2001 496 Mag losing power @ medium load.

[OX2]

Hi all

Have had 19 years of flawless running on my 496 mag. A little over 300 hours. Two weeks ago, would not go much over 3000 RPM (more load dependent than RPM really). Acted like power was falling off hard, intermittently and if I didn't back out, it would eventually stall.

HP fuel pump has always been very noisy, but it now sounds sick (tone not constant when ign first turned on, like it used to be). I checked fuel pressure with engine off and at idle and it is to spec. Have not run boat with fuel pressure guage on it, but due to sound of HP pump, going to do both pumps (HP pump appears to need removal of port motor mount to get to cool fuel setup).

I ordered Diacom and hooked that up yesterday, only with ign on, engine off as I currently have fuel pickup out just to see if it was clogged (it's not and there is no anti-siphon valve). It says seawater temp was 431.6 degrees. I don't think I have that sensor, so is that what Diacom (and/or the 555) will put out/display when that sensor is not there? I also have no codes, other than Smartcraft codes, which I don't have.

In my searches, it sounds like one other possibility is a cam sensor that is just starting to go. Is it true that this sensor may not throw a code, when it is only first starting to go?

Finally, I have enclosed a pic of the stickers on my computer and was wondering how I order the correct pumps?

Thanks in advance

OX

 

[Fun Times]

It says seawater temp was 431.6 degrees. I don't think I have that sensor, so is that what Diacom (and/or the 555) will put out/display when that sensor is not there?
Finally, I have enclosed a pic of the stickers on my computer and was wondering how I order the correct pumps?

You're correct, if you don't have the a sea water temp sensor / or not plugged in, that would be about the reading you'll see.

To order engine parts correctly, you'll want to find your engine serial number sometimes you may find it on top of the black plastic engine covers with a sticker on it and/or down next to/just above the engine starter on the starboard rear side of the engine/boat like noted in the following link, https://www.perfprotech.com/mercury-...-parts-catalog <<< Then input your engine serial number up top of that link for "your" engine parts catalog.

If you haven't recently with this engine model, at 300 hours it's usually time for new correct OEM newly recommended spark plugs only as this engine is known to do odd things when they start to go bad. < And you're right, it's sort of the same concept with the fuel pumps, CAM & Crank shaft sensors all going weak....Also be sure the spark plug wires aren't leaking spark to ground/under the ignition coils as this engine model often does that too...Look for a light show while running at night and/or listen for spark snap type sound.

That mentioned please don't just throw parts at it do to me mentioning some common issues with this engine or component parts, try to fine the cause of the issue.

Good luck.

 

[OX2]

Appreciate all the info. Have done spark plugs, will check spark leak sometime at night.

Believe I found the problem. After cleaning and putting back fuel tank pickup, new rubber EFI rated hose
from pickup to low pressure fuel pump, and new fuel filter/water sep, the primary pump won't prime.
Took filter back off and it is dry. Tried direct battery current to pump and it does not feel/sound like it
is doing anything. 12V comes off harness for that pump for 5 seconds after ign turned on, so it is getting power.
Looks like both low and high pressure pumps are discontinued from Mercruiser,

Cam sensor is also discontinued (seems to be 2001 only) on that, then changed again for 02/03, then changed a 3rd time for 04+. Have to change timing gear and cover to use 04 sensor, ugh!! Rock auto has an aftermarket one that supposedly fits 2001, GM 3500 8.1. Was thinking of buying that and crank sensor now, just in case, someday (if available).

Yes, I try to to just throw parts at things, hence why I bought Diacom right up front. But the low pressure pump not running makes perfect sense. HP pump was pulling fuel through LP pump I guess, but not enough when it got to a certain load.
Idles/runs perfect up to that load.