7.4 Mercruiser Engine Life Expectancy

[Bondo]

Ayuh,...... It'll run til it don't,......

Once knew a charter captain out on the northeast corner of lake Ontario, that had a 7.4l with 5,000 hours, 'n near perfect compression,....

Most boat motors don't die,..... they're Killed,.....

 

[Lou C]

Need more info, is it closed cooled or raw water cooled, salt use or fresh, has the exhaust been maintained, etc. If closed cooled and the exhaust has been maintained you've got a lot more life in that engine assuming compression is good and no head gasket leaks.
Even if raw water cooled in salt, I have over 20 years on this block (my use and the previous owner, before that it was in fresh water), I replaced the heads 4 years ago but that was due to a previous overheat (cracks) not rust though. Since I've had it the exhaust has been changed on a 5-7 year schedule, I'm on my 3rd exhaust system in 19 years of seasons.

 

[Scott Danforth]

I started this thread back in 2006 I did end up buying a single engine cruiser with 7.4 MPI Mercruiser. Its approaching 700 hours. Manifolds, risers, flappers all replaced. Oil pressure still in spec etc.....again I ask in a single engine 28.5' cruiser how much longer will it last.

with proper maintenance, about another 4000-5000 hours, or at that rate you are going (29.2 hours per year) about 172 years (seriously, the norm is 50 hours per year). use your boat more.

however parts may not be available in another 30 years, and truth is most boat motors are murdered from lack of maintenance or parts become NLA.

 

[Rick Stephens]

If you take a pickup like mine, which is a 2000 with 7.4L, it has 140,000 miles on it and I am thinking about doing its first plug change. 140,000 miles is usually around 25 MPH average - which is 5600 hours. Engine still has no leaks and runs same as it did when new. It got parked most winters before it got turned into a welding truck for my business. If maintained, you have loads of life left usually.

That said, I have seen iron piston rings wear out in some motors fairly early. Compression tests tell that story easily.

 

[msd58]

Need more info, is it closed cooled or raw water cooled, salt use or fresh, has the exhaust been maintained, etc. If closed cooled and the exhaust has been maintained you've got a lot more life in that engine assuming compression is good and no head gasket leaks.
Even if raw water cooled in salt, I have over 20 years on this block (my use and the previous owner, before that it was in fresh water), I replaced the heads 4 years ago but that was due to a previous overheat (cracks) not rust though. Since I've had it the exhaust has been changed on a 5-7 year schedule, I'm on my 3rd exhaust system in 19 years of seasons.

Engine is raw water cooled. Used most of its life in fresh water and in blackish water. Risers and elbows changed in 2012 and manifolds done in 2017. Exhaust flappers replaced in 2021 as preventative measure along with oil lines. Oil changed once per season. Plan on doing compression check before putting boat away for the season. Motor does have some valve train noise. Performance of this mpi motor seems fine with no power loss over the years.

Now - I have been told that since this boat is single engine express 28.5' cruiser that the motor works much harder than lets say a 33' express with twin 7.4s thus my concern. .....On long trip I typically run at 3650 to 3700 rpm at 26 - 27 mph maybe be less depending on water conditions ect. I hope the this is enough info

 

[msd58]

Engine is raw water cooled. Used most of its life in fresh water and in blackish water. Risers and elbows changed in 2012 and manifolds done in 2017. Exhaust flappers replaced in 2021 as preventative measure along with oil lines. Oil changed once per season. Plan on doing compression check before putting boat away for the season. Motor does have some valve train noise. Performance of this mpi motor seems fine with no power loss over the years.

Now - I have been told that since this boat is single engine express 28.5' cruiser that the motor works much harder than lets say a 33' express with twin 7.4s thus my concern. .....On long trip I typically run at 3650 to 3700 rpm at 26 - 27 mph maybe be less depending on water conditions ect. I hope the this is enough info

It's now 2024. The engine has close to 800 hours on it. Oil pressure good, same as it always was. Compression 140-145 psi. Last time compression was tested at 325 hours it 160-175 psi cold.

 

[Scott Danforth]

Take it out and run it hard. Numbers should come back up. However your gauge could be off too

 

[msd58]

Take it out and run it hard. Numbers should come back up. However your gauge could be off too

The first set of numbers were cold. Second set I don't know if it was hot or cold when mechanic took them.

 

[cyclops222]

Moderator Edit: Rule violation of rules #1 & #4.

 

[pursuit2460]

I have a 2000 5.7 FWC w/800 hours. Most of it life was fresh water then it came down to S.FLA. Dont know how much time it was down here but it was a single owner boat and well taken care of. The only issue was that 2 spark plugs had rusted away and the heads had to be taken off. Did both heads and running great. Last PSI test was around 190-180 about 3 weeks ago.