Knocking sound from Mercruiser 140

[padn]

I changed rotor - but missed to place it properly. When I cranked the engine the rotor stopped somwhere in the distributor, causing a misfire (explosion backwards out of the carb) and a sudden stop of the engine while cranking (starter bolt broke). After changing starter bolt and replacing rotor and distributor cap, I started the engine up again - it ran well - but there was a knocking noise that seemed to come from a valve.
I ran the boat for approx. 15 min at full speed (5000 RPM) when the revs dropped to 4500 and later 4000 rpm. The noise had become worse. Got home on 1000 RPM.
Thought it was a broken valve lifter (caused by the misfire) - all 8 lifters was changed. Started the engine up again - but there was no change - At idle there is hardly any knocking - it runs smoothly - at approx 1000 RPM the knocking starts.
Can anybody help me solving the knocking problem ?

 

[Bondo]

Re: Knocking sound from Mercruiser 140

Just a WAGuess,............ You've bent a Rod.......

 

[padn]

Re: Knocking sound from Mercruiser 140

Don't think so - I checked the rods when I changed the lifters - they seems OK.

 

[Coors]

Re: Knocking sound from Mercruiser 140

Checked for spun bearing? Something is in a large bind somewhere.

 

[180shabah]

Re: Knocking sound from Mercruiser 140

Let me see if I understand this, first you discovered the knocking sound, then you decided to run wide open for fifteen minutes?????

 

[Robj]

Re: Knocking sound from Mercruiser 140

Start by pulling each plug wires to determine which cylinder it is coming from. When the wire is pulled and there is no knocking, that is where your problem is. When you broke the starter bolt was it because it was hydrolocked?? If so, I think Bond-o may have been referring to a connecting rod, not a push rod. From my experience valve noise is more of a clacking, not really a knock.

Good luck and have a great day,

Rob.

 

[padn]

Re: Knocking sound from Mercruiser 140

"Let me see if I understand this, first you discovered the knocking sound, then you decided to run wide open for fifteen minutes?????" --- PADN: Not very wise - but yes - didn't realize the seriousity before the revs dropped (I've had an ignition problem which I was testing).

 

[180shabah]

Re: Knocking sound from Mercruiser 140

You had an "incorrectly timed" combustion event that broke a starter bolt, now the engine is knocking. I agree withe the above posters, it is in the bottom half of the engine. You can play with it a little more if you would like, but it is still going to have to come out for any repair.

Next question, What engine are we talking about???? I assumed 4cyl, cause you said "all eight lifters were changed", and I hope it was a volvo cause you were running 5000RPM.

 

[padn]

Re: Knocking sound from Mercruiser 140

It's a 4 cylinder Mercruiser / GM 181 - engine.

If it is to any help, the knocking noise dissappears for some seconds when I add oil while the engine is running.

 

[180shabah]

Re: Knocking sound from Mercruiser 140

OK, which one???
WOT for the 3.0L is 4200-4600
WOT for the 3.0LX is 4400-4800
Either way 5K was over-reving. You need to read through the Factory Service Manual, I am sure you don't want to destroy your next engine.

Here is a free online copy courtesy of DonS

Mercruiser Service Manual Number 13

 

[padn]

Re: Knocking sound from Mercruiser 140

It's a Mercruiser 140. vintage 1979 (guess it equals the 3.0L).

I thought it was not possible to damage the engine under load (maybe it was only 4800 RPM) - but anyway it was more than WOT 4200-4600

 

[180shabah]

Re: Knocking sound from Mercruiser 140

I thought it was not possible to damage the engine under load

Why wouldn't it be possible? Perhaps it is time to call a mechanic.

 

[padn]

Re: Knocking sound from Mercruiser 140

I all ready have - thanks for your help - though still I am looking for a reason for the knocking sound - if I've over-reved the engine, maybe the power loss has nothing to do with the knocking sound which were there before the 5K.

 

[180shabah]

Re: Knocking sound from Mercruiser 140

though still I am looking for a reason for the knocking sound - if I've over-reved the engine, maybe the power loss has nothing to do with the knocking sound which were there before the 5K.

Again see above

You had an "incorrectly timed" combustion event that broke a starter bolt, now the engine is knocking.

This is the REASON for the knocking sound

Over-reving was subsequent poor judgement, but essentialy irrelevent.
You have a bent rod, damaged bearing or pin or something in the rotating assy.

 

[padn]

Re: Knocking sound from Mercruiser 140

Again see above



This is the REASON for the knocking sound

Over-reving was subsequent poor judgement, but essentialy irrelevent.
You have a bent rod, damaged bearing or pin or something in the rotating assy.

Sounds like a new engine is what I need.

 

[padn]

Re: Knocking sound from Mercruiser 140

Hi All

Thanks for your help - For your information I had a spun rod bearing, that caused the oil pressure to drop - which killed all the other bearings as well. Now I had the rods and the crank shaft overhauled and installed over size bearings, replaced valve lifters, replaced compression and oil rings and installed a new cylinder head. Almost a new engine ..... but it seems like there is still coming a knocking sound at approx 1000 RPM - not at idle 700 RPM and not over 1200.

The engine runs fine - plenty of power and has been running for 2-3 hours at different speeds with out change in the sound of noise - the oil pressure is 40 psi at idle and 60 psi at 4000 RPM

The sound is not metallic sound as before its a more "dry" knocking - (a little like a diesel engine).

Can anybody tell me whether the description is normal (or what the problem is / could be) or should i fear another melt down ?