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496HO: would IAC cause this? 1 engine lagging intermittently lagging 100-400 RPM

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496HO: would IAC cause this? 1 engine lagging intermittently lagging 100-400 RPM

Old 06-23-2014, 08:20 PM
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Default 496HO: would IAC cause this? 1 engine lagging intermittently 100-400 RPM

2 early model (2003) Mercury 496HOs
Couple hundred hours.
Regular maintenance.
No major issues previously (knock on wood)

As of a couple weeks ago, I have one motor that is down on RPM anywhere between 100-400 RPMs, compared to other engine; at mid-range (1000-4000 RPMs seems to be where problem manifests)

So far:
Changed spark plugs (plugs looked pretty good, one slightly lean, one a bit darker than the others, but nothing glaringly wrong)
Changed fuel/water separator/canister
Changed inline fuel filter (had a nasty rattle to it when pulled)
Checked injector pulse test with Rinda scanner; all appear to be firing correctly. (have not pulled and checked flow)
Checked forced misfire test with Rinda scanner, each cylinder when misfired makes the engine drop 500 rpm or so when throttle is set at 1500 RPM or so.
Checked fuel pressure at the rail (on both engines) at multiple RPMs, and readings are consistent across both motors.
I've also made sure that both throttle cables are synced with the throttle bodies & TPS.

After all of the above, the problem seems better, but I'm not 100% certain it's gone.

I am thinking of swapping the IAC from one motor to the other.
Thoughts on this?
The IAC on the troublesome engine passes the IAC self test with the Rinda scanner, but maybe it's actually having issues and hasn't died yet?

What am I missing? Any thing else you can think of?


I am going to swap the IACs tonight, and run the boat tomorrow; and see what she does under load.
I do have two separate fuel tanks. I could potentially swap fuel supply from one engine to the other by rigging up a couple of temporary lines.
But I don't believe fuel/tanks/pickups are the issue as all fuel has been consistently administered to both tanks.

Last edited by Sydwayz; 06-23-2014 at 10:34 PM.
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Old 06-23-2014, 08:24 PM
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I would quit paying so much attention to the tachs...
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Old 06-23-2014, 08:29 PM
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I doubt it, did you nick a prop? Any whay you can get actual fuel pressure running? Gauge with long hose, let me know if you need one.
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Old 06-23-2014, 08:33 PM
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Originally Posted by Expensive Date View Post
I doubt it, did you nick a prop? Any whay you can get actual fuel pressure running? Gauge with long hose, let me know if you need one.
Problem exists in neutral sitting with drives in the water at the ramp. It's not the props.

I am borrowing a gauge from a buddy right now, Tim. It has a 2 foot hose which works for stationary testing.
As soon as I charge the rail, it pops up to 40 and stays there. It dipped maybe 5 psi when I stabbed it up to 3000 rpm but came right back up. I am going to run the boat tomorrow and see. The other (good) engine did the same thing.

How much are the gauges Tim? It might be a good thing to add to the tool collection.

I am trying to make a poker run this weekend.
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Old 06-23-2014, 08:49 PM
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So its at idle or at mid range, im confused ?

If its at mid range, I don't see how the IAC could be the problem as its not being used anymore as far as I knew ?

If its at idle its possible theres some crud built up in there which is nothing some Electric Parts Cleaner couldn't take care of.
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Old 06-23-2014, 09:34 PM
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Merc inline metal fuel filters are junk. The inline filters are for the boost fuel pump to filter the larger crap out so it does not hit the boost pump. That filter is rated a tad more than 100 microns. Once the filter breaks up internally they no longer filter a darn thing or cause a fuel restriction in return can hurt the boost fuel pump.

You need to know what your fuel pressure readings are thru out all rpm ranges. Also check for fuel restrictions using a vacuum gauge Merc spec is 2 hg MAX. Air leaks in the fuel system causes high vacuum readings and can fluctuate rpms.

Here is how we check fuel pressures running the boat with one person plus WITHOUT a long hose attached to the fuel pressure gauge. We use the standard fuel pressure gauge mounted securely and then set up a GO PRO camera in the engine compartment. Run the boat on the water, then afterwards watch the go pro as it was pointed at the fuel pressure gauge with a lite source in the engine compartment as well.

If your fuel pressure holds a steady needle thru out the rpms plus at the recommended psi, IMO you might need your fuel injectors flow tested and clean along with new basket filters. Also could be a possible weak fuel pump related for the lagging rpms especially if your rpms fluctuate most likely the boost fuel pump . If you want to post a complete readout of diacom report and post it that might help as well.

Last edited by BUP; 06-23-2014 at 09:44 PM.
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Old 06-23-2014, 10:39 PM
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At 1700 RPM on good engine, the "bad" one is behind at about 1400 RPM (with throttles even).
At 3000 RPM on good engine, the "bad" one is behind at about 2700 RPM.

That's as far as I tested in neutral today at the ramp. (Tide was too low this afternoon; could not launch.) I will launch and run it tomorrow and see how it does. I did buy a boost pump today.

I am thinking it's time for injectors to come out and get cleaned at end of this season anyhow, if they make it that long.
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Old 06-23-2014, 10:47 PM
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Also needed to add if your rpms fluctuate and so does your fuel pressure readings then most likely its your fuel pump(s) A hurt boost pump can really over work the high pressure fuel pump so the longevity of it can be greatly reduced. As you might already know the 496's hate to run lean as they all are cast internals.
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Old 06-23-2014, 10:52 PM
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I've replaced the boost pumps once a few years ago due to some garbage fuel that tore them up.
If I still see issues tomorrow with this lagging engine, put the new boost pump in?

Thanks for the help everyone.
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Old 06-23-2014, 10:59 PM
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Originally Posted by Sydwayz View Post

Thanks for the help everyone.

No problem glad I could help....
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