View Full Version : 1st generation 502 EFI

09-01-2011, 02:24 PM
My friend/mechanic Henry has a Baja 240 sport. Back in the early 90's his friend who owns one of the largest Mercruiser dealership(s) in the States had a few Mercruiser 502 EFI's that just came out. So he replaced the 260/alpha with this package for basically peanuts as these engines were factory test engines. Again this guy got stuff from Mercruiser because he was so large, etc. Well a year or 2 later his ECU or ECM whatever they call it went south. When they sent it back to Mercruiser for replacement the company sent out their "Men in Black" to his bud's marina. So it goes these were experimental EFI engines and were supposed to be destroyed. What they did for my bud was they gave him a brand new EFI 502 with the 2nd gen Bravo one, racing throttles and a few other gadgets and a mean warranty in exchange for what he had....no brainer. Well since then his engine will only rev to 4750 not the 5050 or 5150 it used to, no matter what prop he uses it is now a 65mph boat, not 72. They obviously gave him the wrong ECM but he doesn not want to spend the $$ for a new one as the 240 gets pretty squirley above 65 anyway. Has anyone experienced an ECM that limits their throttle that low? He literally can put a 19 prop or a 25 and it goes 4750 and the speed varies...obvioulsy hitting a rev limiter.

09-01-2011, 02:39 PM
maybe the tach is off and it is really hitting 5050 or 5100 or whatever the limit is supposed to be...

09-01-2011, 02:48 PM
Apologies if I'm missing something, but if the tach is off and it is actually reaching that higher 5100ish RPM, wouldn't the speed be about the same as it had been before?

I'm curious, since 4750 is in the peak power RPM range for the motor (at least I think it is, or close), if your friend has tried propping UP, not down, to see if close to the original 72mph is achievable at 4750 but with more screw.

I openly admit to not thinking much before posting this. I may well be missing something.


09-01-2011, 03:44 PM
4700-4750 is the 454/310 hp rpm limit

09-01-2011, 04:00 PM
4700-4750 is the 454/310 hp rpm limit

Interesting--my 310 HP 454 said 4400-4600 for peak power.

Either way, I don't think I was as clear as I should've been.

What I'm thinking was that the factory peak RPM recommendation for the 502 was 4800-5000.

Is that true?
If so, how much below peak HP would you really be at 4750? (I'd think a little, but not a ton.)
Most important, isn't the cheapest solution, if this a a rev limiter issue, to prop up? Such that maybe for the price of a prop, he can get back to I dunno, maybe 70?

09-01-2011, 04:12 PM
he asked what the rev limit was,,, not the peak power number :)

Dr. David Fleming
09-04-2011, 10:37 PM
The 502 electronic is called MEFI which means Mercury Electronic Fuel Injection. This system comes in three different forms. The MEFI I and MEFI II used a computor with different end terminals than the MEFI III. The first and second types also came in a metal box on the top of the engine. MEFI III just had a bracket bolted to the same location on the exhaust manifold. All these computor systems are visible right on top of the engine.

The MEFI engine computor is entirely programable if you have the software and can link your PC laptop to it. You can change just about any aspect of the engine performance and the MEFI II and MEFI III have special provisions for supercharging the engine.

With these modern computor ignition designs it is possible to drive the boat and tune the engine fuel curve and rev limiter while underway. To really do this effectively you need an "oxygen sensor" installed in the exhaust system while you drive and tune. There is a pipe plug in the cast iron manifold that can be removed to do this.

Since this type of tuning is beyond the scope of most marine mechanics you might consider one of the major engine tuners like Tyler Crockett Racing Engines - they have the full access ability to adjust the MEFI system. I believe you can just send the computor to Tyler and he will reprogram it - but you will have to be able to tell him on the phone what you want and what the engine is all about. Or if you have the bucks you can pay Tyler's air fare and he will fly to your marina and fix it while you drive your boat. I believe several other tuners in the USA will do this also.

Several of the later MEFI systems were encripted by Mercury and no one has been able to adjust them except the factory.

09-04-2011, 11:43 PM
Again...with a 19" it hits 4750, with a 25" it hits 4750. So it goes from 58mph to 65mph and NEVER breaks 4750.

09-04-2011, 11:50 PM
Again.... he has the 454-310 ECM

09-05-2011, 11:14 AM
That would make sense.

Dr. David Fleming
09-05-2011, 11:57 AM
The 310 hp 454 Big Block Chevy uses the same MEFI computor as the 502 and 454 MAG MPI engines. The only difference is in the programing. The MEFI III engine computor is expensive but is a very fine instrument.

The 502 MAG MPI is all forged components but is limited in its strength by the stock GM valve gear. Merc Racing replaced these components with Crane Cams valve gear.

Stock cam on the 454/502 MAG MPI is slightly more agressive than the 310 hp 454. both engines make power peak below the 5000 rpm limit. I would suspect that if you friend has experienced that much power loss the "test engine" he gave up had different components than the one he received back.

The 502 MAG MPI that I have in my 1999 22ZX will run in the 5200 rpm range or make more performance by proping it to run 4800 rpm range. The dyno horsepower curve for the 502 shows 20 to 40 more horsepower at the lower 4800 rpm setting. Rpm does not alway equate to performance unless you can breathe the engine at the higher speed. Cam profile is the whole issue.

Your friend ought to verify if he has the correct MEFI computor for the engine he has.

By the way the 502/454 MAG MPI engines all used the same two stage throttle - an absolute dream for smooth low end performance at all lower speed and throttle settings. The 310 hp 454 engine (which is basically an automotive engine) used a large single throttle that advances very quickly more like an on/off switch. Made throttling at low speeds much more tricky.

09-05-2011, 02:59 PM
call mark boose,eddie young or trler crocket,send the ecm to one of them and have one of them raise the rev limiter..