88 Searay 300 twin MPI 5.0s

nickzlb77

Seaman
Joined
Sep 10, 2010
Messages
58
Hey everyone,

Picked up my 300 with twin MPI 5.0s/Alphas. One motor was completely broken down to the short (starboard) and other in working order. Tuned up the port motor, replaced T-Stat, raw water impeller, and all hoses. Runs @ around 180 deg @ 1100 RPM. Anytime I push it over 1800 (8-10mph) begins to overheat. In neutral can run at high RPM without any overheating issues. Assuming it's just too much to move for the little guy but curious to hear other opinions.

Also outfitting a 305 HO auto motor for donor shortblock. Will MPI top half bolt up accordingly?

Thanks for your time and any humor that will probably be added at my expense :)
 

HT32BSX115

Supreme Mariner
Joined
Dec 8, 2005
Messages
10,083
Re: 88 Searay 300 twin MPI 5.0s

Hey everyone,

Picked up my 300 with twin MPI 5.0s/Alphas. One motor was completely broken down to the short (starboard) and other in working order. Tuned up the port motor, replaced T-Stat, raw water impeller, and all hoses. Runs @ around 180 deg @ 1100 RPM. Anytime I push it over 1800 (8-10mph) begins to overheat. In neutral can run at high RPM without any overheating issues. Assuming it's just too much to move for the little guy but curious to hear other opinions.

Also outfitting a 305 HO auto motor for donor shortblock. Will MPI top half bolt up accordingly?

Thanks for your time and any humor that will probably be added at my expense :)

Howdy,


Overheating issues (when it happens at higher power settings) are usually due to low flow of cooling water.

Notice I said higher power, NOT higher RPM. High RPM unloaded (in neutral) is not synonymous with high power since the engine is not really working.

If you have a clogged raw water circuit anyplace from the raw water pump to the manifolds and/or risers it can restrict flow and cause an overheat.

It may not overheat at idle or medium power at all.

Look to oil/power steering coolers, sea-strainers, t-stat housing, clogged (with rust) manifolds and risers.

If it's a salt water boat, and the manifolds and risers are severely clogged, the block, heads and circulating pump may be similarly clogged with the same rust.


Regards,


Rick



Using a 5.0L automotive engine might work ok but only if you use the marine camshaft and do some other things (like brass core-plugs etc) The automotive cam will not be suitable at all.

If it's a flat-tappet engine, you should replace the cam and followers at a set with the correct Mercruiser (or equiv) MPI cam and followers etc.
 

nickzlb77

Seaman
Joined
Sep 10, 2010
Messages
58
Re: 88 Searay 300 twin MPI 5.0s

Howdy,


Overheating issues (when it happens at higher power settings) are usually due to low flow of cooling water.

Notice I said higher power, NOT higher RPM. High RPM unloaded (in neutral) is not synonymous with high power since the engine is not really working.

If you have a clogged raw water circuit anyplace from the raw water pump to the manifolds and/or risers it can restrict flow and cause an overheat.

It may not overheat at idle or medium power at all.

Look to oil/power steering coolers, sea-strainers, t-stat housing, clogged (with rust) manifolds and risers.

If it's a salt water boat, and the manifolds and risers are severely clogged, the block, heads and circulating pump may be similarly clogged with the same rust.


Regards,


Rick



Using a 5.0L automotive engine might work ok but only if you use the marine camshaft and do some other things (like brass core-plugs etc) The automotive cam will not be suitable at all.

If it's a flat-tappet engine, you should replace the cam and followers at a set with the correct Mercruiser (or equiv) MPI cam and followers etc.


Hey thanks for the reply. As far as the cam you mean to tell me I cant spin this thing to 8k? :joyous:
Just finished the plugs gaskets cam lifter swap etc. Summit actually has a grind comparable to original Spec.

Regarding the overheating on the one "good motor". So far Ive only checked hoses, strainers, t-stat housing and replaced impeller. Housing looked pretty bad so I don't doubt that I may find some serious restrictions at the other locations mentioned.

I guess my only question is (assuming a completely intact cooling system and correct prop) would simply running a boat this size @ about half throttle on the single port motor ever cause it to overheat (this was an explanation given to me by marine mechanic close by).
 

HT32BSX115

Supreme Mariner
Joined
Dec 8, 2005
Messages
10,083
Re: 88 Searay 300 twin MPI 5.0s

Hey thanks for the reply. As far as the cam you mean to tell me I cant spin this thing to 8k? :joyous:
Just finished the plugs gaskets cam lifter swap etc. Summit actually has a grind comparable to original Spec.

Regarding the overheating on the one "good motor". So far Ive only checked hoses, strainers, t-stat housing and replaced impeller. Housing looked pretty bad so I don't doubt that I may find some serious restrictions at the other locations mentioned.

Yeah.....8K might be problematic!!:rolleyes:

I guess my only question is (assuming a completely intact cooling system and correct prop) would simply running a boat this size @ about half throttle on the single port motor ever cause it to overheat (this was an explanation given to me by marine mechanic close by).

If the cooling system is flowing properly, NO power setting should cause an overheat! Methinks the marine "mechanic" is mistaken!

You're not flowing enough water to cool the engine, the t-stat is stuck, the circulating pump (rotor) is defective, corroded away etc... OR the manifolds/risers are so clogged with rust as to restrict flow overboard.


The engines should turn rated RPM at WOT. (for those engines, around 4500-5000) Your manual should indicate the max RPM. If the RPM at WOT is significantly lower than the bottom of the recommended range, you'll be 'lugging' the engines and could cause damage eventually.

Ideally, they should turn *close* to the MAX RPM for the best performance.

Is this a raw water cooled boat? Salt?
 
Last edited:

nickzlb77

Seaman
Joined
Sep 10, 2010
Messages
58
Re: 88 Searay 300 twin MPI 5.0s

Yeah.....8K might be problematic!!:rolleyes:



If the cooling system is flowing properly, NO power setting should cause an overheat! Methinks the marine "mechanic" is mistaken!

You're not flowing enough water to cool the engine, the t-stat is stuck, the circulating pump (rotor) is defective, corroded away etc... OR the manifolds/risers are so clogged with rust as to restrict flow overboard.


The engines should turn rated RPM at WOT. (for those engines, around 4500-5000) Your manual should indicate the max RPM. If the RPM at WOT is significantly lower than the bottom of the recommended range, you'll be 'lugging' the engines and could cause damage eventually.

Ideally, they should turn *close* to the MAX RPM for the best performance.

Is this a raw water cooled boat? Salt?


Thanks, sometimes other opinions do a lot for the sanity because I couldn't wrap my head around his theory. Raw water, salt. And considering the motors were installed together I think its a safe bet that whatever destroyed the starboard motor occurred on at least some level to the port based on what I saw in the housing. It actually gets up and goes surprisingly well for a boat that size on the one and will reach close to max rpm. Unfortunately it will overheat almost immediately at that RPM. From the beginning a flow issue is the only one that made sense to me.

Thanks again :)
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,465
Re: 88 Searay 300 twin MPI 5.0s

Thanks, sometimes other opinions do a lot for the sanity because I couldn't wrap my head around his theory. Raw water, salt. And considering the motors were installed together I think its a safe bet that whatever destroyed the starboard motor occurred on at least some level to the port based on what I saw in the housing. It actually gets up and goes surprisingly well for a boat that size on the one and will reach close to max rpm. Unfortunately it will overheat almost immediately at that RPM. From the beginning a flow issue is the only one that made sense to me.

Thanks again :)

Ayuh,.... I agree with Rick,...

The manuals up in Don'S Adults Only section have flow charts to help ya understand,...

All ya can do is check different points in the system to see where yer gettin' Good flow, 'n where ya ain't,...
 

HT32BSX115

Supreme Mariner
Joined
Dec 8, 2005
Messages
10,083
Re: 88 Searay 300 twin MPI 5.0s

Thanks, sometimes other opinions do a lot for the sanity because I couldn't wrap my head around his theory. Raw water, salt. And considering the motors were installed together I think its a safe bet that whatever destroyed the starboard motor occurred on at least some level to the port based on what I saw in the housing. It actually gets up and goes surprisingly well for a boat that size on the one and will reach close to max rpm. Unfortunately it will overheat almost immediately at that RPM. From the beginning a flow issue is the only one that made sense to me.

Thanks again :)

Oh boy..........If your manifolds, blocks and heads are clogged with enough rust to cause overheating.......


It spells repower. New engines. (or rebuilt with NEW or NON-rusted used blocks/heads)
 

nickzlb77

Seaman
Joined
Sep 10, 2010
Messages
58
Re: 88 Searay 300 twin MPI 5.0s

Does have that repower ring to it doesnt it?... If all goes well with the starboard installation going to get going on the port immediately. No shortage of good 305 longblocks around here. Honestly I got the boat for next to nothing, rewired bow to stern, reupholstered, and upgraded all lights and electronics. The fuel injection setups and drives are in good shape so if I have to swap both long blocks its not the end of the world.

Thanks again!
 

HT32BSX115

Supreme Mariner
Joined
Dec 8, 2005
Messages
10,083
Re: 88 Searay 300 twin MPI 5.0s

Does have that repower ring to it doesnt it?... If all goes well with the starboard installation going to get going on the port immediately. No shortage of good 305 longblocks around here. Honestly I got the boat for next to nothing, rewired bow to stern, reupholstered, and upgraded all lights and electronics. The fuel injection setups and drives are in good shape so if I have to swap both long blocks its not the end of the world.

Thanks again!

After you get both engines done with new manifolds, it might be a good idea to consider closed cooling.

If I didn't say it before, (except for the risers, it completely mitigates the problem permanently.
 
Top