Merc 4.3LX lower unit knocking

[chef1962]

I have a '96 Mercruiser 4.3LX with an Alpha 1 Gen 2 drive unit, the original power package in my '96 Larson 174SEi. The boat and motor have probably less than 250 hours (it didn't come with an hour meter). The lower unit makes a knocking noise when turned hard over in either direction and the trim is down. It has done this since I bought it new, but very mildly. I had to turn the wheel all the way to the stops for it to happen, and thought it was normal. I just avoided forcing the wheel all the way over. Last summer the noise became more pronounced. Specifically, I didn't have to turn the wheel as far over before the knocking began, nor did it have to be trimmed quite as far down. I took it to a Mercruiser authorized shop where they replaced the gimbal bearing and U-joints. Next time I used the boat, the noise was LOUDER. If the lower unit is trimmed down more than halfway in its normal range and the wheel is turned more than halfway to either direction, the knocking begins, and gets progressively worse the lower the trim and more hard the turn. It used to be that it only occured during idle speeds, because that was the only time I turned the helm that far, but now it happens during normal maneuvering at speed.<br /><br />They pulled the lower unit again, and it looked like the aft yoke on the U-joint assembly had been coming into contact with one of the tabs, or "dogs," of the retainer nut on the upper gearbox. They tried to grind down the dogs a bit, but to no avail. I don't dispute the need for the new gimbal bearing and U-joints - they needed replacement - but they clearly didn't fix the problem, and the technicians are scratching their heads. The alignment was perfect; in fact, they said the alignment bar went in more easily than many new boats. After months of waiting, a Mercruiser tech support person finally called the shop. His only suggestion was that the transom was weakened, causing the unit to travel farther down than it should. I think this is extremely unlikely. I don't think it is rotted because the boat has always been trailered (never spent a night on the water) and I've seen no evidence of any leakage or other condition that would allow water damage. I have also seen no evidence of any kind of collision damage that might result in a weakened transom.<br /><br />The techs all say they've never encountered this. Ordinarily, noises such as this occur when the lower unit is trimmed UP, causing more binding in the driveline. The conventional wisdom is that the noise should abate then the unit is lowered and the driveline is at its straightest.<br /><br />Sorry for the long post.<br /><br />Any suggestions?

 

[Northern Eclipse]

Re: Merc 4.3LX lower unit knocking

Got me stumped, but has anybody looked to see if the coupler is good. I was told was that the sealed u-joints Merc was using didn't fair to well, when I last changed my U-joints I put the greasable one's in. The yoke should not be making contact with anything, did anybody check the yoke for straitness or if the pinion gear in the upper gear case is shimmed correctly or if all other shafts are strait. The first I/O I had made a awful clunking while underway , I later found out the big washer behind the prop was the wrong one and even though the prop nut was tight it still allowed the prop to move back and fourth on the shaft, another point is shift cable adjustments on alpha's must be bang on, or the will make noise. When I bought my Wellcaft I had a rattle that sounded like it was in the driveline, drove me nuts, I later discoverd it was my swim ladder, tightened up a few bolts, noise gone. Be persistent with Merc, and if they think your transom is weak, make them prove it, hopefully someone on this board as had the same issue has you have now.

 

[chef1962]

Re: Merc 4.3LX lower unit knocking

We haven't checked the coupler yet, though one of the techs at the local shop suggested that maybe there is too much grease in the coupler, causing a hydro-lock-pressure sort of thing that doesn't allow the shaft to slide in far enough. According to this tech, when the drive unit is trimmed down, the shaft goes deeper into the coupler. If there was an obstruction or other issue at the coupler that interfered with the shaft's travel, it would exert pressure toward the drive unit, forcing the U-joint assembly back into the retainer nut. According to this same tech, the shaft is also pushed deeper into the coupler as the drive unit is turned to either side. If that is correct, the problem would be at its worst when the drive is trimmed down and turned all the way to one side, which is exactly when my noise is at its worst. He said it surprised him to learn that the shaft actually moved INTO the coupler when turning, rather than being pulled outward. Another suggestion from the Mercruiser rep is that the unit is simply being allowed to travel too far down. My guy at the shop, however, says that this unit does not have an adjustment for downward travel, so that theory wouldn't work.<br /><br />I have to assume that the pinion gear and other shafts were checked, but I can't be certain.<br /><br />I guess one of my most pressing questions is whether something inside the drive unit gear case could be causing this, or does it sound like the problem lies on the engine side. I'd hate to pay to crack open the gearcase only to find that it was the coupler that was causing the problem, and I'd hate to pay for pulling the motor to check the coupler or transom only to find that the problem lay elsewhere.<br /><br />I can say that the boat's performance hasn't been impaired. It runs just like it always has. The only manifestation of the problem has been the knocking noise. I guess I can just keep running it until something fails, but, again, I'd hate to allow the driveline to chew itself to pieces if it could have been avoided by replacing a $50 part.<br /><br />Some additional information that may, or may not, pertain to the issue: I replaced the aluminum prop with a stainless steel prop 2 years ago, going down 2" in pitch. I was warned to give it a little more time when shifting between forward and reverse gears because of the increased inertia of the ss prop. I try my best. The only other modification was the installation of a Stingray wing on the drive unit about 5 years ago. It allows me to stay on plane at lower speeds, helps pull my fat butt out of the hole faster during deep water starts on my slalom and causes the boat to ride a little higher on plane at speed.

 

[achris]

Re: Merc 4.3LX lower unit knocking

Hi chef1962,<br /><br />For what it's worth. I HATE those stingray things, with a passion! I know I'm probably going to upset a lot of people by saying that, but so be it. This is a forum after all. <br /><br />The problem with them is the way they work. They push the back of the boat up, hence lowering the bow, giving you a lower planing speed et al. They push the stern up by pushing the outdrive up, and loading bearings and surfaces that weren't design to take load in that direction. I have seen some pretty impressive damage caused by them. We're talking busted cav plates, hinge pins chewed up and in one extreme case, the gimbal ring broken (for all you pro-wingers, no he didn't hit something). One possibility is the the wing is pushing the leg up and throwing the unis out of alignment. It may be a longshot, but at this point I'd be looking at everything, no matter how unlikely.<br /><br />As an aside, those things are considered dangerous over here. We have some pretty big swells roll through, their last landfall was Africa. When you're racing down the face of one of these, the last thing you need is some stupid piece of plastic trying to bury the nose of the boat into the bottom of the next one. <br /><br />Chris..........

 

[chef1962]

Re: Merc 4.3LX lower unit knocking

Originally posted by achris60:<br /> Hi chef1962,<br /><br />For what it's worth. I HATE those stingray things, with a passion! I know I'm probably going to upset a lot of people by saying that, but so be it. This is a forum after all. <br /><br />The problem with them is the way they work. They push the back of the boat up, hence lowering the bow, giving you a lower planing speed et al. They push the stern up by pushing the outdrive up, and loading bearings and surfaces that weren't design to take load in that direction. I have seen some pretty impressive damage caused by them. We're talking busted cav plates, hinge pins chewed up and in one extreme case, the gimbal ring broken (for all you pro-wingers, no he didn't hit something). One possibility is the the wing is pushing the leg up and throwing the unis out of alignment. It may be a longshot, but at this point I'd be looking at everything, no matter how unlikely.<br /><br />As an aside, those things are considered dangerous over here. We have some pretty big swells roll through, their last landfall was Africa. When you're racing down the face of one of these, the last thing you need is some stupid piece of plastic trying to bury the nose of the boat into the bottom of the next one. <br /><br />Chris..........

I have wondered if the Stingray places too much stress on the lower unit, but I haven't regarded it as too much of a problem since any number of technicians have seen it installed on my boat and nobody has implicated it. Plus, the knocking occurs, and has always occured, at idle speeds, when the wing would have no measureable effect. Another reason I hesitate to blame the wing is that the boat exhibited very mild knocking both before and for several years after the wing was installed. It had been bolted to the boat for years before the knocking became worse. This is not to say that the wing might somehow have a hand in all this, I just haven't seen a direct correlation.<br /><br />I haven't encountered the handling or danger issues you describe, though. Of course, my boat is a 17' runabout that only sees a calm, inland reservoir. I don't see many swells of the magnitude you describe. If I do, it means the dam has burst and I'll get to try my hand at white water powerboating...

 

[qystan]

Re: Merc 4.3LX lower unit knocking

Chief 1962, <br /><br />Its no consolation but I am suffering the same and still trying to fix it.<br /><br />In my case, I bought the boat last Nov. Did a service and found the upper housing seal leaking at the retainer nut.<br /><br />Changed all bearings (corrosion), seals, U-joints, both yokes, mid section not changed, drive gears were in perfect condition and hence not changed. After installation I get the knocking noice exactly as you described. <br /><br />Did a noise trace, put a pole against the lower unit, upper housing, bell housing, transom, coupler and engine block. Noise was definetely coming from the transom, all other areas gave only a turning whine. Replaced the gimbal bearing, no help. Try the trace and see where's your noise coming from.<br /><br />One thing I found was that the gimbal ring hinge pin holes were enlarged. It has freeplay, I can get it to rattle by lifting on the cavitation plate. Is this the cause, I don't know yet but I have bought a replacement ring and will replace it soon (not too soon, its quite a job). At the moment, getting all the tools, parts and other preps ready. Hate to be high and dry for days while waiting for spares.<br /><br />Any help/suggestions greatly appreciated.

 

[achris]

Re: Merc 4.3LX lower unit knocking

Something just occurred to me. When the gimbal bearing was replaced, was it a genuine Merc bearing and were both the bearing and carrier replaced as a unit? I know it's far more common than it should be, but a lot of bearings get replaced into the old carriers. The dealer thinks he's saving money (yours or his is up to you to decide).<br /><br />Chris...........

 

[qystan]

Re: Merc 4.3LX lower unit knocking

Hi Chief1962<br />Any new developments from the mechanics and did you manage to trace the source of the noise?<br /><br />Did not mention that in my case the noise is only present while in gear, in neutral its quiet.

 

[chef1962]

Re: Merc 4.3LX lower unit knocking

I haven't had a chance to have it looked into yet. The boat is stored 40 miles from my home (near the lake we frequent) and the various shops I plan to canvas are scattered over several counties in the area, so it will take me some time to get any information. I plan to bring it back to my house this week to clean it up and get ready for use - it performs fine, it just makes an unnerving noise. I still plan to take it on the lake soon. Just in time for that $2.00 / gallon gas (not that that engenders much sympathy from you guys in Australia and Singapore).

 

[qystan]

Re: Merc 4.3LX lower unit knocking

At 2 bucks, do they do mail order for gas? Over here it evaporates at US$3.50 a gal.<br /><br />Good luck with the drive and do keep us posted. I will let you know when I finish with the gimbal ring replacement (right now it works and sure hate to lose the season to this repair). My mail is the user name at singnet.com.sg, do let me know of developments.

 

[qystan]

Re: Merc 4.3LX lower unit knocking

Chief<br />An update on the knocking noise.<br /><br />The u-joint cross bearing threw off 1 cap, rattled and crunched up about an inch of the u-joint bellow flange at the transom end and 1 tab off the upper gear housing retainer ring. This was changed by a mech in Jan.<br /><br />Decided to do my own fix this time. While removing the cross bearing, found that 1 of the caps is missing 1/3 of the bearing needles and had developed a 1/4 inch crack.<br /><br />Replaced the cross bearing, trailer engine run showed promise, no noise. Now to get it back into the water to see how it fares under load.