Is this low? THe reason I have a lot of steam out of the exhaust, The temp rises then the then falls once the t state opens Any help would be appreciated.