On that motor you set base timing in timing mode and the computer adjusts it from there. As everyone else has said, its normal to see the eradic changes at idle. If your mechanic didn't know this, take his expertise in this application with a grain of salt. I had a similar problem with a 99 454Mag and sold the boat after replacing many of the things you did. The next owner eventually replaced the wiring harness and fixed the problem. My motor was laying down when over 3/4 throttle. Sometimes it completely died. We replaced sensors. We put an external fuel pump and guage on it. Replaced the cool fuel assembly. Ran from an alternate fuel supply, etc. At the end it was the computer killing the motor after getting a bad signal somewhere it the loop due to the faulty harness.
As for the timing, it was eratic with all RPM's, it never did smooth out. Shouldn't it smooth out at upper RPM's?
I have twin 454 Magnums, model year 2000.
I also went through this entire exercise -- changing lots of parts, lots of time under scan tools -- with Teague and Pfaff essentially giving up, both giving me advice to replace the engines (7 years old, 700 hours, "That's why people sell used boats, etc). LOTS of $$$$$$$$$$$ and lost time boating and interrupted adventures.
However, it was obvious to me that there was no intrinsic problem with the engines -- they idled smoothly forever, often ran flawlessly, but sometimes alarms would go off and/or the engines would work strangely (including timing going all over the place). Restarting the engines often cleared the problems temporarily.
Finally, I had the wiring harnesses on both engines replaced. Mercruiser just took the serial numbers of both engines, and delivered new ones. It took Pfaff days to install them -- its a tight engine compartment, side by side engines. Again, $$$$$
But since then, the engines again work like new.
When I looked at the harnesses, both had different problems with different pins on different connectors. Some wires had corrosion under the insulation, only visible after the harnesses were removed.
Basically, your engine reads all the sensors all the time, and then adjusts all the actuators all the time. If the signals from or to any sensor or actuator are not correct, then all sorts of wierd stuff happens.
When EFI works, its great. When it does not, its almost impossible to correctly diagnose: any symptom can be due to any sensor or actuator or, as in my case, the entire wiring harness.
You, along with an ealier post has me wondering if I may have a harness issue. Just the inconsistant nature of this problem. Most of the time the boat runs perfect, but then without warning, it falls off, & I can't isolate as to when or what makes it happen, it just happens.
Not real sure what I can do, if anything to narrow it down a bit farther to show the harness as being the "culprit", but I may have to dig a bit further than just checking the grounds. You say your biggest issue was corrision? When I peeled back the wire loom & checked each ground, I found no corrision at all, & each one checked out fine, but I didn't completely remove the harness.
I plan on replacing the module, it's only $50, & if that proves not to be the problem, then, maybe the harness is my next step....I do appreciate all your input on this.
Ed, the white compound is actually there to promote heat transfer from the components in the module to the module mounting plate, we use it all the time in the electronics business, it is almost as nasty to use as anti-seize compound.
There are components in the module that will overheat and fail in the absence of a good heat sink.
OH! Sorry, I stand corrected. I knew it did something for heat, but I always assumed it was the heat from the distributor housing that would harm the module.
See, even at my age I learn new things! Thanks.
Is this what is provided with the module, or do I need to get it from somewhere else?
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