Logo

Engine knock 1986 260

swellraiser

New member
"Okay, here we go...

Starboardside engine knocks. 1986 Mercruiser 260 Alpha 1

Cold starts no problem, knocking get louder as engine warms up , then goes away. Rev the engine t0 2,500 knocking got louder.

Thinking (praying) fuel/sparkplug ping. I then added Seafoam 1/3 into the carb, no change then 1/3 into crank case. Knocking got quiet, then disappeared at Idle. Rev the engine to 2,500 still no knock, idle down and knock appears and then disapears.

Engine does not over heat and maintains a good tempature. No power loss. Engine sounds great otherwise.

One thing to note - my oil readings are different from one another - different sending units? What should the pressure gauge be reading?

I was told to baby the engine, more frequent oil changes and additives etc, If I replace an old Item with new items I may create more broken parts.

okay - clues, suggestions. And no I am not sure where exactly the knocking is coming from. I just don't like knocking engines - thanks for your helps."
 
"Well Steve, you kinda of solv

"Well Steve, you kinda of solved your own problem, when you put sea foam in crankcase and it got quietier, sounds like you may have a bad bearing on the bottom end!! I would try to find problem before too more hours or you might ruin the engine it can possible be rebuilt now, but if it throws a rod you ruin it. This is a great place to find a answer, Bondo, Bt Doctor, Guy and a whole hoard of high quality mechanics frequent this site.. Do a compression test and post results."
 
"Oil pressure: 30 - 55 PSI @

"Oil pressure: 30 - 55 PSI @ 2,000 RPM. The 1986 engine is probably very tired. Knocking that comes from carbon buildup in the combustion chamber doesn't come and go. It will make the engine shake violently and knock loudly continuously. Your problem is most likely worn rod and main bearings and general engine fatigue.

Time to repower and not flatulate around. You might as well rejuvenate both engines while you are at it. Perform the compression test, both dry and wet. It will tell you how the valves and rings are doing."
 
"Cheifalan, Great website lin

"Cheifalan, Great website link on reading the vaccuum test.

Thanks all for the input. I am going to be posting the results of compress and vaccuum once I get them done - possibly this weekend."
 
"The first order of businees w

"The first order of businees would be to determine if it is a detonaion knock, lifter noise or a bearing knock. A rod bearing knock can be isolated by reving the engine in neutral until the knock starts and then disconnecting one plug wire at a time. Without the cylinder firing, the bad rod bearing will usually quiet considerably or make no noise at all. Detonation or spark knock is is usually not present at idle speeds or at closed throttle. A 3 foot length of PVC pipe can be used to make an effective "stethoscope" to further isolate the problem. Lifter noises are easily found be running the engine with the valve covers removed and using a rag to cover one rocker arm at a time until the noise disappears."
 
"Like bob says use a pvc tube

"Like bob says use a pvc tube to listen,Is it coming from the top or bottom??Also feel its bearings,whats the oil pressure @idle?"
 
"Okay, I am not a mechanic by

"Okay, I am not a mechanic by trade, but I don't mind busting a knuckle or two.

PVC for listening, compression and vacuum checks - great ideas.

Run the engine with it's valve covers off....in my mind wouldn't oil get everywhere?

and "Feel" the bearings? How would one do that?

The engine is still in the boat...please esplain lucy...

One more thing what's a dry compression test vs a wet one?

Thanks Guys..."
 
"[b]"Run the engine with i

""Run the engine with it's valve covers off....in my mind wouldn't oil get everywhere?"

Ayuh,... Some,... But that's what Rags are for...

"and "Feel" the bearings? How would one do that?

The engine is still in the boat...please esplain lucy... "


Gotta leave that 1 for Russ... Kinda tough still in the boat...

"One more thing what's a dry compression test vs a wet one?"

Dry is just as the motor has been shut off...
Wet is when you add a tablespoon of Oil into the cylinder,+ retest..."
 
"Simple things first!
Warm e


"Simple things first!
Warm engine and learn to re-set rocker arm clearances. Like some have suggested, this may be a rocker arm clattering.... and with little experience, you are not seasoned to know how to eliminate or determine this.

Compression is seldom affected by a failing connecting rod bearing that is just beginning to make a noise, IMO. So compression testing may not render usable data in this particular case. Still not a bad idea to do, though, if you want this information.

On some occasions, a mechanical fuel pump and/or fuel pump push rod can cause a noise as well.

P of E... process of elimination... one item at a time!

."
 
Will the vacuum test show the

Will the vacuum test show the sticky lifter?

Guessing hmmmmmm .

For all we know it could be a water shutter / flapper flopping back and forth.
 
I know the anticipation is kil

I know the anticipation is killing me too...couldn't do any work this weekend got rained out...sorry guys will try again next weekend.
 
"I can't really hear it fr

"I can't really hear it from here so it's hard to tell! Listen to the experienced guys for testing things out and isolating your problems. Like Guy says doing the comp test will let you know if the rings and valves are in good shape. If your knock turns out to be a bad main bearing then it might be worth it pull the engine, pull the oil pan and try and replace the bad bearing. It's a heck of a lot cheaper than replacing the whole motor if it doesn't need it.

testing, Isolating and elimination is the cheapest way to go instead of replacing things willy nilly."
 
Re: Engine knock

OKAY an update - finally - I took the boat out and new problem arises, overheating at Hi rpm in port engine :-(. If you want to jump to my new questions, just skip the update...thx

So, I buckle down and replace all 4 risers, immediately I notice a difference - what I thought to be WOT was 3500 RPMs, Well a little riser change proved me wrong and the engines took a little longer warm up. Hurray for improvements.

All excited for this new found power, I was chopping at the bit to experience my new found RPMs. BTW knock still present.

I get the boat all fueled up, take her out and get her on plane, slow down for some seaweed, and then go to reengage...we'll new noise, hissing sound from carb on the Starboard side, same engine with the knock...

A complete lose of power, engined sounded like crap. I putterd at a slow 4 -5 knots back to the slip. The engine sync was all whacked out erratically, no forward momentum what so ever. I was thinking I must of spun the prop?

So, I decided to call in the professional mechanic...Engine compression tests all good, 150-165 across the the board both engines. The Mech said the hissing is obviously a carb problem. Vacuum test on this carb riddle problem well not so good. He also pulled each spark plug wire and seems only 4 cylinders were getting any spark...he said probably due to the carb (Rochester quad-jet) only feeding 4 cylinders.

The Mech also replaced the prop just to rule out any question and quoted me a price on a carb cleaning/rebuild, which I did myself. (Learned a new skill).

Installed the carb, all went very smooth thus far, engine cranks and starts, knock still present btw, the mechanic said the knock sounded like a stickylifter, but wasn't for certain...He admitted that the owner is better in determining these sounds, but to him sounded like a sticky lifter. :-\
--------------------------------------------
The mechanic found out that my oil filter was barely on and seeping oil into the bilge, now oil had always been in the bilge ( Just bought the boat a was cleaning it the best I could)

Q's:
1. Oil Gauge reads High always has. What are some things I should do to fix?
2. I am concerned with the engine sync seems way to bouncy and more difficult to maintain synced engines - I have yet to take the boat out under a load to see if my carb issue was my problem to the massive lose of forward momentum. Where do I begin to fix a engine sync issue?
 
Back
Top