1987 4.3 Merc, Blow by and knocking

[Evinrude Boater]

Started the engine and sudden increase in blow by and knocking caused me to idle a few minutes to the launch and pull the boat out. Cold compression as follows:
1 - 155, 3 - 155, 5 - 135
2 - 165, 4 - 160, 6 - Too low to register
A couple squirts of oil in #6 brought it up to about 50 psi, my gauge starts at 50 psi.
The spark plugs are normally tan and I would describe them as dusty. These look quite different. 1 and 4 seem to have a shine and 5 is rather sooty. #6 looks rather normal yet that cylinder has the worst compression.
What do these plugs reveal? Do they indicate water in some of the cylinders? I'm thinking a leakdown test is the next action for #6. Is 135 psi in #5 acceptable.

 

[Biged007]

Re: 1987 4.3 Merc, Blow by and knocking

What is this an endemic has this engine been running all along? I don't like the Knocking thing.

 

[JustJason]

Re: 1987 4.3 Merc, Blow by and knocking

Yep, a leakdown is your next step. But if your getting 0 PSI then you either have a big old hole in the piston, or the valves aren't opening/closing correctly.

 

[Biged007]

Re: 1987 4.3 Merc, Blow by and knocking

Yep, a leakdown is your next step. But if your getting 0 PSI then you either have a big old hole in the piston, or the valves aren't opening/closing correctly.

Yeah I really! don't like the Knocking thing.
Hey Jason Pm

 

[Evinrude Boater]

Re: 1987 4.3 Merc, Blow by and knocking

Yeah, it has been running well. The last time I had it out we had the family in it running at various speeds including WOT. It sounded and smelled good. The next time it's knocking from the start and we're overcome with fumes. Hmmmm? There is some compression, not zero. Maybe a little water seeped into the cylinder(s)?

 

[John_S]

Re: 1987 4.3 Merc, Blow by and knocking

I would do a leakdown on #5 as well. Did you hear knocking while doing the compression test? If so, was it every revolution or every other? The latter might indicate valve train vs rod. Also, removing the valve cover might quickly locate issue. If valve is stuck open/bent, rocker and/or push rod could be bent.

 

[Aloysius]

Re: 1987 4.3 Merc, Blow by and knocking

I see no color change on the ground electrode, and flyspecs. Indicates lean or over-advanced ignition. Combustion temperatures have been too high.

This has nothing to do with operating temperature.

Are you running ethanol tainted fuel??

 

[Evinrude Boater]

Re: 1987 4.3 Merc, Blow by and knocking

I changed the timing recently. It was at 6 degrees BTC and I changed to 8 as the manual indicates. I don't think I've had any ethanol blended fuel. In fact it has been on the same tank of fuel all season.

 

[Aloysius]

Re: 1987 4.3 Merc, Blow by and knocking

Initial timing is just a rough setting for TOTAL advance, which is the essential element. Ethanol laced fuel tends to lean out a carburetor, since the ethanol is there strictly as an oxygenator. If you don't have ethanol added..great! However, a plugged jet or low float level can also cause a lean condition.

In any case, time to pull the motor.

 

[John_S]

Re: 1987 4.3 Merc, Blow by and knocking

In any case, time to pull the motor.

Do the leakdown test and evaluate whats wrong before pulling engine.

 

[coheej]

Re: 1987 4.3 Merc, Blow by and knocking

Do the leakdown if you want the practice, but the blowby says you have a ring or piston problem. The knock is most likely lower end problem. As stated above, you might as well pull the motor and check everything.

 

[Evinrude Boater]

Re: 1987 4.3 Merc, Blow by and knocking

The leak down wasn't required. It doesn't hold air. It didn't even spin the engine over when I injected air. I tore it down and found the cylinder wall is scored, pitted and corroded. For interest sake, any ideas how this happened?

 

[coheej]

Re: 1987 4.3 Merc, Blow by and knocking

What did the piston look like? Does the boat sit for quite awhile between uses? Moisture in the cylinder could have started the problem. How did that rod bearing look; you said you heard some knocking?

 

[Aloysius]

Re: 1987 4.3 Merc, Blow by and knocking

I suspect a combination of problems. Water in a cylinder can bend a rod easily on start-up. Running street engines full throttle without addressing carburetion or ignition is the kiss of death. Ethanol tainted fuel makes the problem even worse.

These stock engines aren't designed for sustained high rpm full throttle operation like outboards. Just setting initial timing at idle doesn't cut it..you are ASSUMING that the correct timing at high rpms is occurring. If you assume wrong, damage results.

 

[Evinrude Boater]

Re: 1987 4.3 Merc, Blow by and knocking

What did the piston look like? Does the boat sit for quite awhile between uses? Moisture in the cylinder could have started the problem. How did that rod bearing look; you said you heard some knocking?

In my climate the boat only gets used 3-4 months out of the year. I didn't tear it down any further, it's still in the boat. From my estimation, that damage looks old.
I never run it WOT for more than 30 seconds or so. I've only had it two years so I don't know what's happened during the previous 21 years. I'm trying to learn how to detect and avoid this in the future.
If cooling water leaks into the cylinder, how will I know?
Should I remove the plugs after layup and spin the engine over to expell moisture before starting it?
How does one check exhaust manifolds for leaks on a yearly basis?
The exhaust and intake manifolds are going on the replacement engine so how can I be sure they're good? Should I get them acid washed and professionally tested?

 

[Evinrude Boater]

Re: 1987 4.3 Merc, Blow by and knocking

It's getting worse the deeper I dig. I took the port head off and two cylinders have some corrosion on the walls and lines that looks as though it rusted where the pistons stopped. I thought I saw a piece of the intake gasket in the intake galley but no, it was a piece of a piston laying between the #1 lifters and the block. How can an engine cough up a piece of the piston skirt up above the cam?

 

[JustJason]

Re: 1987 4.3 Merc, Blow by and knocking

It can't, sure it's a piece of the piston?

 

[Evinrude Boater]

Re: 1987 4.3 Merc, Blow by and knocking

It can't, sure it's a piece of the piston?

It sure looks like piston to me. It has the same curvature. It's been tumbling around for a while judging by the nicks in it. I'll have the oil pan off today to look for more souveniers.

 

[Evinrude Boater]

Re: 1987 4.3 Merc, Blow by and knocking

Well, it's confirmed. Piston skirt from #4 broke off and ended up beside #1 lifter. All tolled, four out of six cylinders are damaged in some way. It's a door stop now waiting for replacement. It was a good 23 years. Can't expect more but with proper winterizing by the previous owner it likely would have lasted a good long time more.

 

[JustJason]

Re: 1987 4.3 Merc, Blow by and knocking

Piston skirt from #4 broke off and ended up beside #1 lifter.

How the ell did you get the beans above the frank???? It's a remote possibility a chunk could get tossed up there,,, but the odds are way against that!