Knocking in 140 Merc

[Boater Bill]

Can?t find the source of the knock. I have an ?86 140 Mercruiser (3.0 inline 4 cyl) with the Alpha 1. Starts and runs great. I have a tingy knock that started last week in the driveway, haven't taken it out since.

I?ve tried to isolate the area where the knock is with a 3 foot long steel prybar to the engine and the other end to my ear but nothing evident. The knock starts at idle, and increases intensity as RPMs go up, no slow down or quite on rev up or rev down. Occasionally it wont strart immediately, but within 4-5 seconds it's knocking again. Did the pull the plugs with the engine going test and nothing conclusive ? with 3 and 4 (rear plugs) the noise subsided but not the extent that the technique should result in (especially with 2 plugs pulled). Plenty of oil, and pressure shows well on gauges, no water in oil. Pulled the top valve cover, the side valve cover and the coil wire and turned over and over, nothing stuck, no noise, things moved like they should - nothing conclusive. Pulled the exhaust manifold and took a close look at the lifters, they look normal ? mild carbon buildup but no indications of chuncks that could have fallen and make the noise.

So THEN pulled the engine and the oil pan (no shavings on the magnetic oil plug or in the bottom of the pan), pulled off all the bearings (piston and mains) no abnormal signs and actually in pretty good shape for 22 years old. The camshaft looked in good order as well

The bearing inside the gimble housing seems in bad shape and the plate on the flywheel that has the rubber holding the gear spline seems to be slightly out of center but has a decent amount of play in it (the spline is out of center, not the whole bracket). The ?CV? joint looking thing on the outdrive doesn?t have any play in it and the castle-nut looking housing isn?t loose (one of the local boat guys had mentioned that castle-nut thing, no idea what it does but its not loose).

Would the gimble bearing make enough knocking noise to resonate through the block to sound like a rod or a lifter? I?m sure there is someone who had chased this same noise and found it somewhere strange? My next step is to pull the head and pistons - but from underneath there is no sign of wear on them either.

Any ideas would be helpful. ? Thanks all

Bill

 

[chiefalen]

Re: Knocking in 140 Merc

Yea that noise can travel. You know already the coupler went. Change it e-bay has a few right now.

Gimbel bearing went change that.

You pulled the drive change all the rubber.

You need the manual. Maybe start a new thread and ask Don s for the manual,

Post the s/n and model #. Ask him nicely hes a good guy.

 

[Boater Bill]

Re: Knocking in 140 Merc

Thanks Chief. I'll order those parts for sure.

Someone had mentioned 2 things that could be possibilities. The first was that it could be the Vibration Damper on the front of the drive shaft. I'll take that off and see if there is anything unusual (haven?t noticed if this model actually has one or not). Anyone else ever had this be the issue?

The second is a machine shop said it could be the piston skirt that collapsed and is letting the base of the piston move ever so slightly. He said that you can?t tell by looking? Sounds like a horse trader's comment to me, but at this point I?ve checked everything i can and nothing has come up as a "Here it is!" - just dead ends...

I figure a new upper and lower for $800 parts and labor isn?t a bad deal here in California, but if that doesn?t solve it I?ll be super bummed! Any suggestions or experience on the piston skirt issue?

 

[Uraijit]

Re: Knocking in 140 Merc

Is the drive down while you're running it in the driveway?

Like Chief said, replace the parts that you know are shot, and see if it goes away. Then go from there.

Those knocks can be tough to track down. Good luck!

 

[chiefalen]

Re: Knocking in 140 Merc

Why a new outdrive. Gears messed up? If not put it back on.

 

[Boater Bill]

Re: Knocking in 140 Merc

Sorry, wasnt clear on that 'upper & lower' - not the outdrive - the upper and lower block. I talked to a machine guy and he suggested doing both while i have it out. Sounds like a good plan, but still 800$ i wasnt planning on spending. The other option is making it a row boat with a really good sterio. What's your thought on the block rebuid?

Or could it be the rear transom mounts that are worn and the shaft raising and dropping and rattling the engine around bad enough that its clackin all over the place?

 

[Boater Bill]

Re: Knocking in 140 Merc

Update: 3 years later... I had a friend that knew a guy that knew about engines, he took a look at it and we pulled it, spun it and checked all the smoking guns. We took it to an engine guy and it turned out to be piston slap in #3 and #4. I had spun the engine around enough that the gas has 'washed' out the oil and the piston ended up scratching up the wall of the piston. We had to have the pistons re-bored and ended up having the engine mostly rebuilt.
If you have one of these strange knocks, then it's not an easy fix...best of luck to you all!