Engine Noise at WOT

[kwoolard]

I purchased this boat back in June of this year. It is a 1999 235WA Sea Pro with a 125 gallon tank, 5.7L 350, Alpha Gen II with a 2 barrel mercarb. I am currently running a 19 inch pitch propeller and have a WOT of 3900-4000 rpms with 4 people aboard. Over the past week I changed my plugs, installed new riser and manifold gaskets and serviced my outdrive. The problem I have, which existed before I performed the maintanance work, is that when I push it to WOT I get a rattling noise from the engine. The best way I can describe it is putting a couple of steel BB's in an aluminum can and shaking it. It is only making the noise when I run high rpms, ie., above 3300 or so. Does anybody have any idea as to what this noise is? The engine runs great, idles great, I just can't get the recommended WOT and have this engine noise. I am going to try a 15" prop this week to see how that helps the WOT, but my main concern is the engine noise.

Thanks

 

[Haut Medoc]

Re: Engine Noise at WOT

How did the exhaust shutters look?......
You are running a 19 pitch & what diameter (14)/....
You should be getting 4600 wot with that prop.....
Somethind is amiss.....
I'd do a vacuum test to check for a possible exhaust restriction......
Can you localize the noise?......

 

[79Glastron]

Re: Engine Noise at WOT

What grade of gas are you running? Sounds like its pinging (pre-ignition), have you checked the timing?

 

[MikDee]

Re: Engine Noise at WOT

Be careful with this one, it sounds like preignition. If it continues it could blow your motor. You must find the source, either it's overpropped, overloaded, overheated, running too lean, too much spark advance, and, or, excess drag from something on the hull under water.

 

[kwoolard]

Re: Engine Noise at WOT

The exhaust shutters looked fine....in their proper place, covered with that rubber coating. I haven't been able to isolate what part of the engine the noise is coming from, as I have been at the helm everytime I hear it.

The diameter of the prop, I believe is 14-1/4".

Would a vacuum test tell me whether it is running too lean? The old plugs were black with carbon soot. I assume the vaccuum test should be performed over the range from idle up to WOT?

I will check the timing and the advance tomorrow.

It is definitely not overheating, as it runs around 150-160 at WOT and idle.

I am running regular 87 octane gas. Should I be running premium in this motor?

 

[wire2]

Re: Engine Noise at WOT

You could try 1 tank. If the noise stops, it was pinging.
You shouldn't need premium, but at least you'll know.

 

[kwoolard]

Re: Engine Noise at WOT

I just want to pick you guys brains a little about pre-ignition. From my understanding pre-ignition is the act of the combustion gases igniting before the spark plug fires. Everything I have ever read about pre-ignition states that it is usually not detectable by the ear and causes pretty much instantaneous failure. Would the rattling that I hear not be considered detonation, the ignition of the combustion gases prior to the flame front reaching the end of the compressed gases? If so, wouldn't that be considered spark knock and my spark knock sensor recognizing that and retarding my timing?

 

[kwoolard]

Re: Engine Noise at WOT

I was just reading through some old posts here about vortec heads and octane rating and get the impression that I should be running at least 89 octane in this motor since the vortec heads were originally designed for fuel-injected motors. Is this correct?

Also, if this is actually spark knock that is occuring and my knock sensor is not picking up on that, how do I test the knock sensor.....do I check the resisntance across the sensor?

 

[Don S]

Re: Engine Noise at WOT

Have you actually put the distributor into base timing mode and checked the timing? Then with it out of base timing mode, recheck the timing AND advance timing.
There are tests for the knock sensor and knock modules.
Go to the link below and download the manual for your engine (Should be #24) it will explain how to check the knock sensor and everything else.

http://www-alt.mercurymarine.com/mnetdata/sisdata/srvcovr.pdf

 

[kwoolard]

Re: Engine Noise at WOT

I will check the base timing this afternoon. Should I check the advance timing under load out on the water or in the water just increasing the rpms in neutral?

 

[Don S]

Re: Engine Noise at WOT

In neutral is fine, it works strictly off rpm.

 

[wire2]

Re: Engine Noise at WOT

Most knock sensors are mechanical microphones. You can fool them by tapping on the block with a metal tool, and the engine should slow from ignition being retarded.

But follow Don's advice, check it properly by the manual.
#24, page 4B-16

 

[kwoolard]

Re: Engine Noise at WOT

Sorry for getting back to you guys so late, I have been out of town the past couple of weeks. Got back this weekend and wanted to test the boat out with a different prop. I put a 15" pitch prop on and my rpms are now at 4600 wot, and there is no more pinging or spark knocking at wot. Forgot to carry the timing light with me, so when I got back to the house hooked up the muffs and checked the base timing with the purple/white lead grounded. I have a timing light with the advance dial feature, which works great since the only thing that I can read on my balancer is the machined groove. With the ignition in base timing mode my initial timing at 650 rpms is 28 degrees and advancing the rpms to 1500 rpms puts the timing at 30 degrees. I then disconnected the ground jumper from the purple/white lead and checked the timing, it was at 25 degrees.

I know this is not correct, but how is the motor idling and running so well with the timing set like it is. Or is the machine groove on my balancer not 0 degrees TDC???

 

[Don S]

Re: Engine Noise at WOT

When you put it in base timing mode, all you are doing is LOCKING it so it doesn't move and try to change the timing automatically while you are adjusting.
What it does in base timing has nothing to do with what it does OUT of base timing.

 

[kwoolard]

Re: Engine Noise at WOT

I understand how the base mode operates, but how is my engine running so well with the base timing at around 28 degrees and the spec calling for 10 degrees initial advance?

I purchased a tune up kit, distributor cap and rotor, installed it this afternoon and the timing went to 25 degrees in base mode and to jumping around 28 degrees out of base mode. The contact points in the old distributor cap had some minor corrosion build up, I figured that my be causing some increase in the advance, which it looks like it may have a little.

However, should I adjust the initial timing back to 10 degrees to see how my engine reacts?

 

[Don S]

Re: Engine Noise at WOT

It's NOT running better, just faster and smoother at no load to an untrained ear.
Wouldn't work worth a dime under load.

 

[kwoolard]

Re: Engine Noise at WOT

It runs great under load as well.......am I missing something?

 

[Don S]

Re: Engine Noise at WOT

Probably, but we aren't there and have no idea what you are seeing and doing. Just your words typed here.

 

[kwoolard]

Re: Engine Noise at WOT

OK....

I thought I had explained what I was doing in my previous posts...........just tyring to understand my current timing settings.

Guess I will put it back into base timing mode, adjust it to 10 degrees, and see how it runs, especially under load.

 

[kwoolard]

Re: Engine Noise at WOT

Once the cool weather is over in my area I am going to readdress this timing issue.

Does anybody else have anything to add as to how the timing at idle can be 28 degrees and still run well under load at high rpms?

Does the ICM on a Thunderbolt 5 recognize that the timing is already advanced fully and corrects it by retarding the timing at higher rpms, or does it just add the timing curve to the initial base timing? I guess in other words, if my initial timing is at 25 degrees at idle, at 3000 rpms would it add another 20 degrees to make it 45 degrees at 3000 or would the ICM recognize it is already fully advanced?