Different RPM's

Great Lakes

Great Lakes

Well-Known Member
Oct 11, 2016
1,446
Lake St. Clair - Michigan (MacRay Harbor)
Boat Info
2008 - Formula 400ss
Engines
Volvo Penta D6-350
About mid way through our cruise Saturday evening I felt a small vibration start and then after a few minutes I noticed the RPM's were about 200-300 less on the port side even though the sticks were in the same position. I had to advance port throttle slightly to keep RPM's up. I also noticed there was more steam/exhaust on the port side while underway. Looking back now I also noticed the exhaust fumes while at the dock were pretty bad too.

Both engines had normal temps and the other gauges checked out too.

Any ideas?

-Rich
 
Great Lakes said:
Hmmm, I just had them changed a few weeks ago
Click to expand...

Any chance a boot is loose or not making a good connection then? Sounds like maybe down a cylinder

Edit..maybe not. As I pressed saved I realized your smartcraft would probably alert on that, someone more familiar with SC can hopefully confirm
 
My smartcraft had no alerts. My mechanic was hooking the computer up to it today. His first thought was a stuck open idle control valve. I had a bad ICV on my 320 and it just stalled at idle speed. I'm not sure how an ICV can cause this issue. He said the plugs and wires were ok

What signs would I have if the motor was ready to take a shit on me?
 
If it’s producing power and all other indication are in range, I would think something simple. Maybe even the throttle cable clamp let loose requiring more travel to achieve same rpm.

I changed plugs once and the ceramic separated from the threaded base at full power 10 minutes into the shakedown run. I thought I had blown the motor!

I new plug solved the problem and I never had another issue.

Let’s hope for something simple.
 
Thanks for the hope dt.

I def had normal power but what concerns me is the strong exhaust smell while in the well warming up and idling out. There was also more smoke coming out of the port side while underway too.

My mindset is I'm losing an engine..
 
I wouldn't be thinking something as dramatic as losing an engine based on what you're saying. My 8.1's have pulled that 200-300 RPM difference twice over the last 10yrs. Both times it was a bad plug or wire. The plugs never "look" suspicious, but they just start failing intermittently. Might I suggest swapping the plugs/wires from one engine to the other and see if the problem follows? It likely that one bad plug or wire connection is causing your issue. It wouldn't take much to swap them and see.
 
Could be a bad ignition coil. I believe your 8.1 has independent ignition coils for each cylinder. That would explain the rpm loss, running rough and rich exhaust smell but not the excessive smoke.
 
Regarding your excessive exhaust, if it's more than normal (one side of mine is always more steam than the other) for that engine it is quite likely the result of one or two cylinders not firing. You said yourself that the throttle for the Port engine had to be advanced more just the keep the rpm's up. That alone will cause it to run hotter and produce more steam in the exhaust flow. You may have 6 or 7 cylinders doing the work of 8 plus dragging 1 or 2 non-firing pistons up and down in the process. It definitely labours harder to keep the rpm's than a normally running engine.
 
Before you swap all the wires, how about disconnecting one plug at a time while at idle?

If the engine runs rougher, that plug was firing. If removing a wire doesn't impact how it runs, you at least know you've located the misfire: then look for bad/loose wire, plug or coil pack.

ICV... IAC? In my experience, SC WILL sound an alarm for that, but it didn't impact higher RPM performance...
 
may want to check the fuel injectors also to see if they are all firing....I had an injector go bad and the ECM did a good job of adjusting and covering it up....at idle I tried pulling one plug wire at a time to see if the engine would stumble but it never stumbled after each wire had been pulled and reconnected....the engine would idle smooth and run fine up to about 3,400 rpms but it would not go above that....I had a mechanic put an analyzer on it and he discovered the bad injector....

as a side note there was also a noticeable 'knocking' sound from the engine when the injector failed that sounded like a connecting rod knock which obviously was a cause of concern....as soon as the injector was replaced the knocking went away....

cliff
 
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Vibration + rpm loss + enriched exhaust would seem to add up to a misfire. I think that would also cause a rough idle. Have you tried starting up the engines individually and comparing the idle sound?

If it idles rough. you can pull each spark plug wire one at a time. The one that doesn't make a difference, is the cylinder that has the problem. If they all make a difference it's probably not a misfire. This only works for a misfire at idle. If you only have a misfire under load, I'd be looking for a fouled injector or a weak coil.
 
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bobeast said:
Vibration + rpm loss + enriched exhaust would seem to add up to a misfire. I think that would also cause a rough idle. Have you tried starting up the engines individually and comparing the idle sound?

If it idles rough. you can pull each spark plug wire one at a time. The one that doesn't make a difference, is the cylinder that has the problem. If they all make a difference it's probably not a misfire. This only works for a misfire at idle. If you only have a misfire under load, I'd be looking for a fouled injector or a weak coil.
Click to expand...
At idle, 650rpm (in well and docking) I don’t feel a vibration at all, just strong exhaust fumes. If I had to guess I’d say I start getting the vibration at 1000-1500rpm ish.
 
Great Lakes said:
At idle, 650rpm (in well and docking) I don’t feel a vibration at all, just strong exhaust fumes. If I had to guess I’d say I start getting the vibration at 1000-1500rpm ish.
Click to expand...

ok well, I'd be looking at the weak coil or fowled injector angle. That said, it still wouldn't hurt to check the gaps on your plugs. I know you said they're new, but that doesn't mean they were gapped correctly.
 
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I vote for loose plug wire. VERY VERY VERY tricky to seat plug wires on the plugs in the middle cylinders of these motors. Hard to get that definitive click. I replaced my first set of plugs on this boat a few years back. Ran fine at the dock. But, when coming up on plane one engine was a dog. I dropped to neutral and idle, opened the hatch and heart ticking. Found one plug wire dangling and arcing against the block. I didn't seat it properly (I guess). Shut down, pushed it on again and all was well.

My second time replacing the plugs was done during manifold change, when the manifolds were off. Way easier. NOT suggesting pull the manifolds but if you're in there doing it anyway.... ;-)
 
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