Why do marine engine repowers fail?

George Van Parys

Mar 1, 2000
RO Number
Why do marine engine repowers fail?

There are many answers to this question. Actually to many to enumerate in a who book on the subject, as some of the reasons have not even been discovered yet. Additionally, they vary depending on the type of engine, outboard or inboard (I/O). But for purposes of this article we will deal with the two types aforementioned and the major issues.

The single biggest factor!
There is one overriding issue with repowers that affects all types of engines and installations. The type of boat, engine, use, location, etc is meaningless. The single most common reason a repower will fail is very simple. It is because the person doing the repower did not know what caused the first failure or even more common, thought they knew. This over all is the single most important issue overriding all others in a repower situation. Although it happens, a marine engine almost never just wares out. Can it happen, sure, does it happen, its very rare. Marine engines usually fail due to some failure in an engine subsystem or accessory piece of engine hardware. Lets discuss what can happen.

What can happen?
Lets say when you see the failed engine the head gasket is blown out and the piston is damaged. What caused it? Gasket installed incorrectly? Could be, but unlikely if the engine has a lot of hours on it. Overheated? Also could be, the gasket area overheated because of a blockage or failure in the cooling system. Water in fuel? Sure, water in the fuel can combine with the sulfur in the fuel and form a sulfuric acid that can cause different types of damage. Timing off? Sure, but why? Was it not checked? Is it too far advanced or retarded, causing preignition or detonation. Bad spark plug wire? Why not, internal crack in the wire causes a short engine of the spark plug firing time. How about if this is a two stroke? Bad coil? Carburetor too lean? Bad power pack? You get the idea. We are just talking about a blown out gasket and we have a huge number of reasons.

What does this have to do with the repower?
Lets say your guess on why it happened was that the carb was too lean. So you get the carb rebuilt. You put everything back together. 10 hours breaking in the new engine, and it starts to stumble. It won't run right. Something is wrong. You did everything right. What happened? Simple you were not sure what the failure cause was and you guessed wrong. Now you have then exact same failure in a new engine block and are back to where you were before the repower, except a lot of money has been spent for nothing. You thought it was a carb issue and it actually was an ignition issue, or visa versa. It does not matter now, your new engine is ruined. Bottom line, you have to be sure of why it failed the first time.

What if I don't know?
We will assume you are either going to go with a remanufactured long block for your repower or a powerhead in the case of an outboard. If you are going to attempt to rebuild the original engine, you will have your own set of issues and this is not normally the recommended path for a normal repower.
What if you do not know why and can't figure out why do to the extent of the engine damage.
Ok, this is common and happens often. What you have to do is go on the basis that everything outside of the core is suspect. You will need to test each piece of equipment you add to that engine. Lets list them, and some of the potential issues they would cause:

Distributor and all its components
(Worn shafts in a distributor can throw timing off by creating drag or jerking in the rotor allowing for abnormal combustion)

All ignition sensors
(Also can cause a timing shift allowing for the potential for abnormal combustion)

(If the power supplied is not the correct voltage or is not rectified properly the ignitions system can be thrown off allowing for the potential lead to abnormal combustion. Additionally in the case of a Volvo there is an extra diode in the alternator that directly affects the ignition system and can cause the engine not to shut off)

Wiring harness
(Any short in the harness can have affects similar to those above)

Spark plug wires
(Can cause intermittent firing allowing for gas to build up in a cylinder, then suddenly igniting creating an

Manifolds and Risers
(Can cause overheating and burn out gaskets or piston scoring due to expansion, or lead to abnormal combustion)

Water pumps (raw water and circulating)
(See manifolds and risers)

All hoses
(See manifolds and risers)

All heat exchangers
(See manifolds and risers)

Carburetor or EFI systems
(Can cause a lean or rich condition leading to abnormal combustion)

All fuel supply lines, filters, tanks, and fuel
(See carburetor)

Several times above we mentioned things that can lead to abnormal combustion. Lets briefly discuss that. Abnormal combustion is usually defined as any condition that can produce preignition or detonation. Pre-ignition occurs when the combustion process starts before the designated time needed for peak performance. When the fuel vapors ignite premature, the molecule expansion is trying to push the piston down when the piston is on its way up. This creates a shock wave onto the piston crown and forms excessive heat. The heat created is so intense the piston starts to melt (weaken) eventually leading to major engine damage. Detonation is when the temperature and or/pressure in the combustion chamber is above normal, after the spark plug fires, a portion of unburned mixture will violently explode before the wave like front passes through it. This violent explosion colliding with the flame front is called spark knock.

Ok, you say in the case of an I/O you have this figured out. You're going to go for the “big bucks" and drop in a bobtail engine. Basically all assembled ready to go. You just bolt it down and your set to go. Do you limit your exposure this way? At an extreme cost yes, are you safe? NO. Again, it's a question of auxiliary equipment yet again. Many of the same list, aforementioned.

Problem with fuel in a gas tank
Collapsed fuel feed lines
Bad water pump in an outdrive
Defective boat wiring harness

Basically the list starts all over again. I remember one situation with a repower issue where everything seemed perfect, but the water flow to the engine was restricted as it passed thru a section of a transom assembly that had internal corrosion therefore causing the new engine to overheat, just as the old one did.

With so many issues, how can you win in a repower situation?
There are lots of very successful repowers. We do not mean to spread gloom and doom. What we are saying is you cannot over check your engine systems. Check everything, then check it again, there is no such thing as being too careful. Don't be afraid to ask questions, and to ask to have things double-checked. When you get the repower in go-slow. Don't assume everything is ready to go. Go slow, listen carefully. Anything that does not seam right or sounds wrong or feels wrong, probably is wrong. Have all issues addressed immediately. Do not wait or you will be back in the same situation all over again. Remember to follow the procedure your mechanic or rebuilder gives you for new engine break in. Do not skip steps, do not rush it, follow it to the letter. If they tell you to change the oil after 20 hours do it, if they say to re-torque the heads after the first run do it. I have seen two failed repowers this summer for each of those two reasons just mentioned. These are not made up problems, they really happen, and the new engines really fail. You will feel that failure to check these issues in lost time and lost money.