Howdy-
Looking at a boat that has twin 502 mag efi/mp engines with 900 hours on them. I personally run very few hours per season, and this boat has been very well maintained, so I'm curious as to what the lifespan of these engines are under moderate operating conditions. Anything I should look for or check on these motors? Thanks.

900 hours I would do a compression and leak down test for sure.

454/502 mag efi: I was told 500 / 1000. 500 hrs then rebuild top end. 1000 hrs compleat rebuild.

How ever...... I have seen SEVERAL posts and threads on 502's with 1300+ hrs and have not been touched!

My 502 went about 600 hrs before i blew the head gasket and rebuilt the top end. Then came failure analysis..... The reason I found the head gasket blew was due to leaning out. Leaned out due to plugged fuel injectors. Injectors plugged due to fuel lines falling apart. Fuel lines falling apart due to ethanol in the gas.. Why is there ethanol in my gas?.. O ya government says it has to be in there . So in the end the government blew up my engine!! Ya it is a vicious circle.

Just keep a eye on compression and leak down test as posted above. Perhaps with good maintenance would be a injector cleaning ever few years.

I rebuilt mine at 800 hours and they were perfect inside. I was looking for more power. I ran mine at 4000-5000 rpm most of there life and they looked great. I wouldn't worry about it.

Those motors are very unstressed. Low power level per cube, good internal parts, and a very easy on parts camshaft.

The most undependable part that came with those motors where the outdrives bolted to them. LOL.

Thanks for the replies. I agree, assuming these engines weren't overly-abused, they should be fine. IF the engines test fine, good compression, etc., what do you think the avg. cost of a rebuild (via machine shop) would be for these engines for a stock overhaul (ie., no performance mods) ? I've rebuilt automotive engines, but never marine. Thanks.

Still cold in Northern Ohio..............

I did all my own labor (except rebuilding and machining the actual heads). Start to finish with cleaning and flow testing the injectors, gaskets, o rings, ect it cost me B.O.A.T. ($1000,). Single engine

However i got a sweet deal on the head gaskets (2 for 1) from another member on here in the swap shop.

Still cold and rainy in central Minnesota also, hahaha

I am with you on this one. I have a 502 mag mpi and am doing a top end refresh right now. Doing all the work myself and not doing any upgrades, just keeping it stock and getting the heads completely refreshed, and I am into it for $1000. It is not terrible to do on your own and makes for a new winter project.

I agree on the leak down. Head gaskets would be my biggest concern.

Did you ever buy the boat? I have 1997 twin 502MPI MAGS with 820 hours that have never been touched other than external maintance, Tune ups, sea pumps, 1 reciruculating water pump om each engine do to leakage at the weep hole. I will probably have the injectors flowed and cleaned for the first time this winter. I would say those engines are very reliable. I have noticed no decrease in RPM,performance, or higher oil usage since I got the boat with 240 hours. I know a marine tech thats seen 502 mpi mags with 1400 hours still running.