Mercruiser 350 Intermittent RPM drop while cruising

[siriussr]

Hello iBoaters! My wife and I just recently bought our first boat. a 1996 Maxum 2400SCR that was repowered in 2008. Unfortunately it is now having a problem that the previous owner also had, but said they had completely fixed. The boat will start up and idle perfectly, but while at any rpm more than 1500, will randomly/intermittently drop RPM for about 1/4 second and then go back to where is was. is a 4bbl 5.7 vortec, paired with a alpha one. I have search keywords like stutter, hiccup, stumble, surge, but haven't really been able to find an exact definition of then in regards to what each one means for these engines. when the rpm drops, the only gauge that moves is the tach. I saw a few people recommend to disconnect the tach in case it was going bad, but after doing that I still had the same problems. I also checked the water/fuel separator and there was no water or other gunk in it, but I dont know how old the filter is. The previous owners replaced the coil, distributor, spark plug and wires, and when we took it for a sea trial it ran perfect. The fuel pump was also replaced in late 2017. Could it be that one of those new items failed since then? We have maybe an hour on the boat since we bought it.

Thank you all for the help, and let me know if you need any more info.

 

[alldodge]

First and most often, never, ever trust what the prior owner said. They are selling the boat and want to sell it, so everything thing else should be listened as, if there lips are moving their lying.

Unless they are family then you know their lying

Sounds like a loose connection or bad pickup

What is the motor serial number?

 

[siriussr]

I’m having a hell of a time finding the serial number. I looked by the starter and there is no plate and no cast in serial number. On the other side near the back I see the GM logo and 5.7. It looks like there might be a number there but there is a lot in the way and it’s hard to get to. Is that the number I should look for, or would it be in another location?

 

[Ncherry84]

Do you have the Mercruiser engine cover? That should have the engine serial on it. I believe.

If it was repowered that block tag you're looking for won't be there. I ran into that with repowering my boat just this past couple weeks.

I second what AllDodge is recommending.....there's a loose connection on a wire or something somewhere it sounds like. Can you make it drop RPM at will?

 

[siriussr]

Found it! Was in an unusual spot, just down from the spark plugs on the start board side was the plate photo is attached, and serial number is 92011219.

 

[siriussr]

I have been unable to make it drop rpm. it happens pretty randomly, I hit some pretty big swells and it gave me no problems, but another area was like glass and it did it. Ill check the pickup and look into what that all entails, and follow the power and grounds from the coil

 

[Scott Danforth]

your motor is a reman, so the serial number is in a different location.

check all the connections.

literally start at the battery, disconnect every connection, clean it until it is shiny enough your mother would eat off it, then re-connect, and seal with marine electrical varnish.

work your way to the motor, then up the harness to the helm..... every screw connection, harness connection, etc.

 

[alldodge]

Just have to use 1996 due to reman.

Look inside the distributor at the pickup and see if it has 2 or 3 wires and if its tight

 

[wahlejim]

Read the spark plugs, what do they tell you? Had a very similar situation in my 305 right after the PO "just" changed the plugs. I changed the plugs out and problem went away.

 

[siriussr]

Thank you all for your help! I will clean contacts and see what that does. if no luck there I will replace the ignition components.

 

[siriussr]

Finally had some time to work on it. The sensor in the distributor has 3 wires coming out of it (W/G, W/R, and Black). I took some pictures because the interior parts look a little rusty/corroded but not that bad. Also other things to note- the screws holding the cap on weren’t all that tight- maybe finger tight at the most, and I cant get the rotor off. Any ideas how? I don't see or feel a set screw on it so I thought it pulled straight off. Photos in next post.

 

[siriussr]

[No message]

 

[alldodge]

The rotor is just stuck, there is no set screw.
The cap and rotor look good enough to clean and reinstall.

The 3 wire pickup is the latest type which has the ground wire (black). Check that connection

 

[siriussr]

Got the rotor off, just a bit of rust. Found some random bits at the bottom of the housing that I vacuumed up. I also checked the spark plugs. Do these look ok? Supposedly they are fairly new, probably have 15 hours on them. They look kind of white to me, making me thing the carb is running a little lean.

 

[alldodge]

Yes does look a bit lean

 

[QBhoy]

Check your shift interrupt switch. Happened to me exactly this.

 

[siriussr]

I took a look at the main engine ground, it was somewhat corroded so I cleaned it up. Also took a look at the distributor cap and rotor and found corrosion. I replaced those and the spark plug wires since they were 15 years old and showed some deterioration. Next up if the shift interrupt switch if the problem is still not solved.

 

[AKJohne]

Likely ignition, pretty good rundown on TB V here https://forums.iboats.com/forum/eng...-mercruiser-thunderbolt-ignition-systems-work

The white plugs could be a lean miss, but doubt it would be intermittent. You could check volume output of fuel pump.

My guess would be ignition sensor in the distributor or the module on the back.... possibly the knock sensor. I had the ignition sensor go out last year and the motor had similar issues, however more severe, it had the old style sensor. I now carry spares of all those components as we can be a bit remote at times. Dont let some one talk you into changing out the ignition, The TB V is excellent ignition, except for 2 problems: expensive parts and they like to corrode.

Take the times to go thru the troubleshooting flow chart and diagnose the problem. Changing parts is spendy and frustrating. Have you done a compression test on the motor?

 

[QBhoy]

This is a strange one to me. Maybe wrong, but a maxum 2400 scr is a boat made in the 2000’s. It replaces the 2300 scr of the 90’s. Just my boat geek coming out, haha.

 

[achris]

How many intake manifold bolts do you have? 8 for Vortec, 12 for non-Vortec. If you have 8, and I would think being 2008 (yes, I know it's a reman), it should, then you have the wrong spark plugs in....Those plugs (shown in post #14) are BR6FS. Vortec engines should be running BPR6EFS... If that's the case, that could account for your miss.

Chris.......