So here's a new development. Last week we were out riding and the engine just shut down completely at about 45mph and would not re-start. Mechanic came over and found that the ignition module inside the distributor was shot. We just replaced it about 30 hours ago (hobbs). He also noticed that the distributor shaft wobbled a little. We replaced the distributor a few years ago because the previous one did the same thing, and that solved all kinds of problems.

The suspicion now is that something is wrong with the cam, allowing it to move back-and-forth, causing the distributor shaft to eat away until there's enough play to cause a ground-fault. We think that might also explain why we're cooking ignition modules -- I think we've replaced 4 of them over the past 6 years.

So now, it looks like we're in for an engine tear-down to see why the cam is moving. Ugh!

Hi All -- Latest update...

Mechanic pulled the engine. The cam bearings were worn. Suspicion is that this allowed just enough movement or vibration of the cam to cause the distributor shaft to "wallow-out", allowing it to ground-fault. The cam was not moving back-and-forth as we originally suspected.

There's no evidence of any other wear or misalignment in the engine. Mechanic suspects possible lubrication problem allowing the cam to either not get the lubrication it needs and/or allowing it to get hot enough to soften the bearings.

How can we test internal engine lubrication to determine if that's where the problem is? And, if that's not the problem, what else might cause the cam bearings to wear? The engine has around 720 hours on it.

Are there other things we should look at while we're in there? Any other ideas as to what might cause the distributor shaft to wallow around?

Thanks to you all

Yeh, a worn distributor shaft. Those dist’s do wear out (shaft to housing) this, but typically in higher mileage cars/trucks.

edit in: just reread thread for refresher. But killing ignition modules ? No. Are you using gm or merc modules ? Using, and using correctly, heat transfer paste (usually white) vs typically supplied dielectric grease (normally ckear) . Razor and clean off old stuff ?

Is he installing new gm or merc distributors ? Or aftermarket ?
Has a new coil been installed ? GM or Merc ?

Other than junk new modules, heat is #1 killer of these modules.

I haven’t seen worn cam bearings on something without other damage. I have to ask, what steps in diagnosing did he take to get to this point ?

I see you’re on to another whole host of issues, but I worked on a boat recently that had the engine harness and shift cable zip tied together. In this case the gauges were acting erratic every time you shifted.

Amazing thread, and to finally see the cause...except the ROOT cause (poor lubrication?) is still unknown...
Is the cam out to where you could have it Rockwell tested for hardness?

We're using all Merc parts. I was tempted to use GM parts but the mechanic was more comfortable sticking with Merc only. Everything was cleaned when we replaced the parts.

Troubleshooting this started several years ago. Gremlins have been popping up for a while. The original distributor, when we finally figured out what was going on, would seriously wobble. We stumbled across it being the problem when the mechanic was troubleshooting and accidentally bumped it. When he did, the boat cut off. That prompted us to compare it with the other engine. The other distributor had zero play in it... ie. it didnt move at all. Once we replaced the distributor, all of the problems went away. The "bogging down", random cut-offs, backfiring, etc. All gone. That lasted a while and then problems started showing up again...

This time, they presented as "bogging down" again, and randomly shutting off completely when shifting in and out of gear. Over time, that progressed into complete engine cut-off's at speed. ie. cruising along at 45mph and instant loss of engine... followed by the inability to get it to start again. After that, we noticed the ignition module was shot (ie. it looked burned-up). Replacing it allowed the boat to start and idle smoothy and we could rev it pretty high running on a hose. Took it back to the lake and barely made it out of the marina, power up to get on plane and less than a minute later, complete shutoff and failure to restart again. This time after sitting over night, I tried to start it again. It would act like it wanted to crank but every time it would attempt to fire, it would sputter and backfire like crazy...ie BAM BAM BAM.

By "bogging down", what I mean is I could be cruising along and it would suddenly feel like I was driving through mud. The boat would slowly drop rpm's and speed. Ultimately down from around 4800rpm @ WOT to 4200rpm. Sometimes it would pick back up again, other times it would fail to reach full rpm's again. Eventually it got to where it would never get over 4200 rpm's (under load), but otherwise seemed ok. The engine wasn't backfiring (at the time) and from and engine sound perspective, it sounded like it was running smooth, just not right.

When we looked at it again, and comparing the two engines again, we noticed the distributor was able to move within it's shaft again. The other engine has zero play. This time, the failed engine's distributor didn't move as much as last time, but it moved enough that it was clearly noticeable. Especially when there should be zero play at all.

We've swapped computer modules from the good engine to the bad, wiring harnesses, etc and we just couldn't find an electrical reason for the failure. We've replaced sensors, adjusted timing, and still no lasting luck. We checked fuel pressure, fuel pumps, injectors, spark plugs, plug wires, rotor button, dist cap, engine cooling, water pump, cool-fuel, etc. all ok. The computer shows no codes, which is exceptionally frustrating, but the last time we had a these very similar problems, it never showed a code either.

The original distributor lasted 15 years and ~530 hours. The second distributor lasted barely 3 years (100hrs) and finally failed, from what we can tell, at 7 years and 190 total hours. Covid and having a new baby slowed down my boat use for a couple of years in there, hence the long time tolerating this and the relatively low hobbs count.

Given the 2nd "wobbly" distributor, this led the mechanic to believe something was wrong with the cam shaft. Something was allowing the distributor to move enough to wallow it out and the only other thing connected to it is the cam. His initial thought was that the retaining plate that keeps the cam from moving laterally may have failed allowing it to move front-to-back, ever so slightly moving the distributor and eventually taking off enough material to allow it to wobble.

Once he opened the engine up, he found the retaining plate was where it should be and that the cam had no lateral movement. But... the bearings were worn and he could move the cam ever so slightly around. Something he should not be able to do. From what he can tell, the cam itself is undamaged, but he can see spiral grooves(?) cut into the distributor shaft, showing where it had been moving around.

The only thing we can come up with for all of these problems, given what we learned with the first distributor, similar symptoms, and now another distributor that wobbles is that 1) something is allowing the distributors to move, and 2) if the distributor is moving enough to ground out, that might explain the fried ignition modules, the bogging-down, and possibly the cut-off's when shifting in/out of gear. All of those seem to be electrical/ignition related, so maybe rapid pulsing "shorts" could interrupt the ignition enough to cause these other problems. Even if not, the cam and distributor shouldn't be moving around!

As an FYI, the boat is 22 years old with ~720 hours on this engine's hobbs.

The question now is, what could cause a cam to eat away at it's bearings? With no other signs of damage on the cam shaft itself or anywhere else (thus far), the suspicion is a lubrication failure. What else might it be? How would we test lubrication to the cam with the engine open so we can see the lubrication points for the cam?

Thanks again!

Hi All,

I just thought I'd update you in case anyone has similar problems or is just interested in general...

The main components of repair were the distributor and cam bearings. With the new bearings, the cam has no movement and spins on its axis as it should. Luckily, the cam gears that match up with the distributor weren't damaged so no need to replace the cam. In actuality, the cam looked practically brand new. The bearings took the abuse and definitely spared the cam.

The distributor had to be replaced. The shaft was wallowed out to the point that it would wobble, and that appears to be the issue at the core of all the other problems. That wobble was just enough to cause ignition interrupts that affected power output and shifting gears.

While we were in there, we also replaced the timing chain. Easy to do with every thing opened up, and it had a tiny bit of play in it. The thought there was, maybe that little "slop" in the timing chain was putting weird tension on the cam shaft, causing it to eat into the bearings, eventually to the point where the shaft would move enough to affect the distributor. Who knows, but for only ~$200, why not change it!?!?!

Of course there were new gaskets, new oil, the cleaning of the oil pan, etc. Crazy thing is, the gasket set was the most expensive part of the whole job. Like $900 for a set of gaskets. That's nuts!

On our lake test, the boat ran like a scalded dog. It hasn't run that good in years! Easily reaching top speed and full RPM's, and no shuddering or stalling when shifting in/out of gear.

Anyway, thanks again to all of you who provided thoughts and ideas on where to look throughout this saga.