sleeper_dave

I took my boat out for the second run of the season yesterday. '91 fountain, 502 mag, stock, rebuilt last year. Mechanical water pressure gauge plumbed into the block. This is the first run with a new circulating water pump.

Yesterday when running WOT, the water pressure gauge was creeping up to the point of maxing out, then dropping to about 20-25 psi, then creeping up again. Water temp stayed steady at 160. When i slowed down to cruising speed (40ish mph) the water pressure stayed lower, in the 20-25 range.

The fluctuation doesn't seem right to me, and the WOT pressure seems high. Is this a problem? Is there some type of other problem that can be causing this, a restriction somewhere perhaps? How do i fix it? Should i just install a pressure relief valve, or should i start hunting around for a problem?

Later, when idling, the water temp shot up, when the light came on (redline gauges) and i noticed, it started going down before i could pull over and shut it down. It went right back to 160 and stayed there. On the ride back, i didn't notice the water pressure fluctuating, but i was in rough water so i wasn't watching as closely. The water pressure was still high, though.

US1 Fountain

iTrader: (1)

Check your T-stat for sticking.

MOP

Can't comment on the pressure issue, Had you been idling for a little while or had you just come off the throttle? The temp rising sharply when coming off the throttle is very common. It usually shoots up to near 200 then slowly goes back to normal, what happen is the water circulation slows way down which lets the top end get up in temp. I was a marine mech for a long time I can not tell you how many brand new boat clients came in freaked out about seeing what you did, we came to call it false overheat. I would take them back out and demonstrate that if they slowed down a just little slower it would not happen and not to worry nothing was wrong with their new toy.

Phil

sleeper_dave

I was idling at the time, out of a channel. It could be a sticking thermostat that caused the temp to spike, i guess. I replaced it last year, but that doesn't mean it's still good.

The water temperature is still very cold so the t-stat opening and closing frequently could be causing the erratic water pressure readings.

I think i'll double check the exhaust to make sure i'm getting good water flow out, then maybe try a pressure relief valve.

zt260

Looks like you might have two problems. Definitely check the thermostat. I would pull it out just to test before I just put a new one in. If you replace it I would use a 140*
The water pressure is the other. 25psi is the max you want at WOT, or your head gaskets could fail. At cruise 10-15psi is fine, and as long as there's water coming out the exhaust pipes at idle your OK. If you can’t reduce the incoming pressure, a pressure relief valve might be necessary. Are you using the drive as the water pick up?

sleeper_dave

yea the drive is the water pickup

sleeper_dave

Well. It seems that the merc water pressure relief valve is damn near $300.

Is there another alternative?

MOP

The Merc valve is out of sight and not adjustable it is preset for 30psi, I did a search awhile back on pressure relief vavles and found a really nice brass and S/S fully adjustable ones for under $100. I emailed them and they felt there would be no problems using their vavles in salt water.

https://straval.com/

Phil

zt260

Teague custom marine has some. It's a brass pressure relief valve that is adjustable from 15-30 psi. I have mine on the sea strainer, and it works perfect. It was under $100
Lots of Marine companies make these things, and the price varies widely.

sleeper_dave

Have you tried the straval valve yet, Phil?

What part number did they recommend?