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oprating temp?

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Old 07-14-2013, 07:16 PM
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I have a 1996 xl 150 Merc Mariner. The repair manual explains how to troule shoot and test components of the cooling system but does not say what the engine temp should run at. I just replaced the t-stats, and it says 143 on the package so my guess is they are 143f. Is ths the temp he egine should run at or the temp the t-stats begin to open and if so how high should the temp get?
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Old 07-14-2013, 10:01 PM
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If it gets much over 150-160 I would be concerned. I am not sure most of the gauges are really that accurate tho. They are just indicators, not precision instruments.
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Old 07-15-2013, 07:32 AM
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Is this the EFI version? I had a 1998 175 EFI Mariner and at low speeds it would run more or less in the 150 - 160 range. When at power the temperature would drop considerably to maybe 125 if the water was really cold.

The bypass valve on that motor was different from say a Yam motor. In the Merc/Mariner EFI the bypass valve would open to allow more water to flow through the block, when it was opened, which happened at high RPM's. Thus the cooler operating temperature at power than at idle, which is contrary to conventional thinking.

As I recall the SM provides a test for the thermostat. It is placed into a pot of water and the water heated. At a specific temperature the thermostat is to start opening and then to be fully opened at some temperature.
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Old 07-15-2013, 08:31 AM
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Your max temp will usually be after a 10 minute idle through a no wake and will be withing a few degrees of the stat temp.Throttle up to bring the water pressure above the poppet valve spring rate and the temp will fall to about 120 depending on water temperature.The stats control the low rpm temps and the poppet controls the high rpm temps on a merc.
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Old 07-15-2013, 08:58 AM
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Back in the olden days a 145 temp stick wanted to melt and the 163 wanted to stay chalky. But you can point and shoot for an answer now.
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Old 07-15-2013, 11:57 AM
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Originally Posted by muskamoot View Post
Your max temp will usually be after a 10 minute idle through a no wake and will be withing a few degrees of the stat temp.Throttle up to bring the water pressure above the poppet valve spring rate and the temp will fall to about 120 depending on water temperature.The stats control the low rpm temps and the poppet controls the high rpm temps on a merc.

Good point. I forgot to mention that with my Merc I could see the water pressure fall when the poppet valve opened. Shortly thereafter the temperature would go down.
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Old 07-15-2013, 12:06 PM
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Originally Posted by jethro1 View Post
Good point. I forgot to mention that with my Merc I could see the water pressure fall when the poppet valve opened. Shortly thereafter the temperature would go down.
That's why a pressure gauge and temp gauge are a much needed addition over just the warning alarm.
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Old 07-15-2013, 05:51 PM
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The reason for my original post is that I'm trying to solve a problem that I have created. I was attempting a flow test on the oil pump and in all the details I did not turn on the water to the muffs I had hooked up. really dumb I know. of course the water pump was damaged. The alarm did sound so it got hot enough for that to happen. It ran for about 2 seconds with the alarm on and i shut it down.I turned the flywheel by hand and it was free. I changed the pump with a pump that I had that looked to be in very good shape. when started the engine it starts and sounds like everything is good. but something has changed. The engine temp has changed. at low rpm (idling or trolling) the temp was around 120-130. Only when I powered up and planed out would the engine reach 140 +- I have only had the boat 4 yrs I don't know if the poppet has ever been serviced. Now the temp slowly but steadily climbs up to about 140 and then slowly increases to a little over 160 (on the gauge) and the slowly drops back down to 140 and then starts the cycle over again. and the temp runs between these numbers at all rpms and loads even at an idle. When the boat is in the water I have a good strong flow from the tell tale and you can see it fluctuating with the operation of the t-stats. It seems that there was more water flowing trough the block keeping the engine cooler before at low rpms and under load it always stayed right around 140. my next step i to put a NEW water pump in service the poppet valve and run it with a pressure gauge on the engine, do you think i'm on the right track? maybe the poppet is stuck shut? I appreciate your help.
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Old 07-16-2013, 07:59 AM
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You are on the right track for sure and a new pump may very well solve your problems.Could be a poppet valve problem too,they are routine maintenance items,so replace it.Try it with the new poppet and pump,you might get lucky.The poppet valve kit has a spring calibrated to your motor specs and they are different for various models.May be 6 lbs,may be 12 lbs etc. You should see idle pressure that increases up to about 3000 rpms and then starts to top out as the poppet valve (pressure relief valve) opens and closes.You probably cooked the exhaust plate gaskets under the powerhead when you overheated the motor.They are about $75 for the set of two and if bad, will short circuit some of the cooling water before it has a chance to get to the block.The powerhead has to come off for the gasket change.
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Old 07-16-2013, 08:59 AM
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I really appreciate the help. I really don't want to hear that the power head has to be pulled off, but at this point I really just want the darn thing fixed but do not want to get shafted by the mariner spending three days trying to figure it out. I've heard some horror stories. The short circuit theory sounds very logical. It's very difficult to understand the water circuit by looking at the illustrations in the manual. As I said, before I overheated the engine when I ran it on the muffs, it seemed to take a very long time to reach temp if it did at all. It seemed like there was always a certain amount of cooling water passing through the block and stripping heat away even at idle. when I read about the poppet, I first thought that maybe the poppet was always open a little bit or there has been some leaking by which has been cooling the engine aat low rpms. As I said, now when I start the engine it slowly but steadily increases in temp until the t-stats open but there is not enough water flow through the t-stats themselves to hold the engine at the 143 deg. So the heat builds a little more , openening the t-stats more and allowing enough water through to actually cool the engine down enough to see the risng and flling temps on the gauge on the high side of the t-stat temp, you can also see the changes in flow out the tell tale when this is happening. If the gaskets are damaged and water is short circuiting, is this water that would normally be flowing through the engine all the time?, and the poppet reacts to pressure increases? and the t-stat reacts to temp increases to allow additional water flow through the engine? Is the changing of these gaskets prEtty routine for a good Merc mechanic? Thanks again for your help!
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Old 07-16-2013, 09:35 AM
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Originally Posted by Farfrumbehavin View Post
I really appreciate the help. I really don't want to hear that the power head has to be pulled off, but at this point I really just want the darn thing fixed but do not want to get shafted by the mariner spending three days trying to figure it out. I've heard some horror stories. The short circuit theory sounds very logical. It's very difficult to understand the water circuit by looking at the illustrations in the manual. As I said, before I overheated the engine when I ran it on the muffs, it seemed to take a very long time to reach temp if it did at all. It seemed like there was always a certain amount of cooling water passing through the block and stripping heat away even at idle. when I read about the poppet, I first thought that maybe the poppet was always open a little bit or there has been some leaking by which has been cooling the engine aat low rpms. As I said, now when I start the engine it slowly but steadily increases in temp until the t-stats open but there is not enough water flow through the t-stats themselves to hold the engine at the 143 deg. So the heat builds a little more , openening the t-stats more and allowing enough water through to actually cool the engine down enough to see the risng and flling temps on the gauge on the high side of the t-stat temp, you can also see the changes in flow out the tell tale when this is happening. If the gaskets are damaged and water is short circuiting, is this water that would normally be flowing through the engine all the time?, and the poppet reacts to pressure increases? and the t-stat reacts to temp increases to allow additional water flow through the engine? Is the changing of these gaskets prEtty routine for a good Merc mechanic? Thanks again for your help!
The gaskets in question are on the top and bottom of the exhaust plate under the powerhead.The problem happens when the exhaust plate overheats due to lack of water flow, and the gasket burns out allowing water to leak out of the pressurized side of the system before it gets to the motor.The exhaust plate gets a lot hotter faster than the rest of the motor.Not saying that is your problem,but that would be your next suspect if the waterpump and poppet valve don't cure your issue.The stats control the temp at idle and low speed and the poppet controls temp at high speed.I changed my gaskets myself in my driveway on my 3.0 L V6.Not a real complicated deal,but the lifting was pretty involved.
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Old 07-16-2013, 10:35 AM
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So then it is to say that upon starting the engine there is no water flowing throuigh the block or heads until the thermostats open or the poppet opens. When i have the boat on the water and cruising at 4-5000 rpm the cooling system is acting the same as if it were at slow speed so maybe the poppet valve isn't openining at all and that is because of the short circuiting affect and therefore the lack of pressure needed to open it. It's coming together now, so I will change the parts run and some test. In the meantime do I risk further damage if I use the boat if the water temp doesn't go any higher than it is going now? only until I was able to get the parts and schedule the repair.
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Old 07-16-2013, 10:54 AM
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As soon as you start your motor,the pump is pumping water up to fill the motor block.The water circulates through some passages,but not around the cylinder and heads until the stats open at their set point.When the stats open,they regulate the flow around the cylinders and heads to control the temperature.Excess water is dumped into the exhaust plate.What you are saying about the poppet could be true,but if the poppet were to fail in the closed position,you would see high speed overheat,in the open position,the motor would not come up to temp at low speed.A weak water pump will cause problems because the volume and pressures are not adequate for full range cooling.The gaskets,if bad will cause low system pressures that mimic a weak pump because the water pressure is being lost due to a leak,in effect,short circuiting the flow through the motor.Your manual will have detailed specs on water pressure,poppet valve pressure etc.Adding a good water pressure gauge as well as a temp gauge is well worth the effort,make sure you get the right range of water pressure gauge,because some of the Mercury engines are high volume,low pressure designs the need a 0-15 psi scale to monitor the pressure.
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Old 07-16-2013, 11:04 AM
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Originally Posted by Farfrumbehavin View Post
So then it is to say that upon starting the engine there is no water flowing throuigh the block or heads until the thermostats open or the poppet opens. When i have the boat on the water and cruising at 4-5000 rpm the cooling system is acting the same as if it were at slow speed so maybe the poppet valve isn't openining at all and that is because of the short circuiting affect and therefore the lack of pressure needed to open it. It's coming together now, so I will change the parts run and some test. In the meantime do I risk further damage if I use the boat if the water temp doesn't go any higher than it is going now? only until I was able to get the parts and schedule the repair.
At start up there is a water is pushed up into the block and the heads. There is a small air hole either in the stat or stat gasket to allow water to get up into the jackets. The idea is for the engine to warm at low rpm to allow the combustion chamber to be hot enough to fully burn the fuel. The heat minimizes carbon and other fouling in the chamber.

At low rpm the Diverter (poppet) allows water flow to the back side of the stat to allow them to hold closed so the water pressure from the pump does not force them open. They open and allow flow at a given temp.

When you push the throttle forward you flip the diverter to close the hose that is supplied with water to go to the hose to the back of the stats and open a hose that goes over board. So now there is no pressure on the back of the stat, the water from the pump forces the stat open and it goes through that hose that goes over board.

So you want the engine to run warm (150 ish) at low rpm and cool (120 ish) at high rpm.

Some engines differ, and I am not positive about your exact model but that is the idea.

When you fix it, not only make sure the housings and valves are clear, make sure supply hoses and fittings, and outlet hoses and fittings are free and clear. Most times poppets can be serviced with a cleaning and inspection if you are careful not to damage seals, wipers, or springs. Most are exceedingly simple diverters, because they have a simple job.
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Old 07-16-2013, 12:08 PM
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Be careful in thinking that the way in which one brand/model of motor works is the same way that another brand/model of motor works.

Ain't necessarily so.
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Old 07-16-2013, 12:35 PM
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Originally Posted by robmints View Post
At start up there is a water is pushed up into the block and the heads. There is a small air hole either in the stat or stat gasket to allow water to get up into the jackets. The idea is for the engine to warm at low rpm to allow the combustion chamber to be hot enough to fully burn the fuel. The heat minimizes carbon and other fouling in the chamber.

At low rpm the Diverter (poppet) allows water flow to the back side of the stat to allow them to hold closed so the water pressure from the pump does not force them open. They open and allow flow at a given temp.

When you push the throttle forward you flip the diverter to close the hose that is supplied with water to go to the hose to the back of the stats and open a hose that goes over board. So now there is no pressure on the back of the stat, the water from the pump forces the stat open and it goes through that hose that goes over board.

So you want the engine to run warm (150 ish) at low rpm and cool (120 ish) at high rpm.

Some engines differ, and I am not positive about your exact model but that is the idea.

When you fix it, not only make sure the housings and valves are clear, make sure supply hoses and fittings, and outlet hoses and fittings are free and clear. Most times poppets can be serviced with a cleaning and inspection if you are careful not to damage seals, wipers, or springs. Most are exceedingly simple diverters, because they have a simple job.
OP has a Mercury motor and your description is flat wrong.
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Old 07-16-2013, 01:36 PM
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On this model there is no diverter that I can see. I'm afraid that the problem is with what I I can't see. Well I finally got someone from parts on the phone and ordered a new pump and poppet service kit. I also ordered the exhaust plate gasket just in case so i won't have to wait. I need to get the pressure gauge if I don't have one. I really appreciate all the insight and advice. What i need to do is get the pressure readings but I think we know the answer to that already. If there was not a pressure problem than there would be enough water through the engine to prevent the temp from running that high. If a new pump does not fix it and i have a feeling it won't is there a way to do a pressure test on the block and look or listen for the leak? or do you have to just try taking the power head off and hope you see something?
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Old 07-16-2013, 01:59 PM
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Originally Posted by muskamoot View Post
OP has a Mercury motor and your description is flat wrong.
That's what I say. I don't know about his model. Mercury has different operating perimeters and runs hot all the time? How does a 96 efi Mercury 2 stroke cool?
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Old 07-16-2013, 03:36 PM
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I had a problem with my Mercruiser I/O heating up (305CI, 220HP). It wouldn't go far into the red zone but it would be right on the edge or a little bit into the red zone. Took the old thermostat out and put in a new one, I think it was a 155deg thermostat. Had a hell of time getting the old one out because it was corroded.

Replaced the thermostat and everything looked good for maybe a month and then it started warming up more and more. I first thought it was because the water was getting warmer but when the water starting cooling down it was still doing it. What was weird is it wasn't consistant, sometimes it would heat up and sometimes it wouldn't. Ended up I removed the thermostat totally and the engine runs on the cool half of the temperature guage. The boat also no longer diesel's when I shut it off, that must have had something to do with the engine getting hot.

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Old 07-17-2013, 11:32 AM
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Originally Posted by Farfrumbehavin View Post
On this model there is no diverter that I can see. I'm afraid that the problem is with what I I can't see. Well I finally got someone from parts on the phone and ordered a new pump and poppet service kit. I also ordered the exhaust plate gasket just in case so i won't have to wait. I need to get the pressure gauge if I don't have one. I really appreciate all the insight and advice. What i need to do is get the pressure readings but I think we know the answer to that already. If there was not a pressure problem than there would be enough water through the engine to prevent the temp from running that high. If a new pump does not fix it and i have a feeling it won't is there a way to do a pressure test on the block and look or listen for the leak? or do you have to just try taking the power head off and hope you see something?
You can't pressure test the block as it's an open design.The waterpump and poppet may very well cure your problem.Install them and try it out.If the overheat is still there,pull the powerhead and replace the upper and lower gaskets,clean the exhaust plate holes while you are in there as they tend to get carboned up.Not really a major job,but you need the manual if you are going to do it yourself.
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