How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Charles Your Own Question
Charles
Charles, Marine Mechanic
Category: Boat
Satisfied Customers: 3386
Experience:  US Navy (Ret) Certified Mercury Marine Tech with over a decade of repair experience.
67216606
Type Your Boat Question Here...
Charles is online now
A new question is answered every 9 seconds

I have a mercruiser 496 mag ho, and it won't rev over 3000

Customer Question

I have a mercruiser 496 mag ho, and it won't rev over 3000 rpms. No beeps or alarms changed plugs fuel pressure is good. Plugged in scanner no codes that would affect this situation. So i unplugged the neutral safety switch on the engine and it would rev fine. Put a new switch in still same problem..
Submitted: 8 months ago.
Category: Boat
Expert:  Jason replied 8 months ago.

Hello my name is ***** ***** am I speaking with?

What is the history on this engine, have you ever seen it make full power under a load personally?

How old is the fuel?

Does the alarm work, does it beep every time to you start it?

What code scanner are you using?

Customer: replied 8 months ago.
I'm Brian, yes it used to make full power and run fine. It's in a 37 formula with twin 496 motors. Other motor runs fine so don't think it's bad fuel. No starboard doesn't beep when you turn key on but port does. And I used a rinda scanner not the Mercury ddt
Expert:  Jason replied 8 months ago.

Hi Brian,

Are you using Diacom or the hand held techmate?

Customer: replied 8 months ago.
Techmate
Expert:  Jason replied 8 months ago.

The techmate is something that you really have to know how to use. Are you qualified to use it? Do you know how to read the live data on the tool?

Customer: replied 8 months ago.
I also do this for a living.
Expert:  Jason replied 8 months ago.

Have you done a cylinder drop test via the techmate to see if any of the cylinders were not firing?

Customer: replied 8 months ago.
No I haven't because when you unplug the neutral safety switch it revs fine. Plug it back in and it only revs to 3000rpms. So it's gotta be a sensor or something telling the pcm not to rev higher. It idles fine also.
Expert:  Jason replied 8 months ago.

Does the "gear position" say On or OFF in the techmate when you shift into forward gear?

Customer: replied 8 months ago.
It's the small black box looking scanner that just reads codes and tells you data while running it doesn't have that option to tell you gear position.
Expert:  Jason replied 8 months ago.

It should if you have the correct version...

Customer: replied 8 months ago.
what sensors control the rpms? Do you have any suggestions or have you seen this problem before?
Customer: replied 8 months ago.
Customer: replied 8 months ago.
That was a waste of 26.00 thanks..
Expert:  Charles replied 8 months ago.

Hello, my name is***** you still require assistance on the engine?

Customer: replied 8 months ago.
I already wasted 26.00 on the other guy. He didn't know a whole lot..
Expert:  Charles replied 8 months ago.

There are no additional charges to continue with me. Your original question fee is still good.

Customer: replied 8 months ago.
Ok so have you had this problem with the 496 mag HO engine won't rev over 3000rpms.
Expert:  Charles replied 8 months ago.

Yes, I have seen it. There can be several causes for this. Have you tried to swap the shift switches from one engine to the other to see if the issue follows it? Seems odd that when you unplug it, the engine will rev normally.

Also on the horn testing beep. You will need to get the alarm repaired to see if you are getting a fault.

Customer: replied 8 months ago.
I went and bought a new shift switch and installed it, same result.. I'm telling you I have been working on marine engines gas and diesel for 18 years and never had this issue.. It's like the Pcm is not letting the cam and timing advance when you push the throttle. I swapped computers cam sensors unplugged water pressure sensor and manifold sensors. I have no idea..
Expert:  Charles replied 8 months ago.

You originally wrote that there are no codes that would effect this.. what were the codes that you saw?

Customer: replied 8 months ago.
They were pitot code, steering code, and when I unplugged the manifold sensors and other sensors they popped up on the scan too.
Expert:  Charles replied 8 months ago.

Are the butterflies actually opening all the way still?

Have you looked to see if both sets are opening. I have seen one where the second set of butterflies would not open.

Customer: replied 8 months ago.
There is one throttle body with one butterfly on the 496
Expert:  Charles replied 8 months ago.

Do you know the serial number of the engine?

Customer: replied 8 months ago.
I'm not on the boat at the moment but will be tomorrow. And will provide you with that
Expert:  Charles replied 8 months ago.

Thanks. I will pull up the correct diagrams and also check to see if I can locate any bulletins on the engine.

Customer: replied 8 months ago.
Serial number is ***** family 5M9XM08.1HIO year 2005 mercruiser 496 MAG HO
Expert:  Charles replied 8 months ago.

thanks, ***** ***** the engine, what is the MAT, MAP and Baro readings?

Customer: replied 8 months ago.
I had the cheap rinda scanner on it before and it doesn't tell you much.. So I'm trying to get the ddt scanner tomorrow and see what it says. But look up any computer sensor that puts motor into 90% power.. Broker is coming on wed for a sea trial.. Gotta figure this out.. Can you call Mercury for me and tell them?
Expert:  Charles replied 8 months ago.

Lets see what the DDT says and go from there.

Related Boat Questions