2005 4.3 MPI Misfire or Gimbal / U joint question

[jbourne210]

So, I recently brought a 2005 Stingray 195 RL with a 4.3 MPI Mercruiser Alpha 1 Gen 2 back to life. Motor only had about 55 hours or so, but she'd been sitting for a few years. Cleaned her up, flushed the tank, replaced spark plug, all fluids and filters, and she fired right up, and seemed to run perfectly. Note - the fluids that I drained out looked perfect.

That being said, I've been chasing a vibration/growling that comes on after she gets warmed up. Mostly noticeable in turns. So I'm thinking gimbal and u joints, right? Pulled the unit, and visually inspected. Everything looks brand new in there, and I feel zero notchiness or slop in either the gimbal or u joints. Granted, things can 'appear' to be good and be bad, so I decided that I'll pull and replace soon.

I took the boat out yesterday, and FINALLY found some nice water to try a WOT run, and noticed a few things.
1. Boat only hit 4100 RPM, at an indicated 51 mph, full fuel, 2 people in boat. I've never really been concerned with top speed, as I never run there, but I want to make sure that the engine is running correctly. Those numbers were not backed up with a tach and/or GPS, by the way.
2. After the run, I felt some vibration at all speeds, and regardless of whether I'm in a turn or not.
3. It should be noted, than in the pursuit of slick water, I got the boat into a bit of shallow water. Prop got into some sandy bottom, and cleaned the prop of most of the paint, and put a slight ding in the prop. Barely 1/16" of an inch and hard to see unless you're looking for it.

Could the lack of full RPM and growling be linked? In other words, could a misfire on a 6 banger cause what would appear to be a growling sound? I've had some car engines that would idle and run at low loads just fine, but miss under heavy load, but I would think that a V-6 miss would be more noticeable. Or am I just chasing my tail here, and the gimbal and u joints need to be changed, with an engine alignment performed as well?

Sorry for the long post, but I wanted to give as much info as I could.

Thanks!

 

[jbourne210]

Also, I should mention that I haven't changed the plug wires yet. They 'look' okay, but they are also 12 years old. It's on the list....

 

[wahlejim]

Something doesn't sound right with those numbers. You need to get the tach and speed verified. Also, need to know size of your prop and gear ratio to run any prop slip calculations.

It sure sounds like everything is firing. You wouldn't get even close to these readings if you lost a cylinder on a V6. A simple test to perform to see if all cylinders are firing is a drop test. As it is running in idle, pull each spark plug wire one by one and see if you can tell a difference in the sound of the engine. It will let you know if you have a dead cylinder.

When you inspected the Gimbal and the U-joints, was there any play in them at all? If there was no play, and I mean 0 play, they should be fine if they were greased prior to their hibernation and there was clean gear lube in the lower unit.

I would lean towards your prop. Have you tried different props yet to see how the boat behaves with different blades?

Also wouldn't hurt to go through the WOT sticky on the adults only section.

 

[Rick Stephens]

wahlejim is right on. I'd go after u-joints though. Too easy and it will probably resolve the early 'turning vibration' you experienced. Doesn't take much to bend aluminum prop blades. At this point, it needs repair.

Verify your tach. Install a GPS app on your phone. Get a new prop and have the old one reworked. Get good numbers to test your theory that the RPM is low at WOT.

 

[jbourne210]

I appreciate the info!

Actually planning on doing a drop test when I get home today. I'll also be able to post info on the prop size and pitch, and I'll try to find the gear ration out as well.

Regarding the gimbal and u joint play, I didn't feel anything at all. All felt smooth as silk. Of course, that's not the end all answer, but it's confusing. I should also mention that if I rev the motor (without it being in gear), and turn the wheel from side to side, I don't get the growling. It's only while under a load. Which scares me a bit, as I worry about the unit itself. Still, I agree that the u joints are probably the culprit. I'll probably change them before my next trip out. I've read about folks using automotive u joints in the adults only section. I'll probably go that route.

It's funny that you mention to verify the tach. I've had an issue where the tach will jump high and scare the bejesus out of me. Cruising along at 2500, then it jumps to 4500! Thought I'd spun a prop, but then my brain kicked in and I realized that I wasn't hearing a difference in engine RPM, and I hadn't slowed down. Tapped on the glass and it came right back down. Something squirrely there to look into, so my RPM could be in question.

Thanks again for y'alls help!

 

[Rick Stephens]

The 'Mercruiser' u-joints are just automotive u-joints Merc repackaged. No downside at all.

Boat tachometers are not always the most reliable. They also have a switch on the back to go from 4 cylinder to 6 and 8 and so on. Those switches get corroded and start to cause read errors. Sometimes you can cycle the switch a few times and get them to clean up.

 

[jbourne210]

Roger that Rick, I'll probably change them out this weekend.

​Also, I just got home and checked the prop. It didn't look as bad as I thought. Still, I'm probably going to have it reconditioned, and use it as a spare. Oh, and it's a 14 X 19, four bladed aluminum.

​Thinking about it though, if the u joints are bad, wouldn't they growl at me if I'm turning the wheel from stop to stop while out of gear, given that the u joints are still turning?

 

[Bondo]

Thinking about it though, if the u joints are bad,

Ayuh,.... Welcome Aboard,.... U-joints are sometimes a tough call,....

I'm guessin' that once ya pull 'em, 'n look,....
Yer gonna see little tiny ridges in the cross bearin's, where the needles ride,....

They'll "Feel" nice, 'n tight, but chatter under load,....