5.0 Mercruiser engine knocking

[ronrobbie7]

In my original post I thought that the problem came after I decarbed with sea foam. The engine only knocks at WOT but if I back off alittle it goes away. One other thing I did was put midgrade gas it the boat when I usually put regular. I changed the plugs thinking that maybe seafoam fouled some out but I took it out today and she still knocks at WOT. Other than that it sounds fine and runs fine, no missing, starts fine and spins a 17p ss prop at about 4200 rpms. Thanks

 

[180shabah]

Re: 5.0 Mercruiser engine knocking

Is it knocking or pinging. If it only does it at WOT you are most likely running lean, this will kill your engine rapidly. Start checking into your fuel system, from the tank vent all the way to the carb.

 

[180shabah]

Re: 5.0 Mercruiser engine knocking

OOHH!! I almost forgot, water in the fuel will cause a lean condition also, probably want to check this first before tearing into things too deep.

 

[ronrobbie7]

Re: 5.0 Mercruiser engine knocking

Thanks for the help. Could bad gas cause this problem because it didn't knock before the seafoam and I put half tank of midgrade before I went to the lake(previous owner had always used 87 octane and it ran pretty good so I stuck with it)?

 

[180shabah]

Re: 5.0 Mercruiser engine knocking

yes, bad gas or water contaminated gas can cause this. Or, perhaps your new(ethanol blended) gas has dislodged some trash in the fuel system that has found it's way into the carb.

 

[Daddy O]

Re: 5.0 Mercruiser engine knocking

Check your timing. If it is to far advanced it will spark knock (ping). Also higher octane fuel will help stop the knocking also.

 

[ronrobbie7]

Re: 5.0 Mercruiser engine knocking

Thanks, will try to get someone with timeing gun and check it for me.

 

[180shabah]

Re: 5.0 Mercruiser engine knocking

....Also higher octane fuel will help stop the knocking also.

Higher octane only masks the symptom, it does not fix the problem.

 

[Daddy O]

Re: 5.0 Mercruiser engine knocking

Your're right, but unless the spark knock is severe, it will cool the cylinders, and help keep the piston, or spark plug from melting. I was just giving another aveune of troubleshooting.

 

[ronrobbie7]

Re: 5.0 Mercruiser engine knocking

Finally checked timing and here's what I found. When first started it was 8* btdc and thats what the manual says it should be. After it warmed up it was at about 10* btdc. We adjusted it back to 8* after it was warm. Is that the right way to do it? And could 2* more advance give me the knock?

 

[180shabah]

Re: 5.0 Mercruiser engine knocking

Timing should not change with temperature. Or is this a TBV, in which case the setting procedure is totally different. Must be put into base timing mode first.

If other things are borderline, fuel quality, mixture, etc, then yes 2* could make a difference.

After resetting the timing, do you still have the knock?

 

[ronrobbie7]

Re: 5.0 Mercruiser engine knocking

Yes I do have thunderbolt ignition and don't know how to set base timing first. Have not been on water since.

 

[Haut Medoc]

Re: 5.0 Mercruiser engine knocking

TB IV or TB V?.......
It matters.....​

 

[ronrobbie7]

Re: 5.0 Mercruiser engine knocking

How do I find that, just says thunderbolt ignition on carb cover. It's an 89 5.0 mercruiser with alpha one if that helps.

 

[Haut Medoc]

Re: 5.0 Mercruiser engine knocking

TB IV......
All ya gotta do is put the timing light on the #1 cylinder wire & point it at the timing marks....​

 

[180shabah]

Re: 5.0 Mercruiser engine knocking

So, now the question would be why did the timing advance 2* after the engine warmed up?????

 

[ronrobbie7]

Re: 5.0 Mercruiser engine knocking

At least I know we timed it right. I also noticed the rpms seemed kind of high(if the tach is right between 1000-1100)at idle. The manual says 650-700 rpms idle in forward gear but what should it be in neutral? And can high idle advance timing?

 

[180shabah]

Re: 5.0 Mercruiser engine knocking

Shoot for 650RPM in gear, in neutral it will be what it will be.

Yes, at 1000-1100 the distributor is already adding advance to the timing.


Bring the idle down
Set the timing
readjust the idle if needed

 

[ronrobbie7]

Re: 5.0 Mercruiser engine knocking

Thanks for all the help and fast responses, I appreciate everyone who answered. Will do my final adjustments on the lake and tell how it worked hopefully by the beginning of next week. I assume it's best not to put in forward gear on muffs to adjust idle.

 

[180shabah]

Re: 5.0 Mercruiser engine knocking

I assume it's best not to put in forward gear on muffs to adjust idle.

It won't do any good, there is no load against the prop unless it is in the water.