Need some help ... I had a 2002 Formula 330SS surveyed today with twin 496mags and the port motor had a knock Computer diags confirmed 157hrs. and no other codes worthy of note. It is not a motor knock, but a popping more like misfire or wires were crossed kinda pop/knock, but bottom line it prevented us from running at speed. Motor starts and idles fine.

The only other clues are that a warning alarm is sounding throughout while running that motor and doesn't shut off. When checking the Smartcraft codes there are none present indicating an issue, but yet one manifold shows temperature is reading 257* on the Smartcraft and the temp is NOT 257* (prolly a bad sender?!) ... we confirmed this false temp reading through multiple means:

1) infrared gun - reads 78*.
2) temp gauge at the dash reads perfect and consistent with other motor at 165*.
3) coolant temp is on the money does not elevate ... lastly,
4) by touching with hand and it is cool (not 257*). It feels the same temp as the other 3 risers and manifolds.

So that said, Question: Could this ecm Smartcraft reading of 257* be throwing off the computer on the 496mag and not allow it to run at specs and possibly causing the knock?? The knock is not at idle, but only when trying to get up on plane.

Any input or experience with these symptoms, please offer them! Thanks Chris

had the same problem,after 2 weeks of diagnosing this and that I took it at a shop and we ran it on a dyno,could not get it to knock on the dyno,we found the hub on the prop worn,I never thought the noise was coming from the prop it sounded like the engine was ready to come apart,but once we replaced the hub the knock was gone.

I would also think with the knock sensors if the engine was knocking you would notice a lack of power as well because the ecu would retard the timing.

Thanks boatnt ... Yup, I kinda agree with knock sensor timing retarded deal.

I certainly would/will not rule out anything at this point. As of my departure today, the broker has a Merc certified guy looking at it to diag and correct. I just wanted to bounce off the forum here for opinions.

Possibly an exhaust gases leak near the temp sender????maybe???

That could explain the knock sound and the sensor reading. Just an idea though.

My guess is that a bad exhaust manifold temp sensor, they are cheap to replace and they go bad easily, especially the early all plastic ones. Even though the SmartCraft does not appear to be showing a problem, the all the time alarm would indicate the engine is in a Guardian mode and cutting back on power by switching off cylinders which can hence make the engine run rough and sound like its knocking ( probably drive line slapping) under a heavy load in that big Formula, as well as hold power back from a performance standpoint.

If it were me I would hook up a scan tool to the diagnostic plug and check for any open trouble codes and look in history for any Guardian events.

At about $17 , the sensor if bad is a cheap fix and easy to replace on the engine. No sense passing on a nice bost for a possible $17 bad sensor. If its not a sensor problem and there is some other internal engine problem, you certainly want to know now before you close the deal!

Best Regards,
Ray @ Raylar

The nice thing about a twin engine boat is you got everything you need next to you,if you thing the sensor is the problem just switch them and see if the issue transfers to the othe side

I had the same problem. It was a bad temp. senser in the head. Replaced and solved problem.

Great feedback guys .. ((was waiting for Raylar ))And I absolutely agree with not passing on this deal if all checks out and we eliminate the issue! Thanks again, I will let you all know their finding in the next day or so.

Regarding the highlighted area above in your post, If the Exhaust sensor does not fix it, Then The other thing you should have them look at is the spark plugs, The AC spark plugs that where used for these and other like engines Had some internal problems and made a lot of engines run the same way you are talking about here, Also if thats not it then try running the engine out in the dark and look for sparks from all the coils going to a ground. Also you could check to see if your engine serial number is within this service bulletin or not, And if it is then see if it was checked out, But if it's truly a 2002 engine then you do not need to worry about this at all. Just being helpful, And you did asked for opinions
http://www.boatfix.com/merc/Bullet/01/01_06.pdf


[Also as a FYI, Some one eles had a Question that went like this] The 496HO has heat sensors in each exhaust manifold. Do these read temperature for the purpouse of adjusting the air/fuel mapping or are they simply for temp alarms triggered by lack of water flow in the manifolds?

[It was Answered like this]
I looked it up for you, And a mercruiser service book said this.
There are two exhaust manifold coolant temperature [EMCT] Sensors, One in each exhaust manifold [port & starboard] They are located at the mounting flange between each manifold and it's elbow. Both EMCT Sensors are two wire sensors and their primary purpose is to tell the Guardian program the temperature of the coolant in each exhaust manifold. Both EMCT sensors are thermistors and operate in the same manner as the ECT sensor. A failure in either EMCT will set a specific fault for that sensor.

So with the highlighted part said , That should mean, But i'm not 100% sure that as the temp changes so does the fuel delivery to the injectors like the engine coolant temperture does.

Thanks Boat Teck I will have them check out / replace the plugs as well! Certainly can't hurt, but will use process of elimination and go thru step by step ... and I sure do appreciate the opinion!!

Thanks! Chris