Mercruiser 7.4 MPI Engine alarm

H

hnorup

New Member
Jun 17, 2009
8
Toronto, ON
Boat Info
380 DA 1999
Engines
Mercruiser 7.4 MPI
I have had a problem with an engine alarm (Knock sensor code 44) for the past 4 years on my STBD engine; Mercury certified mechanics have:
1. told me I have no problem
2. told me "it's dirty fuel add stabilizer" which I do (this diagnosis cost me $1,000)
3. told me the knock sensor was not not seated properly (this diagnosis cost me $1,300)
4. changed the knock sensor
5. changed plugs, wires, distributor caps, rotor

I still have the problem (i.e. getting the alarm when I run the engine over 2400 RPM); each time following work on the engine I get approx. 2 hours of normal operation; after #5 I only got 1/2 hour, thereafter it's back to max. 2400 RPM before I get the alarm

Has anyone experienced something similar and have a possible solution

Increasingly frustrated and getting poorer

Thanks
Henning

1999 Searay DA 380,
Mercury Horizons 7.4 MPI
 
Last edited:
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
 
Last edited:
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.
 
No Name Sea Ray said:
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.
Click to expand...

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
 
hnorup, was the knock sensor the problem? I'm in a similar situation as you chasing an alarm where nothing is making sense!
 
hnorup said:
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
Click to expand...
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 :)
 
I sure hope that's my situation.

At which end was the wire frayed? Near the connector or nearest the block/sensor itself?
 
hnorup said:
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
Click to expand...

I replaced the knock sensor and some of the wire on a buddy's 2000 380, and his alarm went away. I too noticed that some of the wire looked bad, that's why I went a little further back with the rewiring of it.

Hope that helps:thumbsup:
 
DaleM said:
I sure hope that's my situation.

At which end was the wire frayed? Near the connector or nearest the block/sensor itself?
Click to expand...
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?
 
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:
 
You must log in or register to reply here.

Forum statistics

Threads
113,170
Messages
1,427,736
Members
61,079
Latest member
capeharj
Back
Top