2100V Mercruiser 5.7 suddenly vibrating/missing?

[2100V]

Hey guys, I was on the way back in the other day after cruising around 3400 RPM for about 15 minutes. Everything was fine, and I started backing down getting close to the dock and at about 3000 RPM I felt a sudden slight loss in power and the boat picked up a vibration. I can still pull through the RPM range pretty strong, but around 2500-3000 RPM it shakes bad. I ran it in my driveway yesterday trying to troubleshoot, and although it's not nearly as noticeable with no load on it, I can still barely feel the vibration and hear what almost sounds like a slight miss in the engine in that same RPM range. It's not intermittent or random, it's a very steady "not right" kind of sound. above 3000 it seems to smooth out, but that may just be b/c everything is spinning fast enough not to hear it.

My troubleshooting so far: at idle, pulled one plug wire at a time and could definitely hear a difference. checked all the wires at the cap and the plugs. checked the oil for water, checked the tranny oil, and the v-drive oil. everything seems fine. engine temperature is normal. no smoke or weird smells out the exhaust. checked the prop and shaft, and both look completely fine.

Thoughts? Thanks guys.

[boatwakes]

Did you determine which cylinder was causing the misfire condition or did the engine react with every wire pulled? Possible stuck injector as you've already replaced the ignition components we talked about before. You can disconnect each injector harness while it's running to see if thats the case. Push in on the retaining wire and pull the harness off and replace by just pushing back on. Keep us posted.

[bigdaddy_houseboater]

I may be mistaken, but the vibration could easily be caused by dropping a cylinder. It could be as simple as a plug, plug wire, cap, rotor, clogged injector.

[2100V]

No, I'm unable to determine which cylinder it is or whether it's actually even a misfire since when I pulled 1 wire at a time at idle, the engine reacted every time. So whatever's happening it's not a complete misfire.

I was going to replace the plugs since they haven't been done in 2 years, and see if anything looks funny on the plugs and try to peek into the cylinder while I'm at it.

After putting all that back together I'll try pulling the injector harnesses one at a time to see what happens. Can I do that while turning ~3000rpm without hurting anything?

[bigdaddy_houseboater]

You can.. except if it's ingnition, ie: fouled plug, you will likely kill the motor, because it would be running on 6 then instead of 7. I pull plug wires with a set of plastic pliers. That way you can do it quick and not kill the motor. But then, my boats carbureted, not injected.

[boatwakes]

I wouldn't do it at 3000 rpm, rather disconnect individually at idle to determine if it makes a difference there and then if/when you find the one that doesn't change, raise the rpm to 2500+. Where are you located? I'm a tech and have a computer with injector tester if you're around so cal.

[2100V]

Thanks, going to try that tonight after changing plugs. I'm in TN, but thanks for the offer. I would take you up on it if we were closer.

[2100V]

Alright guys here's where I am:

new plugs
tied up miscellaneous loose ends around the distributor
pulled one plug wire at a time at idle, and engine reacted to all 8
pulled one injector at a time at idle, and engine reacted to all 8

The problem is still there. Upon closer observation of the exhaust outlets on the back of the boat, I noticed that: the miss is actually there at idle (go figure). It's in the portside of the engine, and the portside is running so hot I can't even touch the water coming out. Starboard side runs quieter, with less "puff puff" of air coming out, and the water is warm to the touch.

So from best case to worst, what I am I looking for at this point? What would cause one side of the engine (maybe just one cylinder) to miss and run hot?

Malfunctioning injector?
Bad oxygen sensor?
Burnt valve?
Blown head gasket? (no water in oil)
Cracked piston? (no smoke coming out)

Keep in mind I was cruising around 3400 RPM when this condition suddenly appeared.

[2100V]

I'm throwing in the towel; I don't have the time or equipment to diagnose it any further. I'll let you guys know how far the dealer rams it up my butt.

[bigdaddy_houseboater]

Alright guys here's where I am:

new plugs
tied up miscellaneous loose ends around the distributor
pulled one plug wire at a time at idle, and engine reacted to all 8
pulled one injector at a time at idle, and engine reacted to all 8

The problem is still there. Upon closer observation of the exhaust outlets on the back of the boat, I noticed that: the miss is actually there at idle (go figure). It's in the portside of the engine, and the portside is running so hot I can't even touch the water coming out. Starboard side runs quieter, with less "puff puff" of air coming out, and the water is warm to the touch.

So from best case to worst, what I am I looking for at this point? What would cause one side of the engine (maybe just one cylinder) to miss and run hot?

Malfunctioning injector?
Bad oxygen sensor?
Burnt valve?
Blown head gasket? (no water in oil)
Cracked piston? (no smoke coming out)

Keep in mind I was cruising around 3400 RPM when this condition suddenly appeared.

Sounds like mine when it gets over 200-ish. Mine drops a cylinder if there's obstructed water flow. If you have an IR temp gun, you can probably shoot the port side and see if the cylinders are all the same temp. It's possiblew you have a blown head gasket and when it heats up, you lose one cylinder.
I dont know... let us know what happens.

[davidwmayfield]

Had similar symptoms.

Messed with spark lugs dist. Cap. Stuff.....did not fix......

Had marine service check again....( got owner (more experienced) this time)

He replaced high pressure fuel pump #861155a3 and expensive micron fuel filter # 35-864572.

Runs great now. He said the filter he replaced is in a hard to find place on engine.

Hope this helps.

[R&T Babich]

...
He replaced high pressure fuel pump #861155a3 and expensive micron fuel filter # 35-864572. ...

What engine do you have?
The filter 35-864572 is for engines with a boost pump which I don't think the 1999-2001 350 MAG MPIs have.
A fuel pressure test at speed would show if the pump/filter are working properly.
The fuel rails on our 2000 21V 350 MAG MPI have an extra pipe thread port where I added a small pressure gauge.
We bought our boat used with very low hours.
It had similar symptoms, but it was hard to tell what was normal because it was new to us.
There was a slight odd vibration which smoothed out at higher rpm.
There was a very noticeable smell of gas in the exhaust at idle.
Turns out it had a cracked spark plug.
Your issue may only present itself under load as in a degraded plug wire or distributor cap.
If you replace the cap get one with brass contacts.
One of our exhaust manifolds is too hot to touch and the other is warm.
I've heard that is normal.
Since replacing the cracked spark plug we've put 200 hrs on it without a single issue.

[2100V]

The dealer looked everything over really well, checked the compression and it was ~175psi on all 8 cylinders, etc. Basically they said there was nothing wrong with it. Skeptical, I put it back in the water and it ran fine. I'm chocking it up to either air in the gas line or I had a stick wedged behind the prop that fell out when I pulled the boat out of the water that day. Who knows. Sometimes it's easier to say it's running fine now and not even ask...