Blown Mercruiser 5.0 Alpha. Low hours. Replacement options
 

Hi All,

I bought my 23 SF late last year and it didn't run great on the test drive. I bought it from my great uncle's best friend (who is very sick) and figured it needed a tune up. Cap, rotor and coil looked bad. 2007 5.0 alpha (carb) with a 2007 alpha one gen2 outdrive. 280 hours on the package. Rewired the boat and tuned it up and found out the compression was bad. 0 on two (opposite) cylinders, 50 on two, 150 on two and 170 on two. I have the option of using a long block and swapping the parts or going all new. I am concerned about the long block idea because I am afraid of what blew the engine so soon. Any advice on a new power plant for my boat?

5.0 mpi
5.7 mpi
5.0 carb
5.7 carb
or this new motor 4.5L https://www.mercurymarine.com/en/us/...specifications

Pretty unhappy with the whole situation but what can you do. I think since I have the opportunity to upgrade I may as well but wanted the opinion of the Seacraft squad.

Thanks

5.7
MPI vs carb is a debate that goes on forever - do what you are most comfortable with.
If you want to go fast look at a 383 stroker; same size & weight but more $ - although it is really on the upper limit for an Alpha drive.
Small block V8 has a lot more parts available than the 4.5 and although a 4.5 is an interesting idea (The 4.5 gets you ~130# weight savings), I would lean towards the 5.7 and not having to push the engine at all.
Lots of marine engine manufacturers (new and rebuilt) out there to choose from so you are not forced into a black motor unless you want it. I have had good luck with Marine Power but look around.

alpha drives are restricted to below 300hp

bravo drives,specifically the bravo I is allways the best choice

with all that:

odds are,the manifolds/risers caused your problem.test running something,and the rpms are not right,nor does the motor"seem right" always think the wost ! "tune ups" rarly cure anything - you need to find out why something isn't right


me,i do not like reman ! I also will not use a long block I like and use new,complete motors only - always the best choice !

my advice:

merc 5.7 MPI motor,new - pull out the old and drop in the new


call this one "experience"...

The bright side; That's a lot less $$$$ than say a new Yam 300 for an outboard 23

I think i would go with a 5.7. parts are allover the place! one thing to keep in mind is the 5.0 and 5.7 outdrive gear ratios are different so if u go 350 you are going to have to get a 1.47 upper for you drive.

Thanks for all of the great advice everyone.

I decided to have them pull the heads and take a look myself at the situation. If the cylinders look good I will go ahead with machining the heads and valve job and see what happens. If the cylinders look good I will have them change the manifolds too. I would rather have a new fuel injected engine in there. Hopefully I can delay that expense for a few years.
They are pulling the heads on Friday.

Good luck! I am rather new to I/Os but I know I was warned about having sufficient height of the elbow in the riser in my 25. My local shop suggests a new flapper in the y pipe, too.

The extension to the riser is not that pricey for a raw water engine. They are a bit dear for a FWC engine.

Not sure if it is as big a deal on the 23 as the 25 as the engine likely isn't as low in the water.

once manifolds are pulled off - it will be apparent if the manifolds/risers caused the problem.

if indeed the manifold/risers caused this condition - the motor was taking in sea water.water in the oil,it may or may not have been present,but,the motor was "burning" seawater,due to valve overlap ...

with all that:

a common problem often encountered,after having the cylinder heads rebuilt - a "valve job"
the motor will have an oil consumption problem.blow by,on the rings will cause this.the seawater in the cylinders causes wear on the rings - just inspecting the cylinders,will not pick this up...if water was in the oil,this causes a problem with the cam bearings,causing a low oil pressure situation ....


lots of "stuff" to all this huh ??

Pulled the heads and there was rust on the two of the cylinder walls and the valves were toasted and mushroomed. Engine is gone. I am still waiting on prices for 5.7 mpi and 5.7 carb. He is recommending the Mercruiser reman units but doesn't know the availability or pricing yet.

Is there a difference on the installation for swapping my carb (alpha) motor with an mpi 5.7? If they can't get it done in time I am debating on doing it myself at my friends shop. I don't want to get into anything too crazy though.
Is the harness the same?
Will the exhaust bolt up?
Do I have to change anything in the outdrive?
Is the fuel pump on the engine?
Is there a return line, which would mean tank modification?

Thanks

My MPI has no return and pump is on the engine. Water separator is on engine, too.

The biggest differences I am aware of to watch out for in a motor swap are the new style rear mount bushing and the new style wiring harness on late model MCM motors:

• Mercruiser eliminated the spiral spring in the rear mount bushings in 2003 and changed the height slightly - that means the height of the fwd mts may change a little. Mercruiser service Bulletin 2003-11.
• I can't remember when they went from the 9 pin round harness connector to the 14 pin connector but there adapters for that.

Thanks for the advice. I currently have the 14 to 9 pin adapter on my motor, which is a 2007. I hope the motor mounts aren't different but I can deal with that when it gets in.

Thanks for this advice! I assume those gears are behind the engine and it is something to replace with the engine out. Is it a pretty simple deal or will I need special tools and skills.

Thanks

Repowered my 23 with a 357 Mercruiser Reman. Was told by rep they used new blocks and bore over to make it a reman. They also add mechanical fuel pump. Bolted right up to transom ass'y. You would have to check the drive ratio to make sure you have a 1.47 set. The gear set is in the Outdrive.
I am very please with the set up. runs like a scalled cat.
If your interest in a 4.5 FWC MPI new left over call Marine liquidaters in Fort Peirce, they had one there in Feb. for 5500.

Thanks Bobbert. The 357 reman is one of the engines I am looking at. It is either that or the 5.7 mpi remanufactured. I guess you had that choice too. How do you like the new motor? Any regrets not going with the MPI?
I have 1.62 gears and have been told that I can re-prop instead of changing them.

Just LOVE my 357. Gets up and goes, very economical. Picked the carb over MPI because we have big Ethanhol issues. Here it can run 5-14% and really does a number on the MPI.
This 357 is a Mercury motor, not a secondary builder. Hot run test, 2 year warranty, etc.

As far as the drive ratio, I would spring the bucks to change to the 1.47 ratio. You could diddle with the prop but it would be a compromise situation.

Buy the way it was a quick Drop in install.

Funny, my mechanic recommends MPI over carb for ethanol reasons.

He said you can run a 25 micron separator with MPI, 10 with carb. Who knows?

Thanks for the feedback Bobbert. After a lot of back and forth I went with the MPI motor although I think I would have been happy with the carb too. Did you use riser extensions on your motor exhaust? I am wondering because we couldn't order extensions for my new engine and it will be about 16.5" above water line I think. My current engine is around 18" and the new one is 1.5" lower on the risers.

While you have the engine out inspect the bottom of the Y pipe (exhaust) very closely where it bolts to the transom housing - the one I removed from my 23 was corroded very badly and the lower bolts had nothing left to attach to.

If remember correctly riser extension not avail because the MPI has a DRY Joint riser, ie water does not flow between manifold and riser. Check spec for riser height above water line. Ditto recommend looking at "Y" pipe attachment to inner ransom.
While your at it REPLACE the flappers in the "Y" pipe.
Good luck, and hold on for a fast ride.

They are available, at least in the catalog- I have priced them for my dry joint MPI. I think another 25 owner has them?

I also have 1.62 gears in my Merc Alpha I.(Tsunami 23) After some experiment with props of many sizes,I ended up going with a 15'-1/2 x 17 Mirage Stainless steel prop.it pretty much works perfect at all rpm ranges.

When I did my restore on the Tsunami,I had planned on going all new with a new engine and new outdrive.Unfortunately reality in the financials hit home,so I went with a donor boat that just went through an engine with a new long block 350 carbed Mercruiser.The guy sold it as the transom was shot,and didn't want to sink any more money into it.It only had 200 hours on it.

I almost hate saying this because I don't want to jinx it,but after 5 full seasons,it's been pretty much trouble free.Personally,I like the carb,(I used a 600cfm Edelbrock marine carb).I know that in an emergency I can rube goldberg the carb if I had a problem,but not so sure on the MPI,i'd probably be out of my league.
Also in the area I fish,virtually all the boats are either older I/O's or inboards,and the one shop I use is an expert on those systems.He also has all the parts on hand and is very reasonable.

Good luck which ever way you decide to go.

Hey BigEasy. Thanks for the feedback and glad to hear your engine worked out so well for you. I ended up going with a 300 hp 357 MPI and it is great. Tons of power, smooth and quiet. I also need to reprop. I just finished the 20 hour break in so now I can figure out what I need. I currently have 1.62 gears and a 14 1/8 x 19 prop. I was at 40mph at 4200 rpm with a lot of throttle to go.

Glad it all worked out for you,I still would love to have something like you went with attached to a Bravo drive,but for now I'll have to stick with what I have.
I also remember being worried about the 1.62 gears,but Father Frank and the guys over at Prop Gods were a big help in prop selection.
In fact when I posted a thread about what size prop to go with,it turned into a fairly long thread about the advantages of those gears.I remember someone mentioning that Mosely preferred them over the 1.50's and other guys who raced did as well.My WOT is 4600 rpms(at least that's what I was told)for the engine I have.A few weeks ago on a flat day with the trim set just right,I was at about 42 mph(gps) at 4500 rpms.
That was with the canvas up which is like a big sail.I never run at wot,I try to keep it around 3200 to 3400 rpms.
If I can find that old thread I post it up just for the info.

That is good to hear about the gears BigEasy. I was told they are weak and I should change them. For now I am going to go with the prop change. 42 mph is fast! With my old 5.0 I was only able to get to 35 at wot. I am happy with the added top speed but like you I will rarely use it. I think the manual says my wot is 5200 rpm.

Took the boat out in the bay here and through the golden gate. Performed fantastic. Couldn't be happier with the ride and the engine. There were five of us fishing in the boat. Dry ride for everyone.