water pressure problem 150 vmax

I have a 150 vmax that seems to have developed an intermitant problem of losing water pressure. I recentley had it serviced an impellor replaced, but in taking it out, it works fine and then for no reason (apparently) the psi drops. I can reduce rpmsto idle, then increase and the psi will come back up. Then run fine for awhile and drop. It seems I can tell when there is a problem because the psi will peg higher than normal for a short time before dropping. I use mostly in salt water and thought the sea grass may be clogging, but no other boats on the water seem to have the problem. Any ideas?

[jb123]

sounds like cavitation, are you using an aggressive prop ??

if you havent done it this yr then changeout the waterpump, stats and PRV.

Thanks, I don't think it is prop as that has worked fine the past few years. Have not changed water pump and I am thinking that your suggestion is right. I have never changed it (2001 motor with about 150 hours).

[BQUICK]

I thought you said impeller was replaced?

Yes, I said the impeller was replaced because that's what the service ticket said. I have since called the service shop and they confirmed the water pump was replaced. I didn't realize the whole pump was replaced, I thought you could replace the impeller without the pump. Sorry to mislead. Service shop is stumped, any ideas?

[DMSOUTH]

Sounds like the pressure relief valve spring to me......as the RPM's increase, the prv opens and dumps excess water through the exhaust. Valve or spring could be sticking.