Offshoreonly.com - 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.
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