97 350 vortec knocks only under a load

[lime4x4]

I have a 1986 286 searay 350 mercruiser with alpha outdrive. Engine was replaced a few years ago with a 1997 350 vortec engine. 2 weeks ago i had a pretty bad overheat. Melted the exhaust couplers. Replaced the water pump, exhaust couplers, shift cable. Risers and manifolds looked ok. Started engine up on the hose at home and found engine wanted to lock up when idling. Drained the exhaust manifolds and removed the risers and found water in the manifolds. So ordered and installed new manifolds and risers. Removed the water from the cylinders and drained and refilled the engine oil. Changed oil and filter 3 times till i got nice clean oil. This weekend while out after running the engine for about 2.5 hours on and off plane several times. When the wife went to get up on plane the engine just shut off and wouldn't restart could smell gas thou. Got towed back to our slip and found i had no spark. Coiled failed went and got a new coil. Took about 20 minutes of trying till she started. Ran real rough for about 5 minutes or so. just thought cylinders were loaded up with fuel. The next day we head back out. The first thing i noticed is there is a very slight vibration coming from the engine just thought it was still loaded up with fuel yet. So she goes to get the boat on plane at around 1800 rpm's the engine starts to knock like a rod bearing. Back off the throttle put the boat in neutral and shes tacs up to 4K and no engine knock. Put back in gear and as soon as it hits 1800 rpm's the engine starts to knock again. If i have a bad rod bearing shouldn't the engine also knock when revving it up? But she is quite all the way up to 4K just when there is a load on the engine she starts to knock. I've attached a link to the you tube video and u can hear the knock towards the end of the video. just don't want to pull the engine just to find there is nothing wrong with it. I did check the oil and it's nice and clean yet so the block or heads didn't get cracked from the over heat.

https://youtu.be/UddTQygYwxc

 

[alldodge]

Suggest doing a compression test, and do this after the engine has warmed up. Most engines will not knock in neutral, it just is not burning that much fuel.

 

[gm280]

I had a Chevy 350 do that on my truck once and it would work okay in neutral as yours does. I turned out to be the thrust bearing gone at the rear crank shaft location. But only in gear, forward or reverse. Just saying...

 

[lime4x4]

had some time today to mess with the boat. Found the timing is all over the place. Boat was hard starting today. Once i got it started and warmed up i checked timing at an idle. It was at 8 deg then 32 deg. It will not stay at one spot. Plus the tach was bouncing around between 500 rpm's and 1500 rpms and u could hear the change in the way the engine was running. At 2k rpm's the timing mark still isn't steady it's all over the place. I'm gonna try replacing the pickup sensor in the dist. It already has new coil,plugs,wires,cap and rotor

 

[Rick Stephens]

Reading your initial post, first thing came to mind was your exhaust shutters. If you cook your rubber couplers you also undoubtedly smoked the shutters. You didn't mention it, so figured I'd ask. You'll also want to check the water pocket in the outdrive.

 

[NHGuy]

Timing all over the place, tach bouncing...pull the distributor cap ,twist the rotor back and forth. Then remove the distributor and check the gear on the end of the distributor shaft. If you are really lucky it's messed up.
But. If it's a TB V, not IV, that could explain the timing moves. The V seeks highest advance at steady speeds or when accelerating. TB IV is engine speed dependent. 1997 could be either.

 

[alldodge]

timing at an idle. It was at 8 deg then 32 deg. It will not stay at one spot. Plus the tach was bouncing around between 500 rpm's and 1500 rpms

Need to check wires, cap, rotor and plugs

 

[lime4x4]

It's thunderbolt IV The boat is a 1986 but the engine block and heads are 1997. The exhaust shutters are fine.

 

[alldodge]

You still have the knock?

 

[lime4x4]

yes it still knocks yet. i might have 2 seperate issues with the boat. It only knocks in gear with a load. At times it sounds like a metal knock and at other times it saids like real bad detonation. I went back up to the boat and pulled the dist the gear is fine. Something is causing the timing to jump around. So it's either the pickup sensor or the module. I'm gonna try a pickup sensor and if that doesn't fix it i'm gonna install the voyager marine ignition system which is basically a thunderbolt V setup

 

[NHGuy]

Well you can also go through all the Electrical connections and sand em shiny. Wouldn't hurt.

 

[biggjimm]

If your timing is jumping all over the place I'd expect to see some detonation at times. I'm not familiar with the thunderbolt systems but sounds like whatever controls the advance has gone wonky.

As far as your knock goes, I've experienced a couple main bearing failures & sometimes they'll only knock under a load, depends on which bearing it is I guess.

It's hard to tell from the video clip though, couldn't hear it real good & my speakers leave a little to be desired, but it didn't really sound like a deep hard knock like a crank or rod bearing usually sounds. But it could just be my bad speakers.