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Sudden Knock - 2008 Mercruiser 4.3 MPI with Alpha One

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mejim

New member
So I recently had a marina do my boots, bellows, trim sensors, gimble bearing, water impeller and such. They contacted me and asked if I wanted a tune up so I said yes. This replaced the cap, wires, and plugs.

All seemed great. I had the boat out yesterday and had no issues, I kept it around 2000 - 3000 RPM max, at 15 - 30MPH max. No pushing, no issues starting or running. It was fine. I was able to shut it off at various docks and started right up. On water just floating around the same, no issues starting or idling. Got back to the launch, shut it off. Got the trailer and it started, idled and ran fine. Took it home and put it in the garage overnight.

I take it out today, start it and it's idling very rough, I backed off the dock and it quits on me. I start it back up and it's idling really rough and quits again. I could barely get it to stay running. I tried getting it in gear to get back to the dock but it just wouldn't want to stay running and would shut off each time unless I was giving it more fuel. Someone helped tow me back to the dock where I tied it up.

I took the cover off the engine, started it and idled rough, so I put it in neutral and revved to about 2400 RPM where I could hear a noticeable knocking sound. At idle and lower RPMs there is no knock. It's quiet as a mouse. Just very rough Idle and it dies out. Only when I rev it up does it start knocking. This was not happening until today. Yesterday it idled like a brand new engine, and quiet but strong.

I called a local on-call mechanic and he said he thinks it could be fuel related. As in a bad fuel pump, filter or some other fuel issue.

I guess it makes sense if it's fuel that would explain why it doesn't want to stay running at idle, but pings / knocks at higher RPM.

If it were a rod knock I think it would be knocking / slapping at idle as well as higher RPM. Also, if it was a rod and getting proper fuel, the engine should technically still run at idle but the knock would be present. That's not happening.

Any ideas would be great!

Thank you!
 
Certainly if you have low fuel pressure one would expect detonation under load. would start with your fuel filter empty the contents into a container see if there is evidence of debris or water. Next would probably measure the fuel pressure. You cant get far into these without needing to log into them to see what's happening.
 
I had a mechanic come and review. Everything, I mean everything performed as though it was a brand new boat. He said the engine was very smooth and ran perfect. No rod issue either.

He revved it up on and let it run until 160 degrees and still no issue. He was running it on hose at about 2000 rpm.

He changed the fuel filter but there was no water in there either.

He had the computer connected as well and nothing reported there and the fuel pressure held properly at the correct PSI.

The thing I’m wondering though, due to how busy it was yesterday, I started it at the dock without thinking when the out drive was up. I told him about that and he said ‘you’ll know if the gimble bearing went. It would blow up.’. He then listened at the rear and said everything sounded fine.

As a last step he’s going to meet me at the dock later to take it for a spin.

I’m thinking if it’s not engine or fuel, maybe I messed up the gimble bearing. He quoted me $270 to replace it if I need to.

Stupid novice mistake maybe. I’ll report back
 
mejim said:
I had a mechanic come and review. Everything, I mean everything performed as though it was a brand new boat. He said the engine was very smooth and ran perfect. No rod issue either.

He revved it up on and let it run until 160 degrees and still no issue. He was running it on hose at about 2000 rpm.

He changed the fuel filter but there was no water in there either.

He had the computer connected as well and nothing reported there and the fuel pressure held properly at the correct PSI.

The thing I’m wondering though, due to how busy it was yesterday, I started it at the dock without thinking when the out drive was up. I told him about that and he said ‘you’ll know if the gimble bearing went. It would blow up.’. He then listened at the rear and said everything sounded fine.

As a last step he’s going to meet me at the dock later to take it for a spin.

I’m thinking if it’s not engine or fuel, maybe I messed up the gimble bearing. He quoted me $270 to replace it if I need to.

Stupid novice mistake maybe. I’ll report back
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yes if drive is way up and you hit it the u joints will knock, even in neutral as drive shaft and everything on a alpha one is spinning except prop shaft.

You probably didn't hurt it at all. If you can take off under full power and it doesnt groan when it turns or make noises you are fine. Live and learn
 
Dieter said:
yes if drive is way up and you hit it the u joints will knock, even in neutral as drive shaft and everything on a alpha one is spinning except prop shaft.

You probably didn't hurt it at all. If you can take off under full power and it doesnt groan when it turns or make noises you are fine. Live and learn
Click to expand...

Well, took it down to the dock with the mechanic. He ran through everything and the boat seems perfect.

I guess that's what I did, I was trying to run the engine with the outdrive too high. I thought it was lower but the trim gauge reports a little less than 3/4 high even when you're outside the drive range. I didn't realize that. So I was trying to start it with the gimbal and knuckle in a poor position. Ugh

I mentioned that I was concerned I damaged the inner workings. But he took the boat out and after a solid run he said don't even worry about it and he feels I have no concern with the bearing. He said the boat was great actually.

I guess I'm ok after all. ID-10T error I guess. Novice mistake.

But a good lesson NEVER to try and start the boat unless you know for sure that the outdrive is down and operating within the correct range. DO NOT rely only on what the gauge is saying.
 
mejim said:
One quick note though, I should mention, I do hear a slight rattle / knock when idling. I don't know if this is form the gimbal or something else.

Video Below:
Boat Idle Sound

Thoughts?
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Sounds fine to my stop worrying about it

I have boated for years in a shallow cove , need the outdrive almost all the way up to get in the dock. Just keep rpms down to idle or just above when in the trailering range.
 
Trim gauges are not all that accurate. Mine works but what I do is time in my head how long it takes to get from full down to half way up. That’s the highest I ever need to raise it depending on water depth. 12 Mississippis to go from full down to full up so if it’s shallow I use 6 Mississippis 1/2 way up is fine on the ujoints. My boat is 36 years old I’ve had it 22 years & to my knowledge they have never been changed.
 
louc said:
Trim gauges are not all that accurate. Mine works but what I do is time in my head how long it takes to get from full down to half way up. That’s the highest I ever need to raise it depending on water depth. 12 Mississippis to go from full down to full up so if it’s shallow I use 6 Mississippis 1/2 way up is fine on the ujoints. My boat is 36 years old I’ve had it 22 years & to my knowledge they have never been changed.
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Yea you’re right. User error. I’m really glad I didn’t break something!

Just took it for a many mile journey without issue. If I get back I’ll consider it a success lol Currently floating many miles away.
 
Ok, I knew I wasn't going crazy!

I had the boat out a few times recently. No issues, until today. Took it out a few miles away from the launch. Shut it down and just hung out for a while. Started it up, moved it and shut it off again. No issues.

Then after about 30 minutes just hanging out I started it up, and it stalled. Started again, it idled really low, barely hanging on, but I got it in gear and upped the RPM over 1k and headed back to the launch. No issues along the way. It performed perfectly normal. I get back to the launch area and decided to hang out there, just in case.

Each time I got close to a shallow / 5 - 7 foot depth area, I'd turn it on and move it. This worked without issue 3 times. The 4th time it stalled at idle. I tried again, got it in gear quick and gave it a little gas and it was able to move without issue.

The last time however, after floating for about 20 minutes, I started it, it stalled. Then I had to again quickly get it in gear and I slowly made my way back to the dock. Going in and out of gear to line it up. Each switch to neutral it stalled out. I started it, and it stalled. And so on.

I got it docked, let it sit for about 5 minutes, then I started it, and it seemed to idle normal.

I went and got the car, and when I got back 2 minutes later the boat was shut off. I didn't try again and instead just dragged it onto the trailer manually.

Why would this be happening? The trim was always in the correct range, and it seems to run fine if you get it in gear quick and get the RPMs over 1k. This also doesn't happen all of the time, only sometimes. But when it does it's ruthless lol

The shop that just worked on it did the gimbal, bellows, water impeller, wires, ignition cap, spark plugs and a few other things.

They also said they 'adjusted the carb', but this is an MPI fuel injected motor. Are there adjustments on these?

I don't know much about these motors, but geez, I just want reliability. Why the heck would it do this randomly?

Any suggestions would be awesome. Thanks!
 
Sounds like IAC valve which regulates idle air flow. If you can rev it in neutral then slam it into gear the valve the filter for it are likely clogged or valve hanging up
 
Dieter said:
Sounds like IAC valve which regulates idle air flow. If you can rev it in neutral then slam it into gear the valve the filter for it are likely clogged or valve hanging up
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Thanks for the reply. The one I have looks pretty new, and the filter is clean. Is there a way to test it?
 
mejim said:
Thanks for the reply. The one I have looks pretty new, and the filter is clean. Is there a way to test it?
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i think if you google it you will find ways to test it which I dont know how to. I think if you take it out you should see it moving just not sure engine can run. Also factory manual will likely have it too.

Pretty sure the IAC valve is an automotive part so you may see some stuff there.

If you have Rinda or Diacomm you should be able to see what opening level it is given. if it is stuck Would think the ECU would try to open it 100% to keep it running as this is essentially the electronic version of an idle speed screw ....
 
Dieter said:
i think if you google it you will find ways to test it which I dont know how to. I think if you take it out you should see it moving just not sure engine can run. Also factory manual will likely have it too.

Pretty sure the IAC valve is an automotive part so you may see some stuff there.

If you have Rinda or Diacomm you should be able to see what opening level it is given. if it is stuck Would think the ECU would try to open it 100% to keep it running as this is essentially the electronic version of an idle speed screw ....
Click to expand...
So I went to the marina that did the work. They took it for a test drive with me and it did the same thing. He fiddled with the shift interruptor switch and the boat then idled properly.

They didn't have one in stock, but they think that's the issue, that the switch is sticking or something. They'd have to order a new one, but I'll be down for 2 weeks!!

Also, they fixed the "knock" but adjusting the motor alignment. They said that the knock was due to the front of the motor being too low. They said they readjusted it upward, with an alignment tool, and the knock stopped. Perhaps too much pressure on the gimbal bearing, and a slightly misaligned angle was causing that? I haven't taken it out since but does that seem right?

I don't think I'll be able to take the boat out until I get a new switch though.
 
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