View Full Version : Full Trottle Knock

09-15-2007, 08:03 AM
350 tbi w/ 900 hrs. At full throttle when making a turn engine will develop a knock. After continuing down the lake the knock will GO AWAY until turning again. I have the engine box and floor out of the boat to check transmission, strut. etc. All look fine. Knock is coming from the top of the engine. I have had my mechanic ride along and he thinks it is pre-ignition knock. If you put your hands around spark arrester while it is knocking the noise is muffled by your hands, plus the cone of fuel coming out of the TBI seems to change when knock start. Zero to WOT it runs as strong as every with no noises. Had lifters adjustments rechecked. Blew a head gasket 2 months ago, had heads Milled and checked and block checked and all were ok. Noise started about a week ago and can not seem to location source of noise. I really do not want to start tearing down just to check ? ? This was suppose to be the last year for this boat, we had our sights on newer vee drive. Getting cold here and would like to get fixed before winterizing boat.
My mechanic is scratching his head ? ? ?

09-15-2007, 08:10 AM
If it is only doing it at full throttle, and the spray from the injector is changing, I would suspect a fuel supply issue caused by the gas sloshing on the turn. Normally, I would not expect a knock to be related to fuel supply, but you never know. Does the '95 have the in-tank fuel filter? If so, has it been replaced in the last 100 hours or so? If not, maybe it is partially plugged and depending on how the gas moves in the tank, you are starving the engine. I am just guessing. Its all I got.

Good Luck.

09-15-2007, 08:49 AM
Have you tried higher octane gas? If the heads were shaved enough, it may have crossed the line as far as compression. Also, if the gas is old and it's still the same as what was in the tank before the boat sat while the work was being done, it may have dropped, which it does over time, anyway. Try it again with fresh, 93 octane mixed with the remaining gas and see if it still knocks. I think it will only knock when you turn into the prop's rotation, because of the added load. Make sure the knock sensor wire is seated and making contact.

Is this a boat mechanic, as in MC or Indmar? If he is, have him hook up a diagnostic computer and look for knock. Also, look for the timing to be pulled back when it knocks. If the timing isn't being adjusted, the ECM isn't getting knock input.

Is it due for a tune-up? Is the knock occurring on a quick turn or a long, sweeping turn? If the tank is low and it's a long turn, it's basically running out of gas until it's done voiding the chamber where the fuel pickup is.

09-17-2007, 04:48 PM
Thanks for input,
We are hooking up scanner tonight and running around lake to see if we can locate problem. Before I submitted post I had topped off tank with 93 octane and added bottle of HEET to eliminated any moisture, we average about 10 gal. of gas a week, so do not think it is old gas. Engine had complete Tune-up when I had head gaskets replaced,2 month ago. Sharp turns bring it on..

09-17-2007, 04:54 PM
If you're using a scan tool, that won't tell much, just if you have stored codes. It won't show any data. I would look at base timing. If that's excessive, you can have detonation at high RPM Also, you may have carbon deposits on the pistons. Definitely possible, considering the fact that the programs don't run these lean.

One thing to look for is the PCV valve hose. If it goes from the #8 intake runner, I would recommend changing it so it attaches to the throttle body spacer, just below the throttle plate. The #8 cylinder can run lean because of this.

09-17-2007, 06:39 PM
Have you checked wiring to the knock sensor? You may also want to try a new sensor (I don't think they're too expensive and any NAPA will have one).

Also, did you change the oil after replacing the head gasket......... and was it clear (not milky - water contaminated)? If you didn't I'd look closely at the oil to rule out any bearing problems.