US1 Fountain

iTrader: (1)

Possibly the stat is not opening all the way, giving you higher then normal pressure. That also will help keeping the temps up in cold lake water. Stock cast iron exhaust?

birdog

You have a stock Merc system. Adding a valve will just mask a problem, not fix it. There is NO reason for a press. valve on a stock system.....Just tryin to help.

sleeper_dave

The thermostat might not be opening all the way. the water is still cold enough that the thermostat probably doesn't get a chance to open all the way before the motor cools down again.

I took the boat out again today. Cruising at about 3500 RPM, the water pressure fluctuates between 25 and 30 psi. The temp gauge will move up a little, then the water pressure will drop to 25 and the temp will fall, then the pressure climbs to 30 or so and the temp gauge starts to move up again. The thermostat is only open for maybe 5 seconds at a time.

If i open it up, the pressure maxes out right away.

If the thermostat being closed is what is causing the pressure problem, how would i fix it? I can't run without a thermostat, the motor won't get warm enough. What else could be causing the problem? I get water out both exhaust's at idle. Should i be dismantling my exhaust and trying to clean out the coolant passages? (if that's the fix, i'm fine with masking the problem for the summer...)

paradigm shift

Was mentioned earlier stock system. Question I have is this still stock after rebuild? With a recirculation pump or cross-over with by-pass. Water should dump outt the by-pass when thermostat is closed. I have ran both systems and I always had to mess with water pressure and temperature fluctuations when running cross-overs with no recirculation pump.

There is a plastic insert in mine that goes in with thermostat. Not sure if it is turned wrong if it would cause high pressure. I know it can cause temperature problems. Not much help just a thought. If it was exhaust I would think it would run high all the time and possibly overheat from not enough flow.

US1 Fountain

iTrader: (1)

Are you sure your stat is good and/or installed correctly? Sounds kinda like it is cycling. When it is closed/closing....temp will increase as will pressure.
Then when it opens, pressure drops and temp will fall.
What stat housing are you running? the 4 hose with brass tee? Also what exhaust?

sleeper_dave

It's a stock exhaust. It's a quicksilver stat, 140 or 145 degrees, whatever the marina sold me when i told them i had a 1991 502. It was replaced last year.

It's a stock cooling system.

When i first installed the thermostat, i did it wrong, and the pressure did weird things, and the temperature stayed bottomed out on the gauge. Once i got it figured out and replaced that plastic insert, the engine got up to temperature and stayed there. At WOT, i always had 30+ psi of oil pressure.

Last year after the rebuild i didn't have as much water pressure (but i was still seeing over 30 at WOT). In the process of checking my ignition timing i noticed that the circulating pump was dumping water out the bottom at high speeds. And i was also seeing higher temperatures (170-180 vs 150-160 now). Grog from this board was kind enough to send me a take-off water pump (he runs a crossover) for the cost of shipping, and i installed that this spring. In addition, i got all new gauges, cleaned the water pressure fitting on the block, and replaced the water pressure hose to the dash. The gauge is more sensitive and responds quicker now.

Water pressure was always high at WOT, but i never thought of it until i read some threads on this board and found out that too much pressure can cause gasket problems.

I think if i took the thermostat out, the pressure might go down into a reasonable range. But i need to be able to run with the thermostat closed also. Water temperatures are still "damn cold" in this part of the country, and if i were running without a thermostat, the engine would never heat up. Running too cold is almost as bad for an engine as running too hot.

US1 Fountain

iTrader: (1)

If you remove the t-stat, chances are you may have very little pressure and no temp. You have no restriction.
A simple check would be to install a cheap tee into the hose going to the ex. Then if your pressure drops, the problem is in your exhaust. I'm surprised your are having high pressure though with the stock stat housing from '91. The newer ones, '98(?) and up, that don't have the 4 hose setup have can high pressure issues. I think the stat will go in the housing 2 different ways depending on on the stat/ plastic ring order. Might double check the order.
Stats are cheap, just a thought. You have a cycling issue for some reason. I'm not familar with that cold of water running to say if that is the main problem.

Is this your setup and you are using all 4 hoses to the exhaust?
http://www.mercruiserparts.com/Show_...ARD+COOLING%29

birdog

You could drill your T-stat to stop the cycling but..Thats still a band-aid.
There are thousands of those out there running just fine...Something must be wrong somewhere....Sorry i dont know what tho !

sleeper_dave

US1 - yes, that's the setup, except i have different oil / ps coolers. 4 hoses going to the exhaust. The spacer and whatnot are in there the right way, if you really look at how the parts fit together there's only one way for it to go. And i did have it screwed up at first, with no plastic thing, and it did wierd things - wierd pressure and temp readings.

I left the boat in the water Saturday, and took it out again Sunday. The problem doesn't seem to be as bad, but the pressure is still high. At WOT, i wasn't pegging the gauge, but i was seeing between 30 and 35. I wasn't seeing as much fluctuation, either. At cruise, i'm still running at about 25 psi.

The differences this time out:
more passengers - 5 on board vs. 2
longer time idling before going fast
boat sat in water the whole night
water temperature still increasing, but not that much - air temp was approx. the same

I'm wondering if there wasn't some sediment or something in the cooling system somewhere that's just working its way out? I double checked before leaving the dock that there is flow out both exhaust pipes at idle.

When this pressure relief valve shows up, i'll install it and that should help some. But it is wierd that the pressure was so high.

Thanks for all your responses, everyone.

US1 Fountain

iTrader: (1)

My pressure pegs the gauges at WOT also. All stock system. I just never got around to installing relief valves. If/when I do, I'm going to weld a couple of bungs in the silent choice divertor valves on the exit side of the valve and dump the excess water there. Don't want to install thru hull fittings.