Volvo 7.4 Gi PEFS S/N: 4012021922 - vibration between 1100 - 1600 rpm - next steps?

[Marineman7777]

I have a 2000 Cobalt 252 with the engine described in the title. I've had it about a year.

Symptoms: all cylinders appear to be running a little lean - light brown on all spark plugs, idles smooth, strong acceleration and has full power, I notice vibration starting about 1100 rpm until about 1600 rpm then much less noticeable the higher the rpm. The vibration is not intense but it is clearly noticeable.

What has been done thus far: Thought is was the outdrive so took it to a Volvo Penta dealer and they completely inspected the outdrive and shafts and all was good but this had no effect on the vibration. I replaced all the spark plugs with no effect. I did not notice any single plug that would indicate misfire (all light brown). Checked for fault codes and there were none logged. From previous threads, I thought the fuel cell may be causing the problem so I removed and cleaned the fuel cell. I found a few particles in the pump screens but not very much. I reinstalled and checked the fuel pressures - 41 psi high and 11 psi low...seems ok (no effect on vibration). Removed the distributor cap and found some corrosion...cleaned it up and the engine ran smoother and the rpm vibration band got smaller. I seem to be onto something that makes a difference. Do you think the next step is to replace the distributor? Suggested tests to do next?

 

[Scott Danforth]

welcome aboard

if I recall correctly, your fuel pressure should be about 41, so that is good

here is a link to the EF engine manual on fuel http://boatinfo.no/lib/volvo/manuals/ef_efi.html#/0

However if your running lean, injectors could be dirty.

I wouldn't replace the dizzy (that would be a huge waste of money), however I would replace the cap and rotor. cleaning only gets you so much. I would also look at changing wires if you haven't done so as most likely they haven't been changed in the last 17 years

 

[tlewis1]

I would assume when the dealer pulled the leg they had a good look at the gimbal bearing too?

 

[Marineman7777]

Many thanks for the quick response! The Volvo dealer completed a full outdrive service including the gimbal bearing. They were aware of the vibration that I was trying to solve. They thought the source of the vibration was the gimbal dipping into oil that was leaking into the bellows. The leak was repaired along with a clean up of the duo props with no impact on the vibration.

The spark plug wires appear to be original so that sounds like a good place to invest some money. Sounds like the cap and rotor are other areas to replace.

 

[bigbrownbuku]

The tan plugs is an engine that is running normally. Explain "cleanup"? Volvo props like to shift on their bush often creating minor vibration.
I have also seen newer type gimbal bearings installed backwards crushing the cross assembly causing a nuisance vibration. The only way to know is look at the gimbal bearing mount, if you can see the install recess for the bearing part its in backwards. Gimbal bearings are universal in that they will all fit in the same spot, some gimbal bearings are longer on one side causing crush if installed toward the drive. If all of that is as it should be you might have a engine coupler that is out of whack. if the vibration persists pull the drive again and run the engine briefly with the alignment tool installed and see how violent the coupler shakes the alignment tool. In a perfect world the alignment tool wont wobble around. If it does shake a bit remove the tool and run the engine and eyeball the female splines of the coupler, you should see the coupler wobble.

 

[bajaunderground]

Duo-prop? SX? Check prop(s)? Maybe you got into something...doesn't take much to create a vibration. Although DP can mask some...

 

[Marineman7777]

When I first bought the boat in August of 2015, there was no vibration but there was a little hesitation when accelerating from idle. Got caught in a pop-up storm with a lot of heavy rain and wind so I had to be more aggressive with shifting during the docking process, however, I still got blown out of position and must have clipped a rock or log. The rear Duo-prop had a bend in one blade tip from the incident. It sat for about a month and when I got back the vibration was present. The vibration was present before the Volvo dealer serviced the outdrive and they were looking for the source of the vibration (specific reason I brought the boat to them). The Duo-props were also fixed at the same time to make sure they were not creating the vibration.

The "clean-up" I did included cleaning with fine sand paper the distributor cap contacts (inside and out) and the rotor contact. I also cleaned with the fine sandpaper the shaft that the rotor was on because it was coated with rust.

I don't think it's the gimbal bearing because the vibration does not seem to change when turning side to side or with trimming the outdrive.

It seems the improvement in the vibration intensity and smaller rpm band with cleaning up the distributor cap and rotor would indicate ignition system causes but maybe that is only a part of this issue.

 

[Scott Danforth]

replace the cap and rotor. any amount of carbon tracing, etc in the cap will cause a mis-fire, or dizzy cross-talk

 

[further]

When I first bought the boat in August of 2015, there was no vibration but there was a little hesitation when accelerating from idle. Got caught in a pop-up storm with a lot of heavy rain and wind so I had to be more aggressive with shifting during the docking process, however, I still got blown out of position and must have clipped a rock or log. The rear Duo-prop had a bend in one blade tip from the incident. It sat for about a month and when I got back the vibration was present. The vibration was present before the Volvo dealer serviced the outdrive and they were looking for the source of the vibration (specific reason I brought the boat to them). The Duo-props were also fixed at the same time to make sure they were not creating the vibration.

The "clean-up" I did included cleaning with fine sand paper the distributor cap contacts (inside and out) and the rotor contact. I also cleaned with the fine sandpaper the shaft that the rotor was on because it was coated with rust.

I don't think it's the gimbal bearing because the vibration does not seem to change when turning side to side or with trimming the outdrive.

It seems the improvement in the vibration intensity and smaller rpm band with cleaning up the distributor cap and rotor would indicate ignition system causes but maybe that is only a part of this issue.

Did you hit the rock or log good enough that it could've bent the prop shaft possibly? I'm dealing with that myself now... Hoping its not that and will be taking mine out tomorrow to test, but it could be possible if you hit something hard enough from what i'm reading. I hit a rock 2 weeks ago and sent out the prop for repair, in the meantime, I used my spare, which has a bend in one of the fins (hence why its a spare), took it out this past Saturday and was getting a vibration between 1500 - 2000 rpms but would smooth out slower or faster than that. Hoping its the crappy spare prop i was using causing the vibration but in the meantime i've read that prop shaft bend is fairly common and it only takes a tiny bit for it to be out and cause an issue.

 

[bajaunderground]

I'll bet you money the prop fix was not to Volvo penta spec? They're are very few shops that are certified or have the proper dies to return to factory spec? What'd they charge for the prop repair? I spent $375 on having my props (F6's) returned to factory spec...they are now my spares as I run F5's at my elevation.

 

[Marineman7777]

The Volvo Penta dealer completely checked the drive and shafts because they were looking for the cause of the vibration. I told them that I clipped a rock or something so we both through the issue was the drive. They sent out the SS props and the cost was $250. There was only one tip bend on the rear prop so I thought this was a little expensive. I have purchase a new distributor cap and rotor and will see if that fixes the problem. If not, I think checking the props might be the next step.

 

[Marineman7777]

I replaced the distributor cap and rotor but it had no additional impact on the vibration. I will pull the DP props off and make sure they are properly balanced. This should have been done already by the Volvo dealer but I'll have them checked.