496 help
 

one of my 496 h.o s keeps beeping twice a minute at idle, with the engine speed jumping about 300-400 each time. Does this about twice a minute under 1200 rpm, but runs fine over 1200 rpm. Merc scanner shows no codes, and everything normal. Does not go into guardian mode either. Tried a new iac, new batteries, and no luck. A little stumped here and looking for help . Thanks guys.

Check owners manual for a list of reasons for that type beep. Could be drive fluid level.

+1. Drive lube monitor. Low drive lube level.

drive lube warning won't cause a 300-400 rpm jump. Low drive lube would also show up as error on scanner.

It would for me, but then again I'm usually in the process of breaking a drive.

My guess as well....

Why would low drive fuild cause the rpm's to jump? I don't see the correlation there.

check your impeller in your water pump,,, there is a water press sensor in the pump,,,, try just unplugging it first with the engine off and if the beeping stops,,,, then thats probably it,,,the rpm could be a piece of the impeller binding in the pump,,,, if its an older 496 HO it could be the sensors in the Exhaust manifolds,,, again unplug and restart the engine

Is the alternator charging good under 1200 rpms?

My educated guess here is that you may have either some loose or slightly (corroded) bad ground wire connections or a bad or loose sensor connection on the harness at the sensors that is possibly intermittent. Try unplugging all sensors one by one and spraying some contact cleaner in each connection and clean and tighten all ground connections on engine.
Also since the idle is jumping up is this actually happening on the engine or are you just reading the jump up at your tach? This jump in rpm if at the tach only suggests the bad ground connection.
You can also consult the Merc diagnostic manual for the PCM555 and see which faults show up as that beep period and sequence.
Don't worry as you will find the culprit.

Best Regards,
Ray @ Raylar

the alternator is charging fine. Always over 12. Drive fluid is fine, I check it before every time out. Also, all the connections look good, and I have used contact cleaner on all the ones I can see. Next step will be,

- check exhaust manifold sensor
-and the impeller and water pump

Also, I have checked the diagnostic manual and nothing really matches up. Thanks for all the suggestions.

If by chance, the rpm racing and the beep are NOT related, then you could check to see if the foam piece is in the throttle body.

If it's missing, it will race at medium idle in or out of gear and will go away above 1200.

That's its job - to prevent varying engine speeds in the range you are talking about.

With regards to the beep, why don't you graph the various parameters and time it with the engine rev?

You may find it that way.

Is the reving fairly consistent? If left in neutral, will it race or rev about the same amount of time every time?

Regardless of how broken up or bound up the impeller could ever become...... There is NO WAY that engine would ever "feel" or notice that type of load. It would idle as normal as no small piece of veined rubber could ever change the rpm of that engine.

you may be on to something with the foam. The surge is consistent with the beep. thanks

If they are related symptoms, the foam will not trip the alarm. The lack of the foam will ONLY cause a racing symptom.

Run it under load with the scanner and graph the sensors.

Check plugs, wires etc. It's real hard to troubleshoot on the internet.

What Merc Scan tool did you use? The DDT can be too slow for certain issues. The symptoms can fall between the refresh rates...

Get someone to help you locally. Good luck -

Check the ignition wires under the dash... after a week of chasing my tail that's what it was. :angry-smiley-038:

if I remember correctly there is a ground stud on the back of the block that is hidden under the water hose that runs across there, my stud was all corroded and those ground wires weren't making a good connection. I sold my boat a few years ago but I think I am remembering it correctly it was a 2002 28 daytona with 496's

ok gentleman, ran the boat today and have a update. First the foam was missing, and a new piece fixed the rough idle, and extra noise i had. Sweet. But, I still get and a temporary surge in rpm any place under 1200 rpm with a beep. It sounds just like when you start the boat from key off. Also, it died a few times at idle when shifting. Any suggestions? Thanks for all your help.

Check the digital trim limit sender on the starboard side.

FYI - the foam is to prevent a slight & consistent engine race; it does not have anything to do with a "rough" idle but you will notice a difference without it for sure.

496
 

ok new update. Purchased the diacom software to help solve my problem and still cannnot trace it down. The problem engine still does the surge. It is worst at 1000 rpm. At that rpm it will surge and throw a warning beep almost every four or five seconds, its just like when you first start a engine. It will beep, rev up, then come down again. Just like the ecm resetting? I scanned it multiple times, one time it threw map hi, crank sensor, and cam sensor codes, but after clearing them and running again with same issues no codes after? Also, after about 40 minutes of using the diacom software the ecm would not connect to the diacom software anymore so i am stuck for now.

Thanks for all the time and help so far guys, I appreciate the help. Also, this is a idle to 1300 rpm issue only. Fine running above 1500. Sorry about the long post..

Have you tried swapping the ecm's to see if the problem follows?

ok everyone, new update. ecms swapped to no avail. Fuel pressures good, no codes, grounds clean, and cannot find loose connections. Still the same symptoms. I may do plugs n wires next i guess, but i did the cylinder test, and all cylinders seemed fine on the diacom. Motor runs fine over 1500 rpm. Also fuel filters fine

What year model is this Merc 496 and have any other mods been done or added?

Best Regards,
Ray @ Raylar

06 models, bone stock

count the seconds inbetween beeps that will give you a direction to go... they also have a problem with the water psi sensors in the manifolds and also in the rear of the motor fuel / oil log ( not shure which one )

A tough one
 

My belief here is that this is an electrical gremlin that is showing itself as an intermittent interrupt on possibly the ecm power up feed and the ecm is cutting in and out on a very short interrupt basis at lower rpms possibly aggravated by the engine vibrations and loading changes. Other than that idea, I am stumped at this point with whats causing this idle rpm spike.
Hope its not in the harness wiring as that is a pain to find and fix.

Best Regards,
Ray @ Raylar

I think I saw a thread somewhere before, mechanic chasing his tail, ended up being plug wires...finally saw them arcing after it had gotten dark outside. The harnesses are a rat's nest in my opinion.

Have you already tried to switch the idle air valves ?
maybe there is a gremlin there that does the engine rpm fluctuating ? Maybe worth a try

yes, new iacs. Thanks for the suggestions. ill keep hunting

try swapping some of those relays around,,,check fuses and relays for corrosion

Im with Raylar on this one,

Do you have access to anything like this?
http://www.powerprobe.com/powerprobe/PPKIT03.html

If you have a voltage drop issue due to corrosion or a bad harness a simple volt meter will not work. My friend had a similar problem on a acura, it would throw all kinds of codes none of which were valid. Voltage to pcm showed fine, voltage with my power probe and a 2 amp load showed less than one volt. A pin was backed out on a harness, causing voltage drop under high load.


Try the plus wires if you wish, might need them anyway. If theat doesn't work clean all main grounds, all main + connectors, disconnect all harnesses and check for pins backed out. Back probe all votage and ground connections to ECM using a power probe or loaded meter.

Good luck

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