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5.0 mpi replacement?

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sbarnett6057

New member
Bought a 2005 crown line with a 5.0 mpi and it is very sluggish. Won't exceed 3000 rpm under load and takes a while to get there. Will barely plane out. I have had it to two different dealers, replaced dist cap, compression is good, no codes, starts and idles fine. No water in the oil, etc. After lots of troubleshooting, the shop has discovered that the engine was replaced with a 5.0 automotive engine at some point. I am told that there are significant differences in cams and other components between the two and they recommend engine swap. I am fairly ignorant on boat stuff. I only know how to fix helicopters. I do have a relative with a fully equipped garage and he is a master ase mechanic on cars. What's my best course of action? If a full engine swap is what I need to do, what's the most economical way to accomplish that? I'm considering stepping up to the 5.7, but will the mpi be compatible? Is the 5.7 pretty much plug and play? Should I stick with another 5.0 mpi? Is there a third option like camshaft swap that would be a good option? I've been looking at some places that have the block with pistons etc but I cannot takes the heads off mine and put them on a marine bottom or I have the same issues, correct? Please educate me on the best and most economical way forward. I really got taken on this deal. It's a nice boat otherwise, new interior and such. Thanks in advance for the advice.
 
Go with a 5.7 litre (350 cu inch) marine short block with heads. Make sure it is for your year boat.

all other parts are transferable to new engine.
305 and 350 all parts such as Intakes, exhaust etc are the same.

What may need to be reviewed is the computer to make sure its for a 350/350 mag vs a 305.

If the motor in it now was swapped and no change from original computer (ECM) if it was a 350 then you may be good.
 
Ayuh,..... Was a compression test done,..?? What are the numbers,..??

I truly doubt the problem is because of an automotive motor swap, as a standard GM crate motor is commonly swapped into boats,...
The "You can't run a car cam" bullship, relates to hot rod cams, not standard run of the mill cams,....
 
bondo said:
Ayuh,..... Was a compression test done,..?? What are the numbers,..??

I truly doubt the problem is because of an automotive motor swap, as a standard GM crate motor is commonly swapped into boats,...
The "You can't run a car cam" bullship, relates to hot rod cams, not standard run of the mill cams,....
Click to expand...
Lowest cylinder was 120, most were 160 to 170. So a normally cammed automotive engine should run fine then?
 
Auto engines are basically the same, BUT, it would depend on what vehicle it came from. If it is a auto engine and it came from a crap car that did not require power then it is not the correct engine for a boat.

Virtually all V6 and Small V8 engines start as truck blocks with cams that are good torque at mid rpms,

A 5.0 or 305 is a wimp engine regardless of what it came From.
They are used as a price control....about $3000-$5000 less than a proper engine for a boat 21ft or larger.

Was the boat originally a 5.0 lt? Or was it a 5.7lt?
 
makomark said:
you think those are good compression numbers?

Do the spark plug insulators have the same appearance?
Click to expand...
I was under the impression that those numbers were OK. I'll confess ignorance here, do you think 120 is the problem? There is no water in the oil, I'd have to pull the plugs, I wasn't there when the compression testing was done.
 
A normal compression test, all cylinders should be within 5-10%.

So if high is 170
Low should be no lower than 150.

If all were 120-140 you may be ok but that says the engine is warn and has a lot of hrs use. Low power but will run ok

120 is to low.
 
I'd agree with Jack as to his final assessment...there is some wear that needs some attention...

Did you ever check the fuel pressure in the rail?
 
sbarnett6057 said:
I am not sure, but I have been reading that that could be the issue as well. I will ask the boat shop that question.
Click to expand...
Ayuh,.... I agree 120 psi is abit low, but not replace the motor low,..... 100 psi is a rebuildable core motor,....

My guess is, it's got a fuel delivery issue going on,....
 
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