Engine quits after being pulled back to idle

[Jimmy Buoy]

The last time I had the boat out we cruised for about 1.5hrs at 3600 RPM back to the marina. Coming to the marina entrance I pulled back to idle and everything was fine until about half way to our dock there was a momentary "beep" and as I glanced at the dash I saw the port tach coming up from a lower RPM - like the engine almost stalled but caught itself while still spinning and fired back up. I assume the beep was due to low oil pressure from almost quitting with the ignition still on...

Took the boat out today and everything went just fine at first. Idled at the dock while getting ready, but once out only took it to 2300 RPM since I was in no hurry heading for the lake. Once out there I slowed down to about 1200 RPM and engaged the auto-pilot. After about 20 minutes I decided to just putt at idle with the auto pilot still engaged.

Sitting at the back of the boat briefly, I heard the same "beep" as before but this time when I looked at the dash the Port Engine had stopped (while in gear). I sheet the other engine down, took the stalled engine out of gear and tried starting it again - it turned over but wouldn't start. Tried it one more time and it started right up and idled fine - engaged the transmission and within a couple of seconds started hearing "marbles" in the tranny and sure enough it was a sign the engine was about to stall - yep, stalled again.

Thinking its the idle-air controller, I increased the throttle position and turned it over. Would NOT start, but seemed to be making some clicking type noises and tried to start, but no go. Checked the serpentine belt due to the clicking noise, but it appears fine.

Decided to try starting normally again and it worked just fine once again. Started the Stb engine too and put both into gear and headed back to the marina at 2200 RPM's - no issues coming back (even once idling down in the marina). Once docked, I raised the ER floor and started looking for any obvious issues - NONE. Started to think it was the IAC valve and was going to swap it with the new one that I carry on the boat. One more try first...

Started Port Engine at the dock and idled just fine - but I heard a clicking that was constant with the spin of the engine. Looking closely at the engine I saw a spark plug boot that was off of its coil just by the exhaust elbow and the spark was visible and making the clicking sound. Shut down and pressed the boot back on - fired it up. No more click and seems fine.

Question - Would having a spark plug boot off of one individual coil cause an 8.1 engine to stall at idle while in gear? I wouldn't think it would cause enough of a power drop to stall a big block engine. Maybe if one coil is not able to fire the plug the engine shuts down? Or do you thing there's another underlying issue???

 

[Escapade]

I would have thought IAC. This is how ours behaved when the IAC went. Best of Luck.

 

[Jimmy Buoy]

I would have thought IAC. This is how ours behaved when the IAC went. Best of Luck.

Thanks for the reply! Once you had your engine quit at idle, would the problem continue every time or would it just be stalling occasionally?

 

[Escapade]

2005, 8.1's Every time we went from higher rpm to idle. It died. New IAC corrected the issue. May be different issue for you but, new IAC fixed ours.

 

[JimT]

Swap your IACs between the 8.1s and see if that solves your problem on h bad engine. 15 minute task. If it turns out you need an IAC I have a Merc IAC #862998 in the box I will sell you at a nice discount.

 

[Jimmy Buoy]

I've always carried a spare on board, so today I took the original one off, installed the new one, and cleaned the old one with parts cleaner. It didn't appear overly caked with crud, but it looks almost as clean as the new one now - will keep it for a spare.

I only had the engine stall twice, but if it's not the IAC the problem will surely persist (unless it has something to do with the spark plug wire that popped off the coil).

 

[JimT]

Ok. If anyone is interested I will sell the new IAC for $80 USD shipping paid US.

 

[Jimmy Buoy]

UPDATE

Not sure what worked - putting the rogue spark plug boot back on its coil or the new IAC, but the stalling problem seems to be gone.

One thing to note is that if you have an auto-engine-sync system, its a good idea to shut it off when periodically testing your engines WOT capabilities (a good test to see if things are operating normally). In my case, the port engine was running on 7 out of 8 cylinders due to the spark wire coming off one of the coils (but I didn't know that). Running it WOT both engines would only reach 4100 RPM. They both matched, so I assumed that I had finally stuffed so much crap into that boat that it was over-weight OR the warm summer weather had finally allowed enough algae growth on the hull to slow me down. The port engine is the sync's master engine and the stb engine is the follower, so the sync was actually reducing the stb throttle to match the WOT RPM's of the problem port engine. If I would have turned off the auto-sync, the stb engine would have raced past the port's WOT RPM and I would have then suspected the port engine as having an issue.

Not sure just how long that spark plug wire was off, but it's been a while since I've been able to run at 4400 RPM… No wonder it seemed like the port engine was taking more fuel!