overheating, what can I rule out?

1988, 5.7 L mercruiser with alpha 1 gen1. 140 deg thermostat.<br /><br />New exhaust manifolds, risers, and ciruclation pump last season. <br /><br />Over the winter new water hose at bellhousing and new impeller. The old impeller was fully intact.<br /><br />Upon first start up of season when idling on muffs the coolant tempature needle rose quickly and steadily and I shut it off right before 200 to prevent any damage. I had confirmed a steady flow of water from exhaust ports. While slightly warmer than supplied water, the exhaust water was never even luke warm. The exhaust risers never felt hot. The engine block felt very hot.<br /><br />My first assumption is that this is a thermostat issue. However, given the recent repairs to the impeller and water hose, I wonder if that could be related. Can rule that out as unlikely though, because I saw such a steady stream coming from the exhaust? <br /><br /> I've also heard that air trapped in the circulation pump could cause this problem. Is thisreally likely and could this have happened when I undid one of the hoses to add antifreeze last fall? If this is true, how can I bleed the air from the system? <br /><br />While I am waiting for a new thermostat, will it do any harm if I remove the thermostat completely to test if the overheat problem still exists?<br /><br />Thank you,<br /><br />nat

Re: overheating, what can I rule out?

It won't hurt to do a quick test on the muffs with the stat out, but don't run it that way on the water, the water will circulate too fast near the sender and show very low temps, but the back of the engine will not get the proper circulation and you can have hot spots without the stat.<br /><br />Trapped air is normally not a problem with a raw water cooled engine. But if you are concerned about it, just make sure the front of the engine is higher than the back of the engine and the air will bleed out. The air gets trapped in the back of the block.

Re: overheating, what can I rule out?

water wiil always be passed because it cools the rubber exaust sections. dosent mean that water is actually getting in the block.was the block prefilled or was it left dry.therm can be corroded closed. was the therm. installed in the correct location.

Re: overheating, what can I rule out?

it shifts into gear? I'd just want to make sure the w/p is turning....

Re: overheating, what can I rule out?

Thanks for all the respones. To answer your questions:<br /><br />The block was drained, and I poured antifreeze into the line.<br /><br />The engine shifts into gear, yes.<br /><br />I am confused about water always being passed because it cooles the rubber exaust sections. Doesn't the water impeller pump water right through the water tube, into the powesteering cooler and on to the thermostat? If I am seeing water coming out of the two holes in the gimbal at a good rate, doesn't this mean that water is definitely being supplied up to the thermostat housing at least? And wouldn't that mean that this is not a problem in any way with the water pump?

Re: overheating, what can I rule out?

to the housing and right out the exaust<br />if the therm. is open or closed.dont take this the wrong way: therm installed right side up?should be spring toward engine

Re: overheating, what can I rule out?

Water comes in from the outdrive, to the power steering cooler, then to the thermostat housing. From there the TS housing always allows water to go to the exhaust even with the thermostat closed. So no problems of overheating the exhaust tubes because of the stat not working. While the water from the pump is putting water into the exhaust, it is also putting a positive pressure on the hose going to the engine circulation pump. which circulates the water though the engine "When" the thermostat is open. When the thermostat is open, it allows cold water from the drive to circulate through the engine, warm up and then exit into the water going out the exhaust.<br />Does all that make any sense???

Re: overheating, what can I rule out?

Thanks Don. That makes perfect sense and it is how I envisioned the system working from my cooling diagram in my shop manual. So then I am correct in assuming that this is not a problem with the water pump in the stern drive not suppling enough cooling water, but rather a thermostat issue most likely, or at least something downstream of that point? This assumption is based on the fact that I see plenty of water coming out of the exhaust and that it is not very warm when the engine is overheating.<br /><br />I will definitely test out the thermostat by removing it from the system. Just trying to plan where to look next if that doesn't fix my problem and understand completely how everything works and you've all been a great help in that. Thanks again.