Mercury Black Max 150 over heating! little help please :)

[dohcdelsol93]

have a mercury blackmax 150. I belive it's about an 88.

compression 120-130 on all cylinders cold.

new thermostats

Physicaly inspected head gaskets when stats were changed.

inspected water pump and looked in great shape.

runs great with ear muffs/idle

as soon as i give it gas in gear it will run a few min (3-5) then over heat quickly.

The pee turns to steam. Up until it gets hot the pee stream is steady and good.

I'm thinking exhaust divider gasket is gone and letting hot gas pass into the cooling ports.




Second question reguarding this, will a leaky exhaust divider gasket cause the engine to run like crap? It will idle fine and when in netural it will rev but when in gear it wants to die when put under load. I just cleaned all carbs (no varnish/trash in them but cleaned them anyway) New fuel lines, drained/flushed entire fuel system, 93oct fresh non ehtanol fuel in it. All plugs are clean and getting good spark on all cylinders.

 

[j_martin]

Re: Mercury Black Max 150 over heating! little help please

The head gaskets are good for one squish. If you didn't replace them, there's part of your problem.

If you have a water pressure gauge, what does it read at what speeds (on the water, muffs starve it for water) If you don't, get one.

And yes, exhaust divider gaskets are critical. Also an overheat tends to crack the exhaust divider, so it needs an inspection and new gaskets.

hope it helps
John

 

[Unbalanced]

Re: Mercury Black Max 150 over heating! little help please

Might be too lean or overly advanced as well.

 

[nbnewbie]

Re: Mercury Black Max 150 over heating! little help please

some history might help to diagnose how long have you had the 150? has it ever run cool? any noteworthy event before it started overheating? do you know actual head temp? both heads the same temp?

 

[hkeiner]

Re: Mercury Black Max 150 over heating! little help please

If you don't solve the problem following the good advice you already received above, you may want to install a water pressure gauge to get a better understanding of what is going on with your motor. The below links may help you if you decide to install a water pressure gauge.

http://forums.iboats.com/attachment.php?attachmentid=25466&d=1238543923

http://www.diy-boat.com/index.php?option=com_content&task=view&id=151&Itemid=49

 

[sschefer]

Re: Mercury Black Max 150 over heating! little help please

The tell tale stream turning to steam tells me you have air entering the system somewhere. A tight system will flow water too quickly for steam to develop.

I feel you already suspect something like that so now you have to try to find it. The Exhaust divider is an easy place to start. Like j_martin said, if you pulled the heads you need get a new set of head gaskets to eliminate them now.

If you open the exhaust chest and notice clean metal then you're close to the problem. If you don't, button it back up, the problem is elsewhere. I doubt it's there because you would have also seen at least one clean piston top when you pulled the heads. It would have been hard to miss.

One really common place to get a leak started is in the exhaust plate under the motor. Corrosion is usually the culprit and often hard to see and diagnose.

Another spot is the equalizer tube that some motors have, (yours may not have it). It comes off the fitting at the top rear of the engine and runs down to the poppet valve. It's supposed to help get air out of the block but if its cracked it will suck air in and often doesn't leak water or steam out. It creates a sort of air pocket check valve.

A stuck poppet valve that's logged cockeyed can also cause you to suck air.

Just some ideas..

 

[Faztbullet]

Re: Mercury Black Max 150 over heating! little help please

inspected water pump and looked in great shape

If you went to the trouble of dropping unit you should have replaced it, they can look good but still be weak. Bad water pump gaskets or a warped base can cause this..

 

[dohcdelsol93]

Re: Mercury Black Max 150 over heating! little help please

have had the motor well over 10 years in the family between my dad and me. Maybe closer to 15.

Had an issue with the oil pump going out about 10 years ago and over heated.
Now oil/fuel is mixed in tank. Has been going for about 10 years like that. With the drought in the south east (Hartwell) it sat out of the water for a few years(maybe 2years).

Got it back running with new batteries/fresh gas about 4 years ago. Ran great with a little work. Kept burning up coils, finally replaced stator and that took care of the burning up coils issues. Ran for a few more years with no issues.

Last year it started over heating but i never got to work on it/put it back in the water.

This year i had some spare pocket change so i decided to get it back in the water.

I defintately think i have hot exhaust gasses coming into the system somewhere. I do not have temp gauges or a water pressure gauge.

If i can't visually find an issue and new exhaust divider gaskets/head gaskets/water pump/popit valve dont take care of the issue i will definately invest in them and go in a little further.

Removing/replacing head gaskets is a fairly straight forward job...what do i have to do to go about pulling the exhaust exhaust chest. Other than removing the foot and doing the head gaskets/carbs/stats i've not had to do much work to this motor, she's been very reliable for the most part.

 

[Faztbullet]

Re: Mercury Black Max 150 over heating! little help please

Nowhere in any of the information you have posted do you say impeller was changed, Mercury suggests they be changed yearly as when setting they can get a permanet curl to them,I would replace impeller 1st before I tried any other repairs....

 

[dohcdelsol93]

Re: Mercury Black Max 150 over heating! little help please

will be doing the water pump assy probably this weekend so long as the local marine shop has them in stock.

but i would still like to know the best way to get to the exhaust dividers to inspect them as it looks like the foot all the way up to the power head will have to be removed to do this.