Mercruiser 350 Temperature Fluctuation

"My father has an Alpha One po

"My father has an Alpha One powered by a 350 Mercruiser. He has a new sterndrive, new manifolds, risers, and thermostat/thermostat housing. The housing looks larger than the original, but I've been told that it is compatible. The new exhaust system was purchased at my suggestion since the motor was running at about 185 after an extended cruise and prior to the R&R of the manifolds and risers. I experienced the 185 degree temperature on the initial cruise after the installation of the new sterndrive.

Unlike the replaced risers, the new ones have a 4 inch exhaust side. The replaced risers had a 3 inch exhaust side. I was told that the 4 inch had less back pressure and might improve horsepower as a result.

With all this new equipment, the motor temperature fluctuates very rapidly from 145 - 210 and then back down to 180 degrees in matter of seconds. I've checked the oil cooler, which appears clear. I've also placed a spring at the end of and inside the raw water intake hose (where it joins the thermostat housing) to ensure against kinking. The intake side of that hose does not appear to be kinked.

Any suggestions would be most appreciated. My dad isn't happy with my advice since the problem has only worsened after much expense.

Thanks,

Art

P.S. This is a raw water cooled motor."

"How about the flappers in the

"How about the flappers in the Y pipe, they can break off and restrict flow that would elevate temps.

Where is the intitial timing?

I would verify water flow, back it into a launch and with engine cold remove the water supply hose at the thermostat housing and point it into the bilge, fire it up and check water flow, if you momentarily rev the engine to say 1500 and try to cover the end of the hose with your hand the pressure should blow your hand off and there should be good flow. I suppose there could be some broken impeller pieces in the system somewhere?

Good luck."

"Listen carefully: 180 degrees

"Listen carefully: 180 degrees is WAY too hot for a raw water cooled engine. It should not see over 150 degrees at any time. Why? Due to a lack of back pressure (there's no radiator cap, as in a closed system) there is nothing to keep water from boiling inside the block--and that will destroy it. (Water boils at 212 F if not under pressure, and 250 at about 15 psi.)


Your problem might simply be that the thermostat is too hot for the application and you're boiling the water in the block. (Note: Steam is a lousy coolant!) That would account for those wierd temperture swings. Put in a 140 F t-stat and see if that helps.

The higher risers will help keep the water out iof the motor, and this isd always good. Any back-pressure loss would be insignificant.

Jeff

PS: The widely held myth that lower temps are required for salt water, raw water cooled applications is untrue. DYI magazine archives has a competent write up on this subject."

"As usual, I'm with Fastje

"As usual, I'm with Fastjeff on this one. Harves has some good ideas too. My first check would be the thermostat. Sounds like it's sticking until it gets really darn hot, then pops open. Pull it out, stick in pot of water on the stove with a thermometer. Heat it up and watch what it does. Like Jeff said, it should be full open at about 140F.
Another check while you have the thermostat out, button it up and run it and see what the temperature does. This will prove or disprove a problem with the thermostat.
One more thought, the temperature indication you have may be the problem. Make sure all the electrical connections are good, and verify what the gage is showing you with a hand held, infra-red thermometer. Those little gadgets are well worth 60-70 bucks at sears... a must have for inboard engineers."

"Thanks to Harves, FastJeff, a

"Thanks to Harves, FastJeff, and Troy. They're all great suggestions and I'm very greatful for your help. I believe we purchased a new thermostat with the housing, but I will check it out as advised.

I'll also do the water flow test advised by Harves.

Thanks

Art"

Let us know how you make out.

Let us know how you make out.

I'll be sure to let you gu

I'll be sure to let you guys know. Thanks again!

Art

"althouth water doesn't bo

"althouth water doesn't boil until 212 degrees at 180 degreed the salt in the salt water water begins to crystalize causing major flow problems, so definately find problem and fix as soon as possible."

"Art, Have you verified that t

"Art, Have you verified that the temp of the motor really changes that much in a few seconds? I suspect your gauge may have an intermitent connection. If you put a ohm meter on the gauge sender(at the thermostat housing), you can watch the resistance change, and just feeling the thermostat housing should tell you if the gauge is working correctly. Also, make sure the thermo housing has a good ground to the block. The gaskets had brass gromets to make a ground connection."

"Just got around to working on

"Just got around to working on the boat. As advised by Troy Martin, I removed the thermostat. The mechanic who did the original work is a blow boat guy, and it appears he goofed. He put the thermostat in upside down. Also, there was no thermostat sleeve in the housing to keep it in place. My guess is that this explains the rapid temp. fluctuation.

I installed a sleeve and a 142 thermostat and now the engine temp. stays @ 142 even after extended operation. However, the risers are still very hot to the touch. Plus, after an hour cruise, the starboard exhaust hose sprung a leak at a seam. I'm fearful that the hot risers are weakening the bellows.

I'm delighted by the consistent engine temp., and I thank all who gave valuable suggestions. There's no way I would have figured it out on my own. Yet, I still need advice about the riser heat.

On this engine, there are no hoses coming from the thermo. housing to the riser, and the elbow has a four inch exhaust. On this engine, the risers get a supply of water from the manifolds.

I read online that when no hoses are flowing to the risers, an unrestricted gasket is used to join elbow with manifold; and a restricted gasket is used when hoses go direct from the thermo. housing to the risers. When the mechanic installed these risers, he used a restricted type gasket - with two smaller holes than the water jacket on the manifold.

I called Boat US and made an inquiry. I was told that the restricted flow gasket is the right gasket and that the full flow gasket is only used when there is a riser extender block installed between the manifold and the riser.

At the end of the day, I find myself confused. I see the logic in using a full flow gasket when the only source of water to the riser is that which flows from the manifold. Can anyone tell me what type of gasket I should use? Is it possible that the wrong gasket is causing the hot risers. Afterall, they've barely been used, and there's no sign of any obstruction.

Thanks again to all of you, and Merry Christmas"