Sick Mercruiser V8

[mnypitboat]

OK so here is the senerio. It is a Mercruiser, either 305 or 350(not sure which) with an alpha outdrive. Not sure if anyone here has any ideas. A couple months ago, we were on the way in and there was a storm chasing us. I was full throttle for about 30 minutes. Then it started tapping and chattering. I backed below 3k rpms and it went away, so I just kept it under that. Then when we reached the dock it died. Turned out to be out of gas. OK so we paddled it up onto the trailer. Pulled it out. Figured it had sucked up some water or something from the bottom of the tank causing that noise, that almost sounded like a loud pinging/tapping.

Next trip out we filled it up with 93 octane, and went fishing. There was an obvious tapping. Seemed to come and go. Been on the water many times since. The tapping is there all the time now. Yesterday coming home from the Glen it got really bad. Tapping/rattling type noise at all RPMs in gear, and as RPMs rev it is a faster tap. It only taps slightly in neutral, even revving it up. It is very bad under load, almost to the point that I wanted to shut it down and call boat tow, but it is barely noticeable in neutral revving to 4k. It is there, but very slight.

The noise is coming from the rear of the engine. We cant definitively tell what side it is coming from or if its centered or ???

Here is what we have done so far. I pulled the rockers and pushrods, because the noise sounds just like a bent pushrod. They are all perfectly straight, and I adjusted the valves, which were way out of adjustment. I pulled the distributor this morning and it is fine as well. It has used about 1 quart of oil in about 50 or so hours, and its still clean from the oil change in January. I topped it off this morning.

The thing is that the engine still runs strong, plenty of oil pressure, temp is sitting about 160, unless I am really on it for a while, but then it only hits about 170 tops. There is 40psi of oil pressure at idle and about 60-65 on throttle. Nothing has changed except this noise I am hearing. It runs virtually perfect.

I am completely stumped. I seriously doubt it is a bottom end issue, but I cant figure out what it is.

Any Ideas?????

 

[Fun Times]

Re: Sick Mercruiser V8

You may have tulip the valves, You might try doing a compression and a leak down test.

Good luck with it.

 

[Bondo]

Re: Sick Mercruiser V8

I was full throttle for about 30 minutes. Then it started tapping and chattering.

Ayuh,... My guess is, it started out as Detonation,...
Now the hardparts are trashed....

Like Boat Tech says,... Compression test for sure...

 

[Bifflefan]

Re: Sick Mercruiser V8

Im going with a wrist pin.
Just seem to fit the discription the best.
A compression test wont reveil this, but do it anyway. I have been wrong before.

 

[Bifflefan]

Re: Sick Mercruiser V8

but it is barely noticeable in neutral revving to 4k. It is there, but very slight.

By the way, DONT DO THAT!!!
That is the fastest way to trash a motor.

 

[mnypitboat]

Re: Sick Mercruiser V8

I tried to, but it seems my compression tester is shot. I either have to buy another or pay someone. I seriously doubt that its got low compression though, simply because it is running so good. BUT I am not ruling out anything. It runs exactly the same, if not better, than before the noise developed. Probably better due to the valve adjustment. It idles smooth, jumps out of the water, runs about 57mph without issue. Before the valve adjustment it would stall out at about 53mph or so.

I will have the compression tested, or test it myself later this week though.

 

[mnypitboat]

Re: Sick Mercruiser V8

Doesn't sound like a wrist pin. It sounds like a lifter or bent pushrod. If it were a wrist pin I would hear it in the bottom end with my long extension in my ear. I don't. I only revved it to see if it would produce the chatter without having to take it on a water trip, so it was quick and made no noise other than a slight tapping.

Could it be something coming off the back of the engine?? Just a thought, because it is obviously coming from the back of the engine, and its not the distributor. Could it be a Gimbal bearing or something taking a crap?? That would account for it being quiet unless its under load I think?? I am not that familiar with boat issues, so I am guessing. I know a gimbal bearing will grind but I havent read anything that says it could tap or rattle.

 

[fossill]

Re: Sick Mercruiser V8

Hmmm..water is gas. Possible it leaned out the mixture and started detonating when you "pushed" it for a period of time. Seen it before on a 5.7L resulting in a "toofed" engine. You really want to back off when it starts to rattle.

 

[SSSuper83]

Re: Sick Mercruiser V8

I would take a stab and say because the valve lash was so far out of adjustment..you have a valve/lifter/cam problem.

It's a TAP, not a KNOCK, right? Sounds like you know the difference..

If the valves were that far off..you might have a cam lobe problem. GM V8's are known to eat a cam lobe or two sometimes...

 

[mnypitboat]

Re: Sick Mercruiser V8

Hmmm..water is gas. Possible it leaned out the mixture and started detonating when you "pushed" it for a period of time. Seen it before on a 5.7L resulting in a "toofed" engine. You really want to back off when it starts to rattle.

Its in the origional post. I did back off to where it wasnt making the noise as soon as I started hearing it.

 

[mnypitboat]

Re: Sick Mercruiser V8

I would take a stab and say because the valve lash was so far out of adjustment..you have a valve/lifter/cam problem.

It's a TAP, not a KNOCK, right? Sounds like you know the difference..

If the valves were that far off..you might have a cam lobe problem. GM V8's are known to eat a cam lobe or two sometimes...

It is a slight tap in neutral and a clatter in gear. To put it into perspective, when I was adjusting valves, I do them while the engine is running and back them off til they clatter, etc etc. The noise is exactly like the valve when it starts clattering, except louder. I have had worn lobes on a Chevy small block in the past as well. I just keep thinking it cant be anything that would cause a miss, like a worn lobe, simply because it is running nearly perfect other than the noise. Again, I will not rule it out just yet.

I am on call for the next 5 weeks, so no time to look into it myself for a while, but I wanna be back out on the water soon, so I am gonna drop it off at a mechanic this week and get the compression checked.

Its been a long time since I have messed with simple engines like this. I guess I need to get used to them again, since I bought an old Jeep with a 304 in it as well. I suppose its time to break out the old school tools, and get the broken ones fixed or replaced soon. LOL

 

[6meter]

Re: Sick Mercruiser V8

From what you've checked, eliminated, how about a carbon deposit rattling around in a combustion chamber. Usually sound more like a rod bearing knock but I'm just throwing an idea out where you were running it hard.

 

[z1nonlyone]

Re: Sick Mercruiser V8

I am going thru a similar problem right now with our 5.7. We thought it was a rod knock due to worn bearings, but turned out to be piston slap caused by some unknown engine builder that stuck a standard size piston in a +.030 cylinder!!!

 

[mnypitboat]

Re: Sick Mercruiser V8

From what you've checked, eliminated, how about a carbon deposit rattling around in a combustion chamber. Usually sound more like a rod bearing knock but I'm just throwing an idea out where you were running it hard.

I thought about that too, but I would have thought that by now it would have broken apart and blown out. I guess the only to know that one for sure is to pull the heads.

 

[SeaBolt1]

Re: Sick Mercruiser V8

Could this be a worn out exhaust shutter in the exhaust system. I have heard that this sound is often diagnosed as a bad rod etc. ??

 

[mnypitboat]

Re: Sick Mercruiser V8

Could this be a worn out exhaust shutter in the exhaust system. I have heard that this sound is often diagnosed as a bad rod etc. ??

Now there is something that could fit the bill. How would I check something like that?

 

[mnypitboat]

Re: Sick Mercruiser V8

I spoke to a boat mechanic today. He said it could be a gimbal bearing too. What I forgot about, and he asked me, was that during the storm on Saturday my bilge filled up to the tune of about a foot. Probably over the gimbal bearing, and right after that is when it got way worse. It happened one other time during the hurricanes when the plug got some crap in it.

If nothing else that just proves to me that I need to get a bilge pump with a float, so when it gets too high it automatically turns on. Right now its only on the switch.

Tonight I am going to start it up and grease the crap out of it and see if it quiets down any. Or even changes pitch. If so, it would prove that to be the issue.

Wish me luck.

 

[mnypitboat]

Re: Sick Mercruiser V8

Could this be a worn out exhaust shutter in the exhaust system. I have heard that this sound is often diagnosed as a bad rod etc. ??

OK the mechanic says this cant be it because it wouldnt flutter on throttle and would quiet down under load rather than get louder.

 

[jtmarten]

Re: Sick Mercruiser V8

Full throttle for 30min - sounds to me like you did some valve train damage. At nearly 5K rpms you're pushing alot of oil, for 30min straight, and not giving it much time to drain back into the pan (the reason you should never use a HV oil pump for marine use without a min 10qt pan). Wouldn't suprise me if you had the bulk of your 5qts of oil sitting in the valve covers slowly draining into the pan. The rear cylinders might have gotten lucky with gravity keeping the oil there, but the front cylinder's valve train would be hardest hit.

 

[mnypitboat]

Re: Sick Mercruiser V8

Full throttle for 30min - sounds to me like you did some valve train damage. At nearly 5K rpms you're pushing alot of oil, for 30min straight, and not giving it much time to drain back into the pan (the reason you should never use a HV oil pump for marine use without a min 10qt pan). Wouldn't suprise me if you had the bulk of your 5qts of oil sitting in the valve covers slowly draining into the pan. The rear cylinders might have gotten lucky with gravity keeping the oil there, but the front cylinder's valve train would be hardest hit.

That is what I am afraid of, but it runs so good, no change in its running, just a noise. It isnt coming from the front though. It is definitely in the rear of the engine. Doing a compression test tonight, and greasing the crap out of the gimbal bearing, then hauling it off to the mechanic tomorrow.