Mercruiser 5.0 MPI idle issues; replaced IAC; bad part? UPDATE **Added Video**

[FliesAndFloats]

Ever since I bought my 2005 used boat with a Mercruiser 5.0 MPI it's given some intermittent idle issues. The symptoms seemed in-line with a bad IAC. Usually it would just require a restart and I'd go on my way, but lately it started getting to where the boat wouldn't want to start and required me to put the throttle at around 65% in neutral while starting to get it going.

Here are my symptoms:

When completely cold, the engine runs fine and idles in gear fine.
When warming up, the engine is likely to quit and can be very difficult to start and get running.
When at operating temperature, the engine runs fine, except when idling around hunting for a place to anchor or loitering to dock it can quit and become very difficult to restart.

I replaced the IAC today with an inexpensive aftermarket unit and the boat seemed to start and run better than I'd ever seen it, then it quit while at the dock and seemed nearly impossible to restart once it started warming up. I got it started and held it in neutral at around 1200-1500 RPM until it reached operating temperature and then it idled fine, except sometimes stalling out once I'd put it in gear.

I'm thinking that I might have just gotten a defective replacement part as the behavior since replacing the IAC seems to be much worse than it was with the old one that was in there. I also replaced the little muffler that is jammed down in the throttle body. I did not clean the large round air filter above the intake.

Any thoughts or suggestions?

 

[Fun Times]

Hi there, do you have your engine serial number handy? The stern drive is a Alpha or Bravo?

Is the throttle body dirty? May want to use some throttle body cleaner to clean it even if it don't seem dirty.

Check your spark plugs to ensure they are not fuel fouled in any way. Fuel pressure will want to be checked so you are aware of its state.

 

[Mischief Managed]

Check for a loose connection on the purple wire between the ignition switch and the ECM. It goes from the igntion switch to pin 5 on the round connector at the engine to pin B18 on the ECM. If it's intermittent, it can have the exact symptoms you have described.

 

[FliesAndFloats]

Update: I went out this morning and did some more diagnostic work and I believe the issue is that something is out of rig with the throttle. Also, I wanted to add that it is a 5.0 MPI with, I believe, a Gen 2 Alpha One outdrive. Here are my symptoms:

Starts and idles fine
Move throttle enough for it to click into reverse gear and then back to idle, the engine quits every time. (see video)
Move the throttle forward enough for it to go into forward gear and then out of gear, but not enough to feel the throttle "click" into neutral, it quits.
Move the throttle forward enough for it to go into forward gear and then click into neutral, it idles fine
If the engine refuses to start or keep running, wiggle the throttle back and forth in the neutral position and try again, it starts fine.

Here's a video of me touching it into reverse and then back to neutral and you can see it quit:

https://www.youtube.com/watch?v=MO8r8B49_9w

 

[alldodge]

Need a code reader but without that;
What is your serial number?
Do you have the carb style distributer cap?
Need to see what kind of fuel pressure you have at the fuel rail
Next time it will not start, disconnect the shift interrupter wire

 

[FliesAndFloats]

I keep forgetting to get the serial number off the engine. I received manuals for a 3.0 carbureted (!?) with serial numbers that match a 3.0 carbureted engine, but I clearly have a fuel injected V8 with a 5.0 MPI cover plate. I'm guessing something got mixed up somewhere in the history of the boat.

I am mechanically inclined and have lots of experience working on modern and old car and aircraft engines, but I am new to boats.

I have the "crab" style distributer cap with the 8 ignition wires coming out horizontally from the side. I replaced the cap and rotor when I experienced some backfiring at WOT which fixed the issue last month. Replacing the cap and rotor had no effect on this issue.

I have not verified rail pressure, but I believe it to absolutely be related to the shift interrupter as brisk shaking of the throttle within the neutral range seems to fix it. I figure it's either a sticking interrupter switch or from reading other posts here, a lower shift cable. When the issue occurs, it idles at a low RPM for a few seconds before just dying and when it was refusing to start and run it would turn over "WIJJIJJIJJIJJIJJIJJIJJIJJIJJIJJIJJIJJIVROOM quit" as opposed to the normal start "WIJJIJJIJJIVROOM continue running"

As another minor note, the button on the shifter doesn't seem to lock the shifter as stated in the documentation. I'm guessing there's a stop that probably broke off at some point in the history of the boat.

 

[alldodge]

If it was the IAC the boat would need to have the throttle advanced to higher rpm to keep it running, so don't think that's the issue.

It sounds to me like the interrupt switch or in combination of the lower shift cable needs to be adjusted or replaced. If the interrupt switch is active the engine will die. The switch should only active for a split second during shifting out of gear and into neutral.

As before, once it won't start pull the wire off the switch

Otherwise other testing is necessary as mentioned previously

 

[Fun Times]

If it was the IAC the boat would need to have the throttle advanced to higher rpm to keep it running, so don't think that's the issue.

It sounds to me like the interrupt switch or in combination of the lower shift cable needs to be adjusted or replaced. If the interrupt switch is active the engine will die. The switch should only active for a split second during shifting out of gear and into neutral.

As before, once it won't start pull the wire off the switch

Otherwise other testing is necessary as mentioned previously

This was my thoughts after seeing the video as well.:thumb:

 

[thumpar]

I agree with the shift interrupt switch causing the stalling. It is most likely a lower shift cable that needs to be replaced. You could try adjusting it. It may still have problems though. The lower shift cable is a normal replacement item.

 

[FliesAndFloats]

Just for some closure, I changed how I operate the throttle with the tested behavior in mind and was able to prevent the stalling altogether. Since there is a few degrees of lost motion between reverse and forward gear I just move the throttle firmly in and out of gear and it seemed to stop the issue. Perhaps it was operator error all along

 

[alldodge]

Maybe :noidea: hope it stays that way and you have good times boating