My guess would be you have a cooling problem. A hot spot in the engine could cause the fuel to pre-detonate and give you your knock code.

Check your water pump, exhaust manifolds, risers, hoses, blockage in the heads or other cooling passages.

 

7.4 Horizons are all closed cooling as that was standard with that engine. Doubt there are hot spots in the cooling system unless you've badly abused that engine and/or overheated it. Assuming that it's has been well maintained...

Did they check and reset the engine timing with the EFI ECU in service mode? That's number one. Number two would be to use the diagnostic tester to read the amount of timing advance the ECU is dialing in and verifying that with an adjustable timing light. My money is on engine timing.

Too many mechanics are parts swappers and too darned few are diagnosticians.

Best regards,
Frank

 

Good points Frank. Although I would not assume that. Something could be caught in a passage.

I assumed the boat that I bought at 2 years old and owned for 8 years had the correct heads on it. It did not as i found out last year, thus part of my repower reasons. Square ports vs oval ports.

And you are right about some mechanics. Some are also relying on the code readers too much and do not go back to the basics- Fuel, Air, Spark, and I'll add cooling.

Had an Audi that was stumbling, Audi dealer could not find a problem with their code reader. Took it to my little mechanic and he found a bad spark plug wire. No other problem. Code reader could not find this.

 

Could be cooling. Timing is more likely. If it's not timing, good to check the cooling system, but non-trivial.

Interesting about the ports. The oval ports are for the standard 310 hp 454. The rectangular ports belong to the high output, 380 hp (Magnum, Horizon) 454. I didn't think that the wrong intake manifold would bolt up to the wrong head. Guess it does. Wonder how they butchered that?

I have a friend who works for a multi-brand dealership. He services mostly Porsches, but the shop also has guys servicing other brands. Interesting when I stop by to see him. There's always another guy asking him, "What do you think this could be?"

Best regards,
Frank

 

Knock sensors can also be fooled by vibration. I would check to make sure everything on and around the engine is tight.

 

Thanks to everyone for your comments; gives me a few more avenues to pursue; I will advise of the solution when I find it.

 

Another thing that can trigger the knock sensor is the grade of fuel you use. I was told that you cant run a higher grade than the 89 octane found at most marina's. Anything higher will cause problems with the ecu and the engine timing. At power up the ecu will self test the knock sensors and if all is well will run based on set parameters. The higher grade fuel is not part of this equation. So I was told.

 

Thanks Chuck; I was told that since marine fuel doesn't have the same fast turnover as street fuel, it looses its octane, since it's sitting in the tanks a lot longer (both filling stations and boats); I add stabilizer to spike it/ maintain the octane.

Henning

 

One more update; this may be it! We noticed that the wire to the knock sensor was in poor shape with many broken strands inside the covering, cleaning it up and reseating the connector, appear to have resolved the issue (for now)

Henning

 

I had the same problem several years ago and they replaced both connectors at the knock sensors after cutting wire back till they found clean strains. This was the second trip after the knock codes were found and sounding the alarm. Never had this problem again in five years now

 

hnorup, was the knock sensor the problem? I'm in a similar situation as you chasing an alarm where nothing is making sense!

 

I sure hope that's my situation.

At which end was the wire frayed? Near the connector or nearest the block/sensor itself?

 

There was rust/corrosion at the connector that attaches to the knock sensor. They replaced that connector, stripping a few inches of wire back to crimp it to.

 

If I remember correctly, the insulation was mostly in tact, but after wiggling the sensor around (after removing it to inspect) I noticed the insulation peeling back but I didn't see any wires connected inside. So obviously the wire corroded away inside the insulation.

 

Great information folks! I can't get a real good look at the sensor because of location. How does it physically attach to the block?

 

Screw in.

 

Screw in.

As in it is threaded and spins on or there is an actual screw to remove that allows it to come off the block?

 

Dale, it screws in like a spark plug, then you connect the little clip to it.

Make sure you have actual "Wire" under that insulation, cause like I said in the earlier post, the insulation pulled back a bit, and there was no wire connected to the clip. Good Luck:thumbsup:

 

Kreole that makes sense. Did you need to put a wrench to it or just get a good grip and twist?

Mine has a gray "pig tail" that's about 4-6" long with a connector on the end of that based on what I can tell from the mercruiser schematics. It's connected to the port side of the block.

 

Wrench it.

The 380 I worked on had V-drives so it is located on the port side, but our motors are reversed.

Our problem was on the port motor and man it was very trying to squirmy down next to that damn holding tank:smt089

 

mine is part 37 in this link:
http://www.mercruiserparts.com/Show...84717&ivar=images/COMMON/2225.png&inbr=8093&bnbr=40&bdesc=Electrical+Components

if you click on the PN, you can get a picture of it.

 

Our problem was on the port motor and man it was very trying to squirmy down next to that damn holding tank

Right - and mine is going to be between the port/sb engine in about 6" of space. I got intimate with my generator last weekend before giving up because I didn't have a good visual on how it connected. I'll be taking down a mirror and a light so I can figure out what tools I need.

 

I'll be taking down a mirror and a light so I can figure out what tools I need.

Good Idea:thumbsup:

Well I'm still on the hard waiting on struts to be shipped later this week. The repair facility is going to start on my bottom job this week. I'm going to put a few more coats of cleaner/wax on her towards the end of the week. Hopefully I'll be able to splash sometime next week.

The oil spill hasn't really affected my boating area so, I can't wait to get back out there. I haven't ridden on a boat in almost a month:smt013.
Actually, I'm on the board at my marina and we did a cleanup this weekend of our marina, but it was JUST in the marina so I don't count a ride in a little 16' flat boat as a "ride":smt009.

 

I was actually going to ask you how you guys were doing with the oil offshore. That's such a disappointment - I love the seafood down there and I'm sure it's going to put a real hamper on things. I couldn't get out this weekend due to 30+knot winds and a tide that had some boats sitting on the bottom. Nothing compared to an oil spill.

 

Follow us on "NOLA.com". That's the Times Picayune's website (Newspaper). That's the best website to keep up with what's going on down here.

I told my wife today, you would think that there would have been some preparation for something of this magnitude because it was bound to happen, ONE DAY:smt013.

 

Very interesting write up. We have been having a problem with an alarm on the port engine. After running a 20 mile lake all of a sudden an alarm goes off. If we shout down the port engine for about 1 - 2 min. and start it up again the alarm goes away. Any thoughs on this. Please help.:huh:

 

Connie G, that's the mystery and I'm trying to narrow down the sources. I promise if/when I find the source I'm posting my specific fix. I have another thread that I started in CSR with a topic of about whether the powersteering fluid can cause an alarm because I was getting so frustrated. Here is that thread. some of it is a little long but I was trying to keep an accurate account of what I was doing. http://clubsearay.com/forum/showthread.php?p=341310#post341310

 

Thanks Dale, please keep me posted. As soon as the boat hits the water we are taking it in to have it looked at.

Connie

 

Connie, you bet. Please let me know what they tell you. I've found a lot of "symptoms" posts with no resolution!