8.1 EFI Loss of Power

I have a 04 North River with a 8.1L EFI that recently the max RPM has decreased from around 4000 to 3600. I have an engine analyzer(don't recall which brand as I type this) that doesn't come up with any codes or anything out of the norm. At an idle the engine seems to run at the correct idle rpm and purrrs like it's supposed to except twice this past trip the idle stepped up to around 1100 for no reason and then corrected itself. Cruise rpm is 3200-3400 and doesn't have any noticeable engine vibrations or misses at cruise or full RPM. The linkage was my first thought but it wasn't that easy, my next step is fuel filter which I'm not sure when was last replaced or would even cause these symptoms. Any other ideas??? Charlie

Well I'm not sure yet what the problem but I know what it's not... The fuel rail pressure is up around 40psi at idle and drops to 22psi at it's max rpm of 3600. I havnt found the specs of what it's supposed to be but I'm guessing that the rail pressure should stay fairly constant.? Having a spare low and high pressure pump I changed out both as well as the filter and am at a bit of a loss as to what the problem could be.... Assuming the rail pressure isn't supposed to drop like this the next step is to see if there is some sort of blockage in one of the lines. Any ideas or tips would be very much appreciated at this point..... Thanks, Charlie

pretty sure the fuel pressure spec is at WOT and should be pretty constant (these do NOT use a regulator tied to the engine's intake manifold that keeps a constant pressure differential across the injectors.

Should be two filters on the engine and the 'pancake' (Oberg) filters usually have a screen at the inlet. any restriction should be easy to find with a vacuum gauge provided you can tie it into the hose safely. Other common items are fouled anti-syphon valve (at the tank's outlet fitting) and a restriction in the tank vent (temporarily opening the fill port will provide ample vent air).

Other items to check - shut off valve fully open, and if a generator is tied into the engine's supply line, shut off the valve feeding the generator. Also, loose hose clamps will let air enter and will give similar symptoms to inadequate fuel flow.

I apologize for not posting what I found the problem to be. That was my biggest pet peeve about this site is there are a lot of solicitations for help but when the problem is found the post dead ends with no fix listed and that is exactly what I did. I have learned the hard way that the high pressure pumps have a serious issue on the North River jetboats with the 8.1L (and many others). Long story short, after replacing the high pressure pump with a used "spare" that came with the boat and having the exact same problem, I ruled out what indeed the problem was. Apparently these pumps have an extremely high failure rate, especially if you are sucking air, the cause being a minute air leak on the suction side or in my case of parking the boat nose low in the driveway with a near empty tank after a trip and having the fuel drain back to the tank to where even after filling up the high pressure pump ran long enough pumping air that I ruined a second one last week. I ended up buying a fuel pressure gauge to narrow down the problem and found that my pressure dropped off when I was in the higher RPM band, hence my 3600 RPM max. This pressure should stay constant and honestly I don't remember what it was. It's been so long that I don't remember the website I went to for diagnostic of the engine (marinepower.com maybe??), it was great! step by step diagnostic..... however, I do know that the part about the fuel pressure is incorrect, I think they cut and pasted from the automotive application which uses different pressures. According to the diagnostic my pressure was right in line (at the lower RPM's, I didn't figure out the drop at higher RPM's until later) .....for a Chevy pickup. Hope this helps someone......... Charlie