Surging / losing rpm

[Dylangrimsley]

Hey this is my first time on here but I have a 2001 bayliner with a 5.0 mercruiser. It will randomly lose rpm and jump right back up. The boat will pull back a little then speed back up. I’ve been chasing this problem over 2 years. Sent it to many mechanics and tore it down twice. I will attach a video of the problem and a list of all things I have changed. Almost a completely new motor at this point. It won’t let me post the video but the best way I can explain is I’ll be cruising and everything’s fine. Rpm’s will randomly drop and shoot back up by itself and the boat will slug back a little then go back to the same speed

 

[achris]

Hi and welcome to iboats.

As one of the frequently contributors to these forums, it's nice to know exactly what you have when we're trying to give advice.

In the first post of each new topic, if you list engine model, year and serial number, that will help us a lot in our attempts to help you.... (or do as a lot of regulars do, list it all in your forum signature. That way, it's always there, and you don't have to think about it. You'll see mine at the bottom of this post.)

I would say to check the correct plugs have been installed.

I'd tell you which were the correct ones, but I have no idea if this is a carbed, EFI or MPI engine....(you listed 'computer/ICM'. Was it the ECU or the Thunderbolt V Ignition control module?)

Chris........

 

[Dylangrimsley]

Hi and welcome to iboats.

As one of the frequently contributors to these forums, it's nice to know exactly what you have when we're trying to give advice.

In the first post of each new topic, if you list engine model, year and serial number, that will help us a lot in our attempts to help you.... (or do as a lot of regulars do, list it all in your forum signature. That way, it's always there, and you don't have to think about it. You'll see mine at the bottom of this post.)

I would say to check the correct plugs have been installed.

I'd tell you which were the correct ones, but I have no idea if this is a carbed, EFI or MPI engine....(you listed 'computer/ICM'. Was it the ECU or the Thunderbolt V Ignition control module?)

Chris........

I’m sorry I’m new to this. It is carbed. It’s my first boat so I’m not super familiar with it. Im learning as I go. I can get a vin number. But I just bought new NGK BPR6EFS spark plugs. It did have ac delco I think gapped at .40. And the picture is what I mean by a computer.

 

[Dylangrimsley]

Hi and welcome to iboats.

As one of the frequently contributors to these forums, it's nice to know exactly what you have when we're trying to give advice.

In the first post of each new topic, if you list engine model, year and serial number, that will help us a lot in our attempts to help you.... (or do as a lot of regulars do, list it all in your forum signature. That way, it's always there, and you don't have to think about it. You'll see mine at the bottom of this post.)

I would say to check the correct plugs have been installed.

I'd tell you which were the correct ones, but I have no idea if this is a carbed, EFI or MPI engine....(you listed 'computer/ICM'. Was it the ECU or the Thunderbolt V Ignition control module?)

Chris........

I’m sorry I’m new to this. It is carbed. It’s my first boat so I’m not super familiar with it. Im learning as I go. I can get a vin number. But I just bought new NGK BPR6EFS spark plugs. It did have ac delco I think gapped at .40. And the picture is what I mean by a computer.

 

[Dylangrimsley]

here’s a picture I have of the motor in my phone

 

[Rick Stephens]

Is the serial number written on the flame arrestor?

 

[achris]

Is the serial number written on the flame arrestor?

I zoomed in, no it's not. He's going to have to look at the serial number plate down near the starter motor.

 

[Dylangrimsley]

I zoomed in, no it's not. He's going to have to look at the serial number plate down near the starter motor.

Yeah their all worn off. I’ll get the number off the side near the starter next time I go by

 

[Grub54891]

Fuel restriction? You changed the fuel pump and filter, But did you pull the anti-syphon valve and pickup tube and check them along with the fuel tank vent?

 

[Dylangrimsley]

Fuel restriction? You changed the fuel pump and filter, But did you pull the anti-syphon valve and pickup tube and check them along with the fuel tank vent?

I did change out the anti syphon valve. But I’m not familiar with the rest. Is there any way I can post the video on here ?

 

[Dylangrimsley]

Video

 

[Dylangrimsley]

I had to shorten the video but this is it

 

[achris]

That looked like an ignition cut. I'd be looking for loose wires, and checking the shift interrupt switch.

 

[Dylangrimsley]

That looked like an ignition cut. I'd be looking for loose wires, and checking the shift interrupt switch.

That’s what I’m thinking a bad wire somewhere. I’ve done changed everything else. I was gonna pull the wiring harness next. I did check into the shift interrupt switch. If you manual tap the spring by hand It recreates the exact same rpm loss but I don’t see the switch being bumped while driving.

 

[achris]

Don't forget the 'Deadman switch' in the control box either...

 

[SternDriveEd]

So just a thought here. If you suspect the Shift Interruptor Switch try temporarily disabling it. First warm up the engine. Shift into forward. Stay at idle speed. Unplug the connector at the switch. Accelerate to the RPM range where trouble is usually found. Let her rip! See if the issue repeats itself. The switch is needed when shifting into gear. It momentarily cuts the power to the ignition system. Once the shift is made, the ignition is restored. Momentarily killing the ignition keeps the lower unit gears from meshing with power applied to them.

 

[Dylangrimsley]

Don't forget the 'Deadman switch' in the control box either...

Can you explain further. Are you referring to the kill switch next to the driver seat or something else. If so what do I need to be looking at.

 

[Dylangrimsley]

So just a thought here. If you suspect the Shift Interruptor Switch try temporarily disabling it. First warm up the engine. Shift into forward. Stay at idle speed. Unplug the connector at the switch. Accelerate to the RPM range where trouble is usually found. Let her rip! See if the issue repeats itself. The switch is needed when shifting into gear. It momentarily cuts the power to the ignition system. Once the shift is made, the ignition is restored. Momentarily killing the ignition keeps the lower unit gears from meshing with power applied to them.

Okay I will try that

 

[Scott06]

So just a thought here. If you suspect the Shift Interruptor Switch try temporarily disabling it. First warm up the engine. Shift into forward. Stay at idle speed. Unplug the connector at the switch. Accelerate to the RPM range where trouble is usually found. Let her rip! See if the issue repeats itself. The switch is needed when shifting into gear. It momentarily cuts the power to the ignition system. Once the shift is made, the ignition is restored. Momentarily killing the ignition keeps the lower unit gears from meshing with power applied to them.

Shift interrupt if functioning properly only cuts ignition momentarily when coming OUT of gear due to back cut on gears. Load on prop keeps it held in gear until ignition interupt unloads the gears engagement teeth momentarily

still worth checking just be careful on water if disconnected as you likely won't be able to get it out of gear unless you Turn it off.

 

[Scott06]

Can you explain further. Are you referring to the kill switch next to the driver seat or something else. If so what do I need to be looking at.

Yes thats next to drivers seat on shifter, make sure it is fully toggled on, can jumper around it for testing. Also worth checking connections under dash for purple wire feeding ignition circuit, you’d be surprised how many gauges etc connection points it daisy chains through On its way back to engine harness.