3.0L rattling/knocking noise coming from the engine

[backacres]

Hello All,

I have an 87 3.0L w/Alpha I outdrive. The past couple of times out, I noticed a rattling/knocking noise coming from the engine (definitely the engine, not from the outdrive). It occurs under load, while pulling a skier for example. The engine performance seems unaffected though. The boat starts and runs great. The noise is intermittent but cause for concern as the noise wasn’t there before. I use 89 octane gas (min) and replaced plugs, points, condenser, cap and rotor this spring. I set the dwell using a dwell meter to the specs in the manual. I then set the timing (on the muffs) at idle. I also made sure the dwell was stable through the RPM range and it was. I verified that the timing advanced with increased RPM’s, but not sure how to properly check the advance. Could this be the cause of the noise? I read from other posts that similar noises may be attributed to:

-Valve lifter out of adjustment.

-Bad rod bearings.

-Flapper valve in exhaust. I also read that the tapping of the 3.0 (in the area of the exhaust shutter) is NORMAL.

- Timing gear on the camshaft. I read that they tend to wear since they are a nylon gear and will make noise under load that will make you think that a rod is knocking, or a lifter is stuck.

Not sure where to start, maybe with the timing advance? How should I check this? Would this problem be a candidate for de-carbing or Marvel Mystery oil? I’m a little leery that these are band-aid solutions. Any thoughts are GREATLY appreciated. Thanks in advance for your help.

-Jeff

 

[Chris1956]

Re: 3.0L rattling/knocking noise coming from the engine

Backacres, Normally over advanced timing will "ping" versus the knock you hear. You also have a mechanical fuel pump that will "tick-tick-tick" that you can hear at idle. The ticking noise is also attributable to lifters/rocker arms.

 

[backacres]

Re: 3.0L rattling/knocking noise coming from the engine

There is no ticking at idle. If the timing is set at idle, what is the procedure to check the distributor advance?

Jeff

 

[zshooter]

Re: 3.0L rattling/knocking noise coming from the engine

Jeff, checking timing advance requires a timing light that can be calibrated for degrees advance. Your auto parts guys would love to get in your pocket again for one! (Maybe you can find a "loaner" through AutoZone???)

Be sure to check for other possible sources of noise, such as a hose connector vibrating against a manifold, or an alternater bolt working it's way into the alternator fan blade (been there). Check your motor mounts to make sure they are tight and don't allow the motor to jump around (been there too).

When you replaced your points, did you have a look at the advance weights/springs, etc. under the points mounting plate? Sometimes they get rusted and crusted or a spring comes off and fouls the advance up. I believe there are settings specified in the Merc manual for spring tension on the advance weights.

The flapper valve typically is only noisy at idle - flapper is held wide open by exhaust after you move off-idle.

Pls keep us posted with your results.

 

[Bt Doctur]

Re: 3.0L rattling/knocking noise coming from the engine

Check the flywheel dust cover for cracks. sounds like bb`s in a coffee can.

 

[Boatist]

Re: 3.0L rattling/knocking noise coming from the engine

Mine did the same thing when it was new. I had to run 92 octain. When I pulled the head to replace a bad valve I cleaned out all the carbon, polished all the valves an seats. When I put head back on I ask if they had thicker head gaskets and put on the thicker one. Since then I can run 87 octain with no problem.

Just to make sure it not timing back off 2 to 4 degrees and do a test run. Try switching to a slightly colder plug.

 

[backacres]

Re: 3.0L rattling/knocking noise coming from the engine

Thanks for all of your suggestions. I'll look into them tonight and let you know if I find anything.

Jeff8)

 

[backacres]

Re: 3.0L rattling/knocking noise coming from the engine

Just an update -

I thoroughly checked everything that was suggested. I noticed my timing was at 8º BTDC and should have been 6º. I’m not sure if that was due to the different tach/dwell meter I was using. When I initially set it, I was using a borrowed meter. Since then, I purchased my own (hate not to have tools there when you need ‘em!). Well, after running the engine on the muffs, the only noise I did hear was coming from the exhaust flapper. This was the same side of the engine that the original sound was coming from when I was out on the water. I was able to feel the tapping at the same time I was hearing it, to confirm the location.

ZShooter, I know you mentioned the exhaust is typically noisy at idle, but is it possible that this was intermittently rattling at certain RPM’s while underway? If this is the case, should this be replaced?

I’m also wondering now what 2º advance at base timing did – is 2º considered to be way off?? How accurate is accurate with regards to timing?

Thanks again all, we are heading to the Adirondack’s this weekend and I will do some more in water “testing”. Sorry for the long post.

Jeff

 

[Chris1956]

Re: 3.0L rattling/knocking noise coming from the engine

Backacres, 4 cylinder inline auto motors tend to have vibrations that V engines do not have. The exhaust system on my 4 cylinder Nissan had sympathetic vibrations at certain RPMs. It was difficult to fix. My neighbor has a 4 cylinder Mercruiser. It has certain vibrations as well.

 

[Boatist]

Re: 3.0L rattling/knocking noise coming from the engine

2 degrees is not far off but if your right at the point where the engine will ping under load or not it may be plenty to cause your problem. If i understood your problem of knocking under a high load on the engine then their no way to test with out taking it out on the water. It is not going to ping or knock at idle or with no load.

Timing when hot under load better done out on the water where you can adjust and take for a run. When hot the centrifical advance will advance a little more as when springs are hot streach a little eaiser.

If your running hot plugs then under load they may be red hot and ingnight fuel before the spark.

Another thing you may want to try is go for a run and when it starts to knock slow down put motor in neutral and shut off. See if it try's to keep running after you turn the key off. Red hot plugs will cause this. Letting it cool for a while then shutting off it will shut off normal.

 

[backacres]

Re: 3.0L rattling/knocking noise coming from the engine

Boatist,

Thanks for the advice. I'll bring along the tools to check timing on the water.

Regarding the hot plugs, I did notice exactly what you mentioned where if I shut it off quickly (after putting it in neutral), it tries to keep running. Other times after I let it idle more (and cool down), it doesn't diesel. I think I will try some cooler plugs as well.

What plugs should I consider?

Jeff

 

[Novice]

Re: 3.0L electronic conversion kit for contactless points

Re: 3.0L electronic conversion kit for contactless points

Any leads for electronic kits to convert a 3.0, 4 cyl 140hp OMC with shift interrupter.

 

[Don S]

Re: 3.0L rattling/knocking noise coming from the engine

Novice,
if you have a question, best to start a new thread and ask your own question. Otherwise your question will get lost in a long line of answers to the original posters question.