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496HO Down on power

Old 07-11-2022, 10:04 AM
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Default 496HO Down on power

Guys....

We've got a PowerQuest280 ('02) with a 496HO/M1ProCharger (supposedly a PQ factory build; boost and fuel pressure gage match the rest of the dash gages, including bezels) and a Whipple Stage 2 ECM tune (NOT factory) that is significantly down on power.

Info dump.... A little history and some details....

We bought the boat a year and a bit ago. It had ~143hrs on the Hobbs meter when we bought it, and an ECM scan confirmed that. It now has ~193. The tattle tale on the GPS says 71MPH, but we've never seen it. I'm sure that was with one person, vapors in the tank and a minimal load. We saw a solid 67-68 with a Bravo28P prop, four people, cooler and a half tank, and I figured the loss was due mostly to the load. All was good. We have since propped down, to 26P, trying to make it a bit more family/watersports friendly, which cost me about 2MPH. Still OK.

Beginning of this year, maybe even before the end of last year, I noticed a change. Transom soot is worse, and different. It's hard to explain, but went from a fairly uniform dark grey film to, just this weekend, a darker film that seems like a cleaner has left streaks in it. Previously perfectly polished exhast tips are now jet black, and cannot be cleaned without serious scrubbing and polishing compound. Top end is dropping off. It used to tach out at ~4600-4700RPM, and that 67MPH or so, but had dropped off to ~4200 at ~55MPH, and has now dropped off to ~4000 at WOT, and about ~50-52 MPH. I asked Dustin to set the rev limiter to 5250 (June of last year). I'm assuming he did so, but have no way of confirming. Boost is off as well. I never see anything more than ~1.5, MAYBE 2lbs of boost, where I was getting ~5lbs initially (Where the 496/M1 ProCharger kit is supposed to be). Fuel pressure seems good. As soon as the boost gage crosses into positive manifold pressure, fuel psi shifts from 40 lbs to 50 lbs. Something else that has sagged is the rev "wall" that the engine hits when taking off, until the hull starts to level off. It used to rev to about 3000-3200 at WOT at takeoff, but now it sticks at 2500 or so until the hull starts to level out, which takes significantly longer now than it did initially. I also see virtually no change in RPM, speed or engine sound from about 2/3 throttle position to full throttle position. I have religiously put at least REC90/alcohol-free gas in the tank. I usually put a 50/50 mix of 89 and 93 in it, since I can't seem to find any 91 anywhere near where we typically boat. We also seem to be going through a small amount of oil, which I monitor obsessively. I ran the serial number past the tech at Merc, and he confirmed the engine was built with the forged crank and rods, but still has those damned hypereutectic cast pistons.

I cannot point a specific "Uh-OH" moment, where something might have happened, strange noise, or anything that would lead me in a specific direction to start looking for something obvious, other than a certain drain plug incident, where we had a fair bit of water in the bilge, but not enough to the point of ingesting any water through the ProCharger intake. We also had a slight engine stutter immediately after start-up a couple weeks ago, where, afterwards, I noticed the radio had gone back to factory resets, including losing the two remote commanders, but that all reset itself to all custom settings, including regaining the remote commanders, after turning the radio off and back on again. Don't know if this would be related at all or not. I suppose it could have been an electrical "event" that got to the ECM, but the degrading behavior had started before that. I might be able to say the digression has gotten worse since then, but I'm not really sure. We went boating with some friends over the weekend. They were trailing us and he told me he could "seriously smell" our exhaust; raw gas. This makes sense, since everything in my limited understanding leads me to a think we are running a VERY rich mixture. I'm sure the ECM tune is expecting more boost than we are getting, so that would make sense, as well. I have not had a chance to read the plugs, but I will soon. I did notice a slight "worn" stripe on the non-working side of the ProCharger belt, but cannot find anything or anywhere it might be rubbing. I also cannot imagine that belt would slip for very long without completely letting go. The working side seems fine. There could be a leak in the intake tract, between the ProCharger and the intake plenum, but I don't know how to verify or rule this out.

I'm beyond thinking something has changed. It's more like something is changing. And I tend to think that's worse.

Any help would be appreciated. I'm hoping some of you engine gurus will recognize some of these symptoms, individually or in concert, and have some insight as to where to look. I'm willing to tow the boat to someone, hopefully relatively close, that could assess the symptoms more thoroughly and knowledgably, but don't personally know anybody willing to looking at it. I think I have a local option for a compression test of the cylinders, but have not pursued that yet. We are located just south of Dayton, OH, so advice on where to take it would be great.

Thanks. Brad.
(937)545-8991

Last edited by Brad Christy; 07-12-2022 at 10:11 PM.
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Old 07-11-2022, 02:02 PM
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Start with the basics. Plugs, wires, fuel filters and a compression test. Verify that all cylinders are firing with a spark tester and report back
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Old 07-11-2022, 02:10 PM
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Compression test and fuel psi test always first. And ^^^^
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Old 07-11-2022, 02:17 PM
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Change the belt for the pro charger. I had similar happen on my 525sc. I used to have to replace my SC belt every other year because it would start slipping and be down on boost
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Old 07-11-2022, 02:22 PM
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Originally Posted by SB View Post
Compression test and fuel psi test always first. And ^^^^
SB,

Working on the compression test. I know how to do it, even if only on an academic level, but I am not properly equipped. Trying to source either the hardware or the labor, or both.

Thanks. Brad.
(937)545-8991
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Old 07-11-2022, 02:28 PM
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Originally Posted by Griff View Post
Change the belt for the pro charger. I had similar happen on my 525sc. I used to have to replace my SC belt every other year because it would start slipping and be down on boost
Griff,

Is this a common thing with multi-V belts? I grew up on a blower motor boat, but it was a cog belt. We only had to change the belt a handful of times over the fifteen or so years we had it, and it was always the result of an incident; blown oil seal, severe backfire ripping the belt, etc... Just curious.

Looking and considering everything.

Thanks. Brad.
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Old 07-11-2022, 02:57 PM
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Originally Posted by Griff View Post
Change the belt for the pro charger. I had similar happen on my 525sc. I used to have to replace my SC belt every other year because it would start slipping and be down on boost
Griff,

Anything in specific to look for as an indicator the belt is bad?

Thanks. Brad.
(937)545-8991
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Old 07-11-2022, 05:31 PM
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I would check/ change the belt.

Is there a lot of belt dust on the front of the engine? It needs to be tight or it will slip. With your boost down, I would check that first.
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Old 07-11-2022, 06:21 PM
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Originally Posted by Brad Christy View Post
Griff,

Anything in specific to look for as an indicator the belt is bad?

Thanks. Brad.
(937)545-8991
Nothing specific that would make it definitive to the naked eye. Mine were 10 rib serp belts.
Belt would be fine on one trip and then a week later would go out and lost half my boost.
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Old 07-12-2022, 06:54 AM
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Guys,

Another question....

Looking through the ProCharger manual, it states that idle fuel pressure should be ~38lbs, and 62-65psi at full boost. I am reading ~40psi at idle, but only reading ~50psi at "boost" RPM. It seems appropriate to adjust the regulator to get proper WOT pressure, but the engine would appear to already be running rich, and increasing fuel pressure would obviously (I think) make it run even richer. Yes/No?

Thanks. Brad.
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