Mercruiser 7.4 MPI Alarm Sounding, but everything seems fine???

[Lucrative]

7.4 L MPI BRAVO GEN VI - L29 (made from 1998-2000)

0L609594

Boat seems to be running great. Water temp running at 150 to 160 all day, gear oil and engine oil levels good. Will run at up to 2800 rpm all day, no issues. Once I get to that 3k rpm mark, its only a matter of seconds before I get an alarm sounding on one second, off three seconds. It will keep going until I turn the boat off.

I have no idea what's causing this as the temp is good, oil levels are good, and the oil pressure is good. My local Mercruiser guy is telling me he can't hook it up to the diagnostic for 3 weeks. Hoping to not wait that long for a simple diagnostic, but I'm at a loss as to what's causing this alarm. Could it be a bad sensor somewhere? Trying to figure out how I can try and diagnose this. Are there any suggestions on how to isolate and test the sensors that throw this alarm?

Thanks......almost there guys. A few months ago I had this engine down to the block. I appreciate all your help getting it running. Feels like I'm pretty close.

 

[muc]

Has it done this since the repairs or did it just start the last time out?

 

[Lucrative]

I have had alarms go off throughout the process. This is the first time I've been able to "rule out" overheating because temp stayed at or under 160 the whole time.

 

[alldodge]

Try unplugging your oil lube bottle and measure the resistance across the two wires, should be open circuit or real high resistance

 

[Lucrative]

I haven't tried that yet AllDodge, I'll give it a try tomorrow and post results. I did just take the boat out and try to make the alarm go off, then disconnect sensors. I disconnected the sensor wires to the gear lube res. and it continued. I then turned it off and brought up to speed again, and the alarm sounded again. Not sure if that is a legit test or not.

Also, I'm trying to make sure I'm locating all the sensors that can possibly cause this alarm. I believe them to be part number 87-805605A-1 (oil pressure switch), 48952 (temp switch assembly), also could be 806193A33 (gear oil reservoir assembly w/ sensor).

I have no idea where the oil pressure switch or the coolant temperature sensor are located on my engine as I can't find a parts blowup with them on the engine.

 

[alldodge]

Not a good idea to remove the sensor leads while the engine is running. You should have a MEFI-3 ECM according to your S/N. The older ECM's where not part of the alarm for the three items you listed, they would close and electrically sound the alarm. The ECT (engine coolant temp) and low oil pressure are the ones which would set a code. The horn will also sound if there is a faulty sensor (one previously mentioned.

You have what is called a soft alarm but the issue is this should only happen below 3000 rpm. The hard alarm would normally happen above 3000 rpm. Your engine temp switch (item 6) is on the thermostat housing and the oil pressure switch (item 16) is below the distributer on the port side rear of the engine.

http://www.mercruiserparts.com/Show...705&bnbr=90&bdesc=SENDERS+AND+HARNESS+BRACKET

Would like to know what the resistance value is on the lube bottle. Can also check the resistance value of the oil pressure switch by taking the blue wire off and measuring from the stud to ground.

 

[Lucrative]

I discovered that my knock sensor on the port side was in pretty bad shape. When I pulled the wire going to it, a little bit of rusty dust came out and the wire looked rusted. The sensor itself looked rough, the plastic nipple on it was jiggiling around and the rubber seal around that was kinda loose too. Could this be the problem?

Havent gotten to the ohms check yet, Ill get to it soon I hope

 

[alldodge]

It's possible but I am unable to verify looking at the manual. You should have two knock sensors, one port and one starboard. The part number is 8M0077616 but the old number is 805544T. I bring up the old number because if you search on it you can find it a Rockauto for $29, but using the new number you have to get a Merc part and the cost is in the $120 range.

Try cleaning it and see if it stops.

Have you tried to go further in the throttle once you hear the alarm to see if it goes to a solid alarm?

 

[Lucrative]

So, I can actually get above 3k rpm if Im going from a stop and then getting up on plane. I was at 3500+ for a bit before the alarm went off. Now that Im thinking of it, it seems like it only really happens after I get on plane. If I take it slow advancing up rpms, Ill plane at about 3k, which kinda coordinates with when the alarm goes off. Dunno if that means anything.

 

[Lucrative]

Also, the alarm has been the same intermittent alarm each time, even when I was above 3k rpm. I havent pushed it too hard for fear of jacking something up, even though it appears all is well. The only difference I noticed was that sometimes there is a higher pitched squeal. No rhyme or reason or pattern (it seemed to me at the time). So a few beeps, then a high pitched beep. Then sometimes it might have alternated high and normal pitch. I dont think that means anything from what Ive seen online, but thought Id mention it.

 

[alldodge]

Get someone or something to measure the voltage at the battery and at the alternator when running.

You really need a scanner

 

[Lucrative]

Ok, I'll check the batteries and alternator. Just out of curiosity, what I looking for with the battery/alternator? I read just over 12V sitting, and 13 or so when running w/ my switch set to both batteries. Hoping to get the boat in the water tomorrow.

 

[alldodge]

Ok, I'll check the batteries and alternator. Just out of curiosity, what I looking for with the battery/alternator? I read just over 12V sitting, and 13 or so when running w/ my switch set to both batteries. Hoping to get the boat in the water tomorrow.

Then this may not be the issue. Looking for a weak battery or ALT. If you don't have a full 5V at the ECM this can also cause some problems

 

[muc]

You really should have the engine scanned by somebody who is very familiar with MEFI 3. That could save a lot of time and money.
But if you can?t, the service manual has a lot of non scan tests that can be done with a good meter.

Interesting that you bring up the knock sensor --- because this ECM is one of the few that is programmed to check the circuit by increasing timing advance until a knock is sensed. If it?s unable to get the engine to knock (sometimes due to high octane fuel) it will set an alarm and enter power reduction mode.

From page 5G-65 of MerCruiser manual #23
?NOTE: if fuel octane is too high, a false DTC 44 may be set.?

 

[Lucrative]

Oh my....I put 93 octane in it. Still probably have 50+ gallons in there. I had no idea I was supposed to run low octane. Always heard that higher octane was better, especially for marine engines.

Does it follow that the knock sensor would cause this particular issue? That it would only go off at higher rpms, or when on plane?

I'm going to replace that port side knock sensor, just because it is questionable to begin with. Gonna try and get one of the ones mentioned by AllDodge, as it's about 1/5 the price for essentially the same thing apparently. I took the boat out today after cleaning the knock sensor, and I still got an alarm. Everything still seems to run fine.

Tested the oil reservoir, got a high resistance when full and basically no resistance when empty, so it appears that is functioning properly.

Batteries are both running at 12.65V at rest. Not sure when running (we had a bad storm go through and didn't get to test everything), but gauge still shows 13+ when running.

Also, I have heard of "guardian mode", but my boat does not seem to do this. I can run at 3k+ after my alarm sounds and don't experience any power reduction when it sounds. Does that mean I do not have a MEFI3 computer in it?

Thank you all for your help and guidance. I can't tell you how much you all have helped me along the way with this boat. Truly, you've been a blessing.

 

[alldodge]

My new motor has a MEFI-3 and was throwing the code 44, which is if the knock sensor can not make it knock. The reason for it was a bad connection on the sensor and/or the sensor. Cleaned the sensor connection and it no longer set the code. When code is set it only reduces the total advance by 4 degrees. On my cruiser I barely noticed it was happening and there was no beeping that went off. It wasn't until the onboard tune was being done that the code was noticed. I'm running 93 but I have to because of a higher compression engine, and did replace the sensor for GP. Have checked the engine with my scan a couple times and the code 44 has not returned.

 

[NHGuy]

Oh you can still knock 93 octane, it happens at higher advance values. The other possibility though is that the system can only advance just so far. So if your engine is really stout, with low compression it might not be able to knock within the advance the distributor provides.
I think you are doing a smart thing to replace that messed up knock sensor.
How much room is in your fuel tank? If you have room, put in a bunch of regular it will lower the octane some.
Or, in a pinch you can remove some, but that's a fire hazard pain in the derriere.

 

[Lucrative]

I've ordered the knock sensor. Question.....is there a way to bypass this sensor so I can tell if this is what's setting my alarm? I'm pretty confident my timing is right in the sweet spot, but I do realize there is value in having functioning knock sensors. I'm just trying to get a solid diagnosis here. I've found the oil pressure sensor and I'm gonna see what the ohms meter tells me on that.

I'd love to get it scanned so I can just know what's going on, but the only guy within an hour+ of me with a scanner has told me I'd have to wait 3 weeks to get it hooked up to the scanner cuz he's busy :facepalm: :lol:.

 

[alldodge]

Don't know of any way to bypass. I'm figuring your boat is on a trailer, and if it is ask if you can hook up the scanner is his lot, and offer $50 for its use. It takes 5 minutes to hook it up and find out if there is any codes (I have one).

You find the DLC connector, remove the cover.
Connect the scanner to the connector and turn the ignition key to ON, (don't start)
Scanner finds the ECM, keep pushing select as things come up.
It will then say something like "scanning for codes"
It will either say no codes or show one at a time what they are

 

[Lucrative]

Ok, small update. Got my new knock sensor in. Installed, but no luck. I did discover that my starboard side knock sensor has no wire running to it. Pretty awesome oversight on my part, but I hadn't noticed this up until this point. It seems as though this boat did not have that connection when I purchased it, does that make sense? Would one knock sensor serve the purpose, as it appears that they run on the same line? Also, I'm not able to find a part number for the connection to the knock sensor so I can run a new one, anyone know where I can get this?

Still have not gotten to testing oil pressure switch and temp sender in the t-stat housing. Hoping to get to it soon. Ran it out yesterday and burned up a little bit of gas, so I'm gonna try throwing some 89 in there before I get it in the water again too.

Man, is that buzzing annoying.:frusty:

Edit: Found someone at a local marina that's got a machine that'll hook it up for me. Gonna do it.