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Old 04-16-2011, 03:47 AM   #1
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Default 1999 2800 SCR engine problems

I have a 7.4L Mercruiser that runs fine for the first half hour or so and then for no apparent reason an alarm sounds and the engine goes into limp home mode. Replaced the port knock sensor (this was indicated by a diagnostics check), then I did a tune up nothing seems to solve this problem. This has been going on for a a couple of years and I have not been able to resolve this problem Definitely takes the fun out of boating. If anyone could help that would be great.
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Old 04-18-2011, 01:10 PM   #2
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What is the alarm doing? A constant tone, or chirping? what is the pattern (e.g. 2 fast beeps? etc) . Have you had another diagnostic check? Drive lube levels is a couple of chirps, overheating is a constant alarm tone, for example. This sounds like an overheating issue. Does it do it after 1/2 hr. at low RPM's or only when up on plane?
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Old 04-18-2011, 04:59 PM   #3
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Thanks for the help. The alarm is a constant beep until I shut off the engine. Once I restart the engine I will not get an alarm until I advance the throttle. I have not had another diagostics done since then. The engine temp is rock solid at about 175 degrees. Alarm sounds when up on plane. I have never checked if the alarm will sound after 1/2 hour while running at low rpm.
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Old 04-18-2011, 05:13 PM   #4
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well..this could be a couple of things....one might be the controller itself....bad connection someplace at one of the sensors....how bout the oil level in the outdrive reservoir.....if it's full..then probably a bad sensor....


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Old 04-18-2011, 05:19 PM   #5
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Is this a fuel injected engine or carb'd? Does it go into limp home the very moment the alarm starts sounding? Does this engine have two temp sensors, one that drives the gauge and another that feeds the computer (if FI'd)? Are the symptoms repeatable every time?

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Old 04-18-2011, 06:22 PM   #6
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well..this could be a couple of things....one might be the controller itself....bad connection someplace at one of the sensors....how bout the oil level in the outdrive reservoir.....if it's full..then probably a bad sensor....


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Which Controller and or sensor? Out drive oil was replaced last year. Had the problem before and after the oli was repaced. Thanks for the help.
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Old 04-18-2011, 06:24 PM   #7
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Is this a fuel injected engine or carb'd? Does it go into limp home the very moment the alarm starts sounding? Does this engine have two temp sensors, one that drives the gauge and another that feeds the computer (if FI'd)? Are the symptoms repeatable every time?

Dan
It is EFI. Yes it goes into limp mode right after the alarm sounds. Don't know about the temp sensors. Don't even know where to look for them. Yes the symptoms are repeatable. Thanks for your help.
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Old 04-18-2011, 07:06 PM   #8
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....how bout the oil level in the outdrive reservoir.....if it's full..then probably a bad sensor....


SP
Mine gives two fast beeps for outdrive oil level. One long constant (very loud) tone for overheating. I'd be curious to see what the temp. guage reads when the alarm goes off. Still 175? I think SS might be onto it.
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Old 04-18-2011, 07:30 PM   #9
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I'm thinking that your temp sensor is bad....or the computer (aka controller) for the efi is getting bad readings....which causes it to go into LHM......

if you can get a merc tech with a laptop and the merc software to plug into it..this might give you a better reading on what is going on....

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Old 04-18-2011, 07:34 PM   #10
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I wish I knew which faults caused the machine to go into protection mode. I'd be a little surprised if drive lube would do it (it did not on my 2004 EFI engine last season) but can see that temp and oil pressure would. Knock sensors - upon detecting detonation, tell the computer to retard the engine timing. I suppose if a KS were acting up it could force limp home mode but I'm just guessing.

A wiring diagram sure would be handy right now.

Regardless, if the engine is actually going into limp home then the ECM is making it do so based on some fault it has identified. Given that, it will have a fault code stored (I think, as long as the batteries have not been disconnected). If there are no codes stored then find out what sub system will cause an alarm and limp home but won't throw a fault code.

Sorry, not much help......

Dan
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