Scan tool results
 

I finally managed to get a scan tool on my 99 454 mag mpi....A quick run-down of what it has been doing all summer long.....Runs great most of the time, but other times, it will just fall off RPM's, (engine won't die). Then will struggle to get over 2000 RPM's. If I let it sit for a few mins, it will run fine...Things done so far...All new sensors, fuel pump, regulator, I've went thru the pick-up tube & vent, & a complete tune-up....Now the results.
Nothing definete, but one thing that was odd, the timing was all over the place. It would go from -5 to +18, it does this constantly, & regardless of the RPM's. I checked for a loose timing chain, by watching the rotor button while barring the engine over, that seems fine. I'm leaning towards the ignition module (distributer mounted)....Does this seem to be a good direction, or...once again, am I way off base???

What scan tool are you using?

I can't tell you more than this, the logo on the top of the tool was "Quicksilver". I had a mechanic run the scan, & he seems to think that the module is a good direction to go. He ran a pretty detailed scan & found nothing out of the norm, except the timing being all over the place.
I know from my personal experience with street rods, even a radical cam'd engine...the timing will smooth out when the RPM's go up (that is watching a timing light). So, I did find it quite odd, that the timing would never smooth out.

I would agree with the module being the next place to check.

Thanks for the input, I'll be ordering one today.

It is normal for the timing to be erratic at idle, it should smooth out as the RPMs increase. Spark control module could be bad or the ground is faulty.

if that has a knock sensor the computor will put the timiing all over the place if the sensor is getting hit just a thought good luck

I did remove all the grounds & cleaned them real good. All were tight & I didn't find any corrision, but however, when I removed the module from the distributer, I noticed a white residue all over the mounting plate, & even under the module. I was able to wipe it off, just not sure if it was some sort of dielectric grease or corrision preventive. I do know that I should put the grease under the module, but should I also cover the entire mounting plate?
As for the knock sensor....It does have one, but the scan tool showed that there was no "signal" that was indicating a knock. So I would assume that the sensor is working correctly?

The ecm is moving timing all over the place to keep the motor's rpm constant. Since it is quicker to change the timming than to have the IAC change.

to check the timing at idle you need to put the motor into timing mode, rinda has the tool that will do this, about 40 dollars.

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.

The white stuff is heat sink compound (thermal dielectric grease) It is important that you re-apply to the back of the module when installing the module.

I have wondered if it could be in the harness, I actually ohm'd each ground wire from the computer connection back to the terminal. Each one tested fine, I even moved the harness around when testing. I know that is not an absoulete test, but with this "issue" I'm trying everything.
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.

Boyyy do I know exactly what you are talking about...Luckily, I never had to be towed back in to the dock, the boat always ran just good enough to get me home. Although, that last Sun, I was worried about it...LOL..
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.

That white residue is a heat block to keep the engine heat from frying the module. You need to re-apply some.

I have seen an exhaust leak fool a knock sensor into retarding the timing.

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?

it usually comes in a little tube or packet with the module