overheating problem

[paul edward]

i have 04 150hp mercury ob 2stroke, that over heat's only while driving at slow speed.
does not heat at idle speed nor max speed. water coming out pee hole is hot, and a strong flow,but does not heat while idling.
changed both t'stats and impeller. housing looked good. could it be the poppet valve? also teach don't work need advice please.

 

[joed]

Re: overheating problem

Replace the impellar. It's probably gotten stiff.

 

[Texasmark]

Re: overheating problem

Engine has 2 thermostats and they are set to 143F with the pee usually plumbed out the stat output meaning the stat has to open to get any kind of flow and when you get it it'll be hot.

At higher rpm's (about 2500) there are popoff valves of much larger volume than the tstat port that open for increased water flow at the higher rpm's and there is ram water pressure from the movement of the lower unit through the water forcing water into the impeller and the blades actually lay back and the impeller acts as a centrifugal pump whereas at low speeds it is a displacement pump.

So, as Joed said, the impeller may very well have taken a "set" meaning at least some of the blades stay permanently in the bent position and at low speeds it can't supply the volume of water required to keep the engine cool. The over temp alarm is set to about 195F if you were interested and resets when it cools back to about 175F.

My 2c,
Mark

 

[Faztbullet]

Re: overheating problem

Do you have a water pressure guage and what are the pressures? As posted in earlier posts it could be weak impeller or trash holding poppet valve open a tad. As for ram cooling there is very little "ram" pressure on a stock lower unit as the faster you go the more the water pulls away from it. Thats the reason the early 3.0 had overheat problems and a raised S/S plate was installed over water intake to "hold" water to intake. The newer 3.0L motors have either a 2 or 4 hole lower which has water intakes in the nose of unit and run upwards of 35 psi water pressure and the new BRP Etech has 4 holes in nose and run about 50-60 psi.

 

[ufm82]

Re: overheating problem

My 150 had it all- poppet, t-stats, water pump, etc. All either new or less than two years old. I added a water pressure gauge since I continued to have overheat issues. I found pressures were off in the mid-range. Replaced the water pump (complete kit, upper housing, lower housing, etc.) and POOF, it was fixed. The gauge is what showed me what was wrong. If you've not changed the water pump yet it's giving you advance notice. Do it before you toast the poor thing.

 

[Texasmark]

Re: overheating problem

As for ram cooling there is very little "ram" pressure on a stock lower unit as the faster you go the more the water pulls away from it.

Well man my 90 has the inlets in the front half of the LU and louvers to help guide the water into the water inlets.....have no idea as to what the 300 has. No doubt the aft half of the LU will have the separation problems you mention.

The theory of operation came from Mercury engineers, not me. They designed these things, reported their engineering applications and why and they should know or go to weather forecasting for employment where you can be wrong 50% or more of the time and keep your job.

Mark

 

[Faztbullet]

Re: overheating problem

Well man my 90 has the inlets in the front half of the LU and louvers to help guide the water into the water inlets...

He has a 150 with side inlets not a 90hp "metric' unit, totally different water pump impeller as yours is a volume not a pressure type.

 

[paul edward]

Re: overheating problem

i replaced impeller, the housing looked fine.

 

[paul edward]

Re: overheating problem

what is the purpose of poppet valve?

 

[j_martin]

Re: overheating problem

The poppet opens at a pre-set pressure somewhere around 3000 rpm. If you have a pressure gauge you can see the pressure drop slightly when it does. It provides increased flow through the power head at high RPMS.

That brings up the subject of design. The cooling system is designed to be adequate for normal operation. That includes properly loading the engine. If it's over-propped or overloaded, it'll tend to overheat in mid range operation. The correct propeller allows the engine to reach it's max RPM at WOT. The design also assumes correct engine tune, timing and fuel mixture.

Also, there's more to the housing than just the looks of the stainless shell in it. The end plates, both built in to the top of the housing, and the base plate are critical. Grooves in them allow a lot of water to bypass the compression action of the pump. Also there's O-rings and gaskets in and below the base that are critical. Usually they're not a problem, but they can be.

hope it helps
John

 

[Riddickulous]

Re: overheating problem

I have an '08 Optimax 115 and had some very odd water/overheating issues today. I'm cruising up the Bogue sound at 5100 rpm, sound temperature is 58 degrees and I flipped through the SmartCraft and made a comment to my buddy that it was amazing to me that an engine could run at 93 degrees and that there must be a ton of flow going through it. Not 5 minutes later an alarm goes off, I quickly scroll through and see it is at the 160 degree point of the alarm and before cutting it I happen to glance back and notice a fairly decent flow out the pee hole.

We sit long enough for it to cool down to about 130, crank it and get a reasonable stream and it seems to be cooling down a little initially. We get underway and I was only able to do about 2000 rpm with the temperature going back and forth between 140-150. Pressure as indicated by the smart craft was about 11psi at this point and the stream seemed to be the same as its been all summer (just got the boat in May). We get a few miles down the sound at this near troll speed and it shot up to 160. I'm not sure if I had flow at this point as I was poking around on my phone looking for the normal water pressure. I let it cool again for a bit and this time no flow out the hole but seemed like I could see vapor/exhaust coming out.

After a tow back to the dock and back to the house I hook up the garden hose and water flows out the pee hole and I hear a couple of clicks as if something was opening or closing. Let it run for 10-15 minutes and I crank it up and after cranking it the flow out the pee hole immediately beings to slow as if the passage is going closed and finally it stops. Motor has 206 hrs on it (20 by me this summer) and likely has never had the impeller changed. Any ideas?

 

[CharlieB]

Re: overheating problem

Redickulous,

Copy your posting and start a new thread, you will get much better responses than hijacking someone else's thread.

Install a water pressure gauge and a water pump rebuild kit.

 

[paul edward]

Re: overheating problem

Redickulous,

Copy your posting and start a new thread, you will get much better responses than hijacking someone else's thread.

Install a water pressure gauge and a water pump rebuild kit.

Did not hijack someone's thread, It was my thread, just needed to add additional information.

 

[CharlieB]

Re: overheating problem

???????
It LOOKED like a different poster, different name, kinda hard to have known it is the same person

You might be surprised how easy it is to suck a piece of drifting plastic trash against a drive, cutting off water flow, impeller and housings begin to melt within seconds. Once the operator notices and shuts down the motor the trash drifts away, restart and the motor cools down, yet the water pump cannot flow enough to cool the motor at certain speeds.

Water pumps fail one of two ways, either not enough flow/pressure at idle, or not enough flow/pressure at speed. Both can be caused by a damaged impeller and/or warped housing(s), upper and lower.

Cooling problems at low speeds may also be caused by a stuck by-pass valve (poppett) dumping excess cooling flow at low speeds.

Either will show low pressure IF you have a water pressure gauge installed.

Install a water pressure gauge, carefully inspect the water pump housings to be sure neither is warped.

Testing on the hose can give slightly skewed pressure results as the hose pressure can 'assist' the pump and cause reads to be slightly higher than actual on the water results.

 

[paul edward]

Re: overheating problem

maybe if I told how problem originated,it would help. I cranked boat at home using ear muffs,and
lost water from muffs, pump problem (house pump). didn't notice until heard motor change sound,
about 5--10 minute later. It never cut off,when I switch it off, it was very hot. I let it cool down and cranked it, it ran fine. didn't discover problem until I put it on the water. I'm thinking might be the poppet, it was very hot in that area. Thanks for any additional help.

 

[joed]

Re: overheating problem

What did the old impellar look like? Was it all there? Did it break apart? Could be chunks of old impellar stuck in passages blocking water flow.

 

[M9.9]

Re: overheating problem

Might want to pull the plugs and using a flashlight check inside for fine metal filings sitting on top of a piston and plug. That would be a sign of an overheating condition w/damage that would continue overheating no matter the amount of water-flow through the jackets.

 

[paul edward]

Re: overheating problem

What did the old impellar look like? Was it all there? Did it break apart? Could be chunks of old impellar stuck in passages blocking water flow.

The blades were slightly bent at tip, other than that it looked fine. There are milky white stains that won't
wash off, that ran down from under the cowl. I used the boat some this summer, it worked fine as long as i stayed out of
mid range speed. tach don't work so i don't know what rpm is mid range.

 

[Texasmark]

Re: overheating problem

He has a 150 with side inlets not a 90hp "metric' unit, totally different water pump impeller as yours is a volume not a pressure type.

Faz, since you added this answer, I browsed the sales brochure for these engines and noted that my pickups and those of the V engines are in two different locations. My apologies sir.

Mark

 

[NO BULL]

Re: overheating problem

Hi Paul i did same thing except warning alarm went off,i pulled the water pump housing apart water pump impellor melted,housing all blisted and melted done everything still runs hot on gauge.What fixed your overheating?