496 HO wont go past 4300
 

Hello everyone,

I have a 496 HO with aprox 70 hrs on it and i am not being able to go past 4300 rpm. Boat is a cigarette 27 with a 22" bravo 1 prob. Ive tried changing water/fuel separator and spark plugs. No change.

here is the diacom screenshot when running at WOT.

PLEASE HELP!!!

http://i1062.photobucket.com/albums/...9at23512PM.png

Has it always done this? any new changes such as prop??

look at the sea water temp............ 431 degreessssssss.... WTF

No it hasnt always done this. and yes i changed prop recently. The boat originaly had a mirage plus 19" when bought it, but it was hitting the rev limiter so i tried a 23 cleaver and a 22 bravo, and was hitting 4600 easily with the bravo then.

yeah, the temp is crazy, probably a mistake of some sort.

When my I bought my boat the guy said it had been doing the same thing and it was a sensor on top of exhaust manifold on left hand side drove them crazy for a while before they finally found it

I don't know alot about motors, but is 22 degrees of timing right? sounds low maybe should be near 29? don't know just a thought. could the sea water temp be telling the computor to back down the timing? I'm lost!

ive been reading that the temp is a default value when some transom sensors are not connected.

the timing also caught my eye but only because of my experience with distributors which advance to 34 deg or more. not sure with ecm injection engines though.

timing is correct on a 496 Ho safe for 87 octane

No mecanich by any means but the weird thing is that , while watching the data playback I noticed that the spark advance is 26 @ 3400 and then drops as the throtle is opened to 22 @ 4300 aprox. Still, i would think that 22 is low even for 87 octane.

dead safe look at the timing at idle
sent my ecm to dustin at whipple did the stage 2 upgrade made a big difference have to run 91

You need to get a fuel pressure gauge on the fuel rail, and check fuel pressure. I have seen this happen before, the rpms will continue to get worse. I would replace both the low and high pressure pumps.

By the way; the lake or sea water temp has nothing to do with the engine performance.

FWIW my 454 mag looses about the same rpms if silent choice is set to silent (exhaust through the prop)--shooting it out the back lets it rev out. I agree that fuel pressure is the most likely issue, but if you have through-prop exhaust or mufflers make sure they aren't restricted.

Start with the basics,I agree.

thanks alot, sounds like sound advice and will proceed accordingly.

Lake/Sea Temp is not used, there is no sensor for it and therefore it defaults at max temp. Has nothing to do with performance. I would put the 19 back on and see what it runs. I would also put a fuel psi gauge and check it.

The motor has fast burn "vortec" style heads therefore they do not use a lot of timing, especially with 87 octane. From a scan tool stand point, it looks normal so either your over propped, have a fuel delivery issue or ignition issue.

hi guys, thanks a lot for the suggestions. I have since found the problem, a bad spark plug cable. Changed it and went right up to 4800.

Again, thanks!

:party-smiley-048:

Ha, that was gonna be my suggestion, either cable, coil or dead plug. Glad it was that simple....troubleshooting aside.