Merc alarm going off - help

[28Eliminator]

My alarm went off 3 times the other day coming down from PC. Cruising and going slow it was fine.. Even seemed ok up to 50 or 60. Got on it a couple times and ran it hard, and after a couple minutes on full throttle the alarm sounds. Each time I shut it down and when I fired it back up after a few minutes it was fine.

Temp, oil pressure and drive oil were all good according to the gauges. Engine didn't smell hot.. Exhaust risers and manifolds were only warm, but the valve covers were pretty hot.

1996 Merc HP 500 cooling system and alarm system. New impeller beginning of the year (seems to be pumping water fine)

Could it be collapsing a hose? Maybe thermostat? Temp was fine, right where it always runs. Any suggestions?

 

[AzGeo]

Just scan it, and know what's going on .......

 

[28Eliminator]

Carb motor. No computer to scan.

 

[AzGeo]

Drive oil, engine oil pressure, water temp ......

Solid beep ? Short regular beeps ?

Often, hitting rough water setting it of would be 'low gear oil' in the bottle .

New impeller was mentioned, but what about 'sand or particles' blocking the engine oil cooler .

It can 'sound off alarm' when hot and then you slow down and that is enough to cool the exhaust, but not the block/heads quickly enough .

I hate to suggest 'test by operation', meaning disconnect the (one at a time) oil or temp sensor, and run the boat .

Why did you shut it off ? Would it not 'stop buzzing' if run at just idle ? It's not 'closed cooling' ?

Unless the engine oil level is low, I still like the 'clogged oil cooler' as the cause . Pull off the hoses, and 'back flush' with a garden hose to check blockage .

If that is not the problem, then you probably have a 'block full of sand' . Turn the pet cocks on each side of the block, (near the pad rail) and run some water into the top of the motor somewhere . (intake manifold) full up EACH SIDE of the block with water and let it 'flush out the block jackets' .

 

[kevnmcd]

Possible crack in the impeller housing that sucks air in at higher rpm's? Or as you suggested, collapsing the hose at higher rpm's?

 

[2FORCEFULL]

My alarm went off 3 times the other day coming down from PC. Cruising and going slow it was fine.. Even seemed ok up to 50 or 60. Got on it a couple times and ran it hard, and after a couple minutes on full throttle the alarm sounds. Each time I shut it down and when I fired it back up after a few minutes it was fine.

Temp, oil pressure and drive oil were all good according to the gauges. Engine didn't smell hot.. Exhaust risers and manifolds were only warm, but the valve covers were pretty hot.

1996 Merc HP 500 cooling system and alarm system. New impeller beginning of the year (seems to be pumping water fine)

Could it be collapsing a hose? Maybe thermostat? Temp was fine, right where it always runs. Any suggestions?

did you look in the drive oil bottle??? sometimes it the oil level isn't high enough or that ender goes bad..

I had one one time that it was the oil pressure sender... had oil pressure but the sender was bad

 

[2FORCEFULL]

Temp, oil pressure and drive oil were all good according to the gauges. Engine didn't smell hot.. Exhaust risers and manifolds were only warm, but the valve covers were pretty hot.

what gauge do you have for the drive oil???

 

[28Eliminator]

Drive oil, engine oil pressure, water temp ......

Solid beep ? Short regular beeps ?

Solid beep.

Often, hitting rough water setting it of would be 'low gear oil' in the bottle .

Bottle is full

New impeller was mentioned, but what about 'sand or particles' blocking the engine oil cooler .

It can 'sound off alarm' when hot and then you slow down and that is enough to cool the exhaust, but not the block/heads quickly enough .

I hate to suggest 'test by operation', meaning disconnect the (one at a time) oil or temp sensor, and run the boat .

Why did you shut it off ? Would it not 'stop buzzing' if run at just idle ? It's not 'closed cooling' ?

I only let it run for about 10 seconds at idle, it was still buzzing, all gauges looked good. Not closed cooling[emoji53]

Unless the engine oil level is low, I still like the 'clogged oil cooler' as the cause . Pull off the hoses, and 'back flush' with a garden hose to check blockage .

I'll make sure they do this.

If that is not the problem, then you probably have a 'block full of sand' . Turn the pet cocks on each side of the block, (near the pad rail) and run some water into the top of the motor somewhere . (intake manifold) full up EACH SIDE of the block with water and let it 'flush out the block jackets' .

I can't imagine it has much if any sand in it. Motor has only been in it for about 4 years (270 hrs approx). It had very little in it when I pulled the original motor (500 hrs, 16 years). I suppose it's possible though.

I used the original HP500 cooling system that also has a circulating water pump on the front of the engine. Could that somehow effect the block cooling different than the bypass system that cools the exhaust if it was failing?

 

[28Eliminator]

Possible crack in the impeller housing that sucks air in at higher rpm's? Or as you suggested, collapsing the hose at higher rpm's?

I have the Hardin marine stainless pump housing, so I doubt it cracked. The collapsing hose I'm thinking is a possibility.... But the temp gauge still said it was cool?? The hoses are all original, I've never replaced one.

 

[kevnmcd]

I have the Hardin marine stainless pump housing, so I doubt it cracked. The collapsing hose I'm thinking is a possibility.... But the temp gauge still said it was cool?? The hoses are all original, I've never replaced one.

Since you were on it what about some bad/old gas causing some detonation or maybe too much timing? That would most likely only show up and the higher rpm's and be ok and the lower rpm's. That would put out some extra heat causing the valve covers to be hotter than usual.

 

[Buster]

I had the same issue on my 496HO. Turned out to be a faulty oil temp sensor telling the ECM the oil was to hot and putting the motor into limp mode. Changed it out and it's been good. Oil temp does not go to a gauge so it was a bitch to find it myself. Needed a comp to read the ECM to find it.

 

[28Eliminator]

Since you were on it what about some bad/old gas causing some detonation or maybe too much timing? That would most likely only show up and the higher rpm's and be ok and the lower rpm's. That would put out some extra heat causing the valve covers to be hotter than usual.

Gas is new... I burn through a lot of it.. Lol. But I did put a new distributor in it and put a shorter curve in it. Total is at 34, which should be fine.... But now you have me thinking. Another thing to have checked for sure.

 

[Bucketlist]

I too had an erratic merc alarm on my 1998 502. Took three trips to mechanic before finally diagnosed and permantly fixed. I had a bad ground on engine wire harness where it connected to the block. Years of moisture under hatch caused corrosion which sporadically affected ground and electronics. Maybe this is your problem?

 

[BajaMike]

Could you have been over revving it??

 

[28Eliminator]

Could you have been over revving it??

No.. It was only turning about 5000.

 

[kevnmcd]

Gas is new... I burn through a lot of it.. Lol. But I did put a new distributor in it and put a shorter curve in it. Total is at 34, which should be fine.... But now you have me thinking. Another thing to have checked for sure.

Is the distributor tightly locked down? Maybe it moved somehow? Just a thought.

And I know what you mean about gas....I burn through a lot of it too. My 2 motors are thirsty suckers! I typically do about 150 miles on the water each weekend. :eek

 

[BDMar]

Call me. I have a few ideas

 

[trex232]

Was the old distributor a thunderbolt with the rev limiter, if you changed it you could be getting an alarm. I changed a 96 years ago and removed that part of the original harness.