90 Mercruiser Overheats - bogs under load

[bairdo3]

I have a 1990 Mercruiser 3.0 with Aplha Gen 1 outdrive. Going on 2nd season with it overheating, bogging and losing power. So far Ive replaced water pump(impeller), new gear oil, new thermostat, removed hoses for any potential clogs, changed oil filter/oil, new spark plugs, added a fuel/water seperator with an additional inline fuel filter, rebuilt carb last year,(but seems to be a mess agian), check fuel pump (new filter), distributor cap/wires were new when i bought the boat last year. Runs great on muffs, have even put lower unit in bucket of water and check to see if it was sucking water and in fact was sucking quite a bit in. Ive taken riser elbow off and only found a piece of metal in outer part. Checked vent hose for clogs. Cant seem to find anything else wrong, and running out of things to check without taking apart manifolds and larger components. Ive done a compression test a few weeks ago which came back great. Lately, the carbs been making a puffing noise while cutting power but seems to fade when i pump the throttle, think previous owner jacked up carb so may get a new one. Anyway, would love some advice on any recommendations as ive search and read almost every post on here regarding this topic, but nothing specific to my situation.

 

[tpenfield]

does it have an oil cooler?

have you checked the water inlet on the outdrive and the inlet hose that leads from the outdrive to the transom assembly?

 

[Scott Danforth]

have you checked timing?

 

[bairdo3]

Ive visually inspected hoses. Those 2 you mentioned i have not removed. The water feed hose on the lower unit looks slightly kink but not enough to restrict water flow..and i have not checked timing, not quite sure how to do so.

 

[bairdo3]

And dont think i have an oil cooler.

 

[kbsunlovers]

What was this piece of metal you found? Could you have a burned-out flapper stuck in the downtube causing an exhaust restriction?

 

[nateo]

You have two issues, overheating and not running good. Start with the overheating problem. When on muffs does a fair amount of water come out of exhaust as it should? i.e. is it circulating water properly? If not then you can remove outdrive, stick hose in water inlet hole and test engine. This is going to be a process of elimination (without replacing parts just yet)

 

[bairdo3]

Piece of metal was about 1.5 in by 1 in. or so...not sure what it is. And ive removed inlet hose by thermostat while having lower unit in water. Water gushes in great. Also alot of water seems to discharge although it only discharges from right side of lower unit. Left side just spits water and exhaust. Is that normal? I should mention when it overheats, motor wont start at all and usually end up draining battery trying. Ive had to get towed in twice because motor got REALLY hot. Carb was steaming and block gets hot...

 

[nola mike]

Piece of metal was about 1.5 in by 1 in. or so...not sure what it is. And ive removed inlet hose by thermostat while having lower unit in water. Water gushes in great. Also alot of water seems to discharge although it only discharges from right side of lower unit. Left side just spits water and exhaust. Is that normal? I should mention when it overheats, motor wont start at all and usually end up draining battery trying. Ive had to get towed in twice because motor got REALLY hot. Carb was steaming and block gets hot...

Yes, the water through one side of the relief port is normal. Nothing else about that is. Might have screwed something else up--sounds like some significant overheating. On my engine, first thought would be head gasket. Not sure how hard it is to blow with an overheat on a 3.0. Compression test? Agree with starting running the motor directly to the intake hose. If it runs ok, you could have a problem in the outdrive. You could have melted the water jacket cover, tube, etc. That metal could be a flapper remnant, but wouldn't be in the "outer part" (water jacket? the flapper would end up in the exhaust stream)

 

[bairdo3]

Did a compressiom test a few weeks ago...came back 120 on all 4 cylinders..ive noticed lately that the carb choke stays open ALL the time. I know this is a problem but could it cause a motor to overheat. Or maybe timing is off??

 

[nola mike]

Choke staying closed could cause the bogging. Open you might just start hard. Make sure the choke is actually stuck open and not flopping around. Overheating would be unrelated.

 

[bairdo3]

Thought so. I can manually close the choke however it remains open..

 

[bairdo3]

I just recently noticed at the very top of motor, where oil goes in..(behind thermostat housing/above spark plugs) there were 5 bolts that were very loose. One was even half way out. What could this cause?

 

[nateo]

I just recently noticed at the very top of motor, where oil goes in..(behind thermostat housing/above spark plugs) there were 5 bolts that were very loose. One was even half way out. What could this cause?

no way of knowing for sure but boats and boat motors have lots of vibrations. Often why timing goes out of wack is because vibrations loosen up bolt on distributor just a wee bit.

 

[Starcraft5834]

this an out in left field perhaps.......your compression is good, water flow normal, engine bogging...under load I assume? how heavy is your boat/size?.. any chance you are running to much prop (pitch),, for the engine/its health/and load its pushing with a 3.0?.... reason I say this is when I first obtained our 20ft cuddy, it's got a 1987 3.0 in it, (likely your same engine) my engine health was good,, I purchased (without any knowledge in this area at that time) a 21p 3 blade prop.. the boat bogged/struggled badly. I then reached out to this forum and learned I had to much prop for that load and engine, down to a 15p 4 blade... now I blow out of the water quickly on plane, the engine runs very well with the combination of thrust/load/engine capabilities I have... this is just a stab in the dark, but perhaps something to look at... lower pitch props produce a great deal more low end punch to push you out of the hole and get skipping on the surface...

 

[bairdo3]

Hmmm. May be something to checkout. Its a baylimer capri and its around 20 ft. Has a three blade prop on it now. How can I be sure if its the correct one?

 

[Starcraft5834]

what's the pitch on it? look around the base of the prop, should be stamped there,, bunch of model numbers usually followed by a /# or -#. if it's above 18, ie- 19, 20,21, or more, it could contribute to your problem.. also if the prop is chewed up,, I assuming you have too (like me) a heavy fiberglass older boat, they usually all get heavier with age,, water finds its way into structural foam, etc.. you don't have to have a huge crack or hole in your boat for this to happen, it just happens over time.... the other thing is what was your TAC reading at WOT- wide open throttle in the first year you had the boat? it should be 4800 or there abouts when your running at full speed skipping on plane.... if you never got to that # (4800) or there abouts, that would suggest to much prop (to much pitch), less pitch and go to a 4 blade prop could help

 

[bairdo3]

I just checked propeller and its a P19. And my tac gauge hasnt worked since ive had boat.

 

[Starcraft5834]

Best to get that tac fixed to know what's going on..... short of that, if your bogging and struggling/losing power to get on plane, your engine perhaps is not providing enough power to turn that 19p prop (3 blade I'm guessing?) to push your heavy boat out of the hole and get it skipping on the water... your engine is likely a little "tired" and needs help..If you willing to spend $125, I would recommend a Solas Amita 4 blade in 15p.. that will give you a lot of low end punch and might just be what u need.. I had the same problem, 3.0 with a 19.5 cudy...considered "underpowered" probably true, however...using the right prop can make a big difference, I doubt anyone here will debate the fact of the benefit received by use of a 4 blade prop compared to a 3 blade.. they do make that much of a difference in low end punch to get u on plane, once there your engine wont have to work so hard, the 4 blade is like "low gear" going up hill, in a car the transmission shifts, boat engines don't shift, its all more gas, less gas, turning the prop faster and faster till your engine reached it's horsepower limit... it's a complicated mixture of boat weight, engine power, prop, load...

as to your tac, you should get that fixed... that's the only way to know your engine is operating in the recommended range...

 

[bairdo3]

I know i need to get tac fixed. Not sure whats wrong with it quite yet. And ill consider getting a different prop, although i seem to get on plane fast and easy as long as it doesnt over heat. I Just ordered a new carb cause the previous owner rigged this one to operate but not operate correctly. in the mean time ive been cleaning out cooling system, testing thermostat and changing filters out. I believe that piece of metal i found in riser elbow was constricting 1/3 of the water flow discharge. Hope removing that helps out. Once carb comes in, ill put it all back together and see how it runs. A few weeks ago i started leaking gear oil out tiny drain hole in lower unit. I suspected o ring to have been broken, however when i checked it, it was completely gone. Any chance this would cause overheating or at least contribute to it? Only asking cause the first two times i took it out this year, it didnt overheat at all and this last time it overheated in less than 5 min.