496 Mag Electronic Control Module
 

Is there a way to test the ECM? I'm being told that this MAY be the problem with my bad running enginebut they do not have the abilty to test the existing one.

A new one is about $1700 and once it comes out of the box I bought it whether it fixes things or not.

interested in a good used one ?

Sounds like the shop does not fully understand the problem on hand. I would NEVER tell a customer to spend money on anything that MAY or MAY NOT be the problem. To me this just shows their incompetence in the matter and you should go else where. While I do not work on marine engines every day I do deal with highly modified and adapted fuel injection systems on NA and boosted applications and there really is NO guess work in it. A sensor will either spec out or not spec out. Anything electrical can be diagnosed and in cases like yours where it is not feasible to actually test the entire ECM and all its circuits other than swapping it out with another known good ECM. Then they should have the confidence that they have checked over all sensors/wiring and mechanical systems and have ruled out all other potential causes and can assure you that buying a replacement ECM will fix the problem. Also, The circuit board can sometimes be repaired but finding someone to do the repair can be challenging.

Mag or HO?

It is a 496 mag. Unfortunately I have no other shop to go to without shipping the boat elsewhere. I am on Prince of Wales Island in Alaska and there is only one shop and they don't seem to be able to figure this thing out.

mtgbiker - I may be interested in a good used one. Email me at [email protected]

Since you're somewhat isolated from other good mech shops, I suggest you invest in a a Rinda hand held Mercruiser scantool at Rinda.com. It will read any active fault codes from the ECM, the last 10 faults kept in the ECM history, real time operating data like RPM's, oil pressure, etc.

They cost $399 with a 496 adapter to plug it into the engine. Also i recommend getting the Mercruiser PCM-555 Diagnostics manual for troubleshooting faults with or without a scantool. Without though is the hard way.

Once you're able to read faults you can really narrow down to what the problem is and it could be just one of the several sensors that's gone bad.

BTW, what are the alarms/symptoms you're having?

It gives off no codes when checked at the shop.

It dies sporadically, only at low throttle or idle. The fuel pumps will run, as if they are trying to prime the engine at initial start up but they will not stop running and motor will not start. Turn off the key and same problem, pumps just run and run.

Now it gets more confusing, I can turn off the battery to the engine and turn it back on and everything works and runs fine for a while then it will die again.


???????

Put a fuel pressure gage on the fuel rail and see what happens. If you see the gage fluctuating and then a pressure loss, it is the pump. A friend of mine had a similar problem. You think both fuel pumps are running by sound but the one in the fuel cooler failed. He has had the same problem with both of his engines and it started the same way with the engine quitting at idle.

kr1276 - Is it possible to get in touch with your friend or the shop who did the work? much appreciate any help...

I can help you but I may not be able to get you back on the water before your boating season ends. You can send me the ECM and I'll check it out for you, no charge but like I said it may not make it back to you before your season ends.

I actually helped my neighbor check his out. I have a fuel pressure gage so we hooked it up to the fuel rail. When you turn the key on, we could see the fuel pressure come up to normal range. We then started the engine and the pressure would hold for a few minutes then the pressure would become irratic and then go to zero. Obviously the engine would die out. It sounded like both fuel pumps were working but the fuel pump at the cooler was quitting. Merc has changed it now to where you have to replace the cooler and pump as an assembly. If the computer isn't beeping, in most cases I have found the problems to take me back to the basics.(fuel and spark) If you do not have a fuel pressure gage then go to a local auto parts store and either borrow or rent one. I will be happy to assist you any way that I can. We have been running 496's and have logged over 1500 hours so we have had our opportunities to learn a few of their weaknesses.

Thanks much for the help. I am out of town for the next week but will work on this when I get back.

Trippin - I may take you up on your offer. I'll get in touch with you when I get back.

i know this is a very old thread but I’m having the exact same problem did you ever figure this out?

The guy hasn't even logged into this site in 14 years. I don't think he's going to be replying.

it was worth a shot though right? i have the same exact problem , do you have any idea what it is?