Can you reset a Mercruiser ECM?

Geoffrey Wallington

Geoffrey Wallington

Member
Dec 23, 2020
45
Norfolk, VA
Boat Info
1999 330 Sundancer
Engines
Tein Mercury 7.4L - 454 Cu. Inch
I have a 7.4l mercrusier that was running rough. After trying to fix a lot of things - Checked and changed the fuel/water separator. The plugs were replaced and then the fuel pump. We are getting 50 psi pressure. Good compression in all cylinders. The engine cranks but won't start now. Arrgghh. The new plugs are dry, so though the engine cranks the injectors are not running. Is it possible the ECM was the issue in the beginning and now is totally wacked? Is it possible to reset it? I am waiting on a mechanic to find diagnostic kit (hopefully next week!), but thought I would try anything else first.
Thanks for any help
 
Yes I do. Checked the distributor and it is clean as well. Pulled a plug and tryied starting to confirm a spark.
 
Sounds like no fuel do you hear the fuel pump running
 
Do you hear the fuel pump relay kick on, I'm actually doing this as well right now. ECM is not turning so no fuel.
For Mercury Horizons these ECMs are hard to find but I did hear they can be flashed.
 
Not sure if it can be reset, But I have a friend that had so crazy things going on and the mechanic said it was the ECM.
They sent the ECM out to be rebuilt.
I know there is companies that will rebuild and test them.
 
Merc has software to load onto a laptop to access the ECM. You can reset faults and histories. I believe you can flash the prom as well. I have heard of black market merc software that can be found online. I’m not sure it’s worth the risk
 
Ok. Thanks all. I installed a new fuel pump. When ignition turns on you hear it charge. I thought that was the issue in the first place. The old pump gurgled. There is fuel pressure past the pump. I removed a line and during turning over fuel was flowing. If it is ECM I was just wondering if it is worth trying a reset. I have heard you can do it in cars by disconnecting the battery and letting it sit for a while. Then reconnect and see if that reset the chip.
 
That myth of auto reset is just that, a myth. If you have confirmed spark on all plugs, you have fuel pressure and compression, it should fire I would think. I don't have any experience with older ECM and am amazed if a motor that old would have issues. Did you mess with the timing/move the distributor? Something isn't adding up here.
 
How strong is the spark? Pretty and blue or a yellow?
 
capecodcruiser said:
That myth of auto reset is just that, a myth. If you have confirmed spark on all plugs, you have fuel pressure and compression, it should fire I would think. I don't have any experience with older ECM and am amazed if a motor that old would have issues. Did you mess with the timing/move the distributor? Something isn't adding up here.
Click to expand...

It isn't adding up with me either. A month or so ago the first thing I noticed was a high idle, up around 1300 rpm. I shut down when we got back and the next day I started it and it was back to around 800, but seemed a bit rough. Then during the trip that day it got worse so we headed back. My first thought was fuel, but the separators were clean and the fuel from them looked good. So we checked the plugs and they looked pretty bad. After changing them the engine won't fire at all now, so we checked fuel pressure and it was zero. The pump had been making a weird noise on power up, so I weirded it must have decided to finally die. Changed that and am now getting good fuel. And good spark. But the plugs are not wet at all after all the cranking, so the mechanic thinks it may be the ECM not telling the injectors to open, since the fuel seems to be making it all the way to them. Any other ideas?
 
dwna1a said:
How strong is the spark? Pretty and blue or a yellow?
Click to expand...

It is an ok spark, but the plugs are dry after all the cranking, so we don't think it is the plugs. The distributor is clean too. With dry plugs the only thing we can think is the ECM is not telling the injectors when and how much to fire (based on throttle position).
 
Before condemning the ECM, I would check for pulses at the injectors. A test light and/or noid light would confirm whether or not the injectors are being commanded to open.

What happens when you spray carb cleaner into the throttle body?
Does it fire momentarily?
 
Espos4 said:
OK. :(
Click to expand...
No kidding. I have looked on the Mercury-Mercruiser site and ECMs are no longer available.
One thing the mechanic said to me that made me scratch my head was that the modules were not interchangeable between stbd and port engines, as the motors rotate in opposite directions. I thought the change in rotation occurred in the gear box to get the counter rotating screws. I was thinking of trying the stbd ecm in the port engine to see if it solved the problem but is this not possible? Anyone know?
 
Geoffrey Wallington said:
No kidding. I have looked on the Mercury-Mercruiser site and ECMs are no longer available.
One thing the mechanic said to me that made me scratch my head was that the modules were not interchangeable between stbd and port engines, as the motors rotate in opposite directions. I thought the change in rotation occurred in the gear box to get the counter rotating screws. I was thinking of trying the stbd ecm in the port engine to see if it solved the problem but is this not possible? Anyone know?
Click to expand...
It’s possible to swap, but you would have to have a device like the Rhinda scan tool to set the ecm up.
 
Geoffrey Wallington said:
No kidding. I have looked on the Mercury-Mercruiser site and ECMs are no longer available.
One thing the mechanic said to me that made me scratch my head was that the modules were not interchangeable between stbd and port engines, as the motors rotate in opposite directions. I thought the change in rotation occurred in the gear box to get the counter rotating screws. I was thinking of trying the stbd ecm in the port engine to see if it solved the problem but is this not possible? Anyone know?
Click to expand...
Counter rotating motor on a 1999 boat? That should be easy to verify. Get down there yourself and see which way they rotate while someone cranks the motor.

there’s a good chance that you’ll find they are both standard rotation.
 
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