2000 Air Nautique 5.8 PCM GT40 running rough

[Spessx]

Hi All

My boat has about 400 hours on it and it's running rough at idle and various places in the rev range when driving. Its running rough enough the the boat is shaking and vibrating a bit. The boat is 100% stock. Oddly enough, when I put the button to rev the motor without the prop engaged it seemed to smooth out a little. Not sure if that has anything to do with it.

Anyway, I'd like to get a sense of where to start with a rough running boat: Here's kinda my list - Is this the best place to start?

1. Check that the kill switch is pressed all the way down (it was cutting out previously because of this). Checked it - not the problem.
2. Clean the dirty spark suppressor/air filter.
3. Clean the throttle body (it looks dirty)
4. Clean the battery cables (it has a new battery, any chance that contact with the battery is causing running issues?)
5. Change spark plugs
6. Change fuel filter

What else should I check? Anything I'm overlooking?

[Jonny Quest]

I would include:
- Dirty fuel injectors
- Distributor cap & rotor
- Plugs wires
- MAP sensor

[Spessx]

I would include:
- Dirty fuel injectors
- Distributor cap & rotor
- Plugs wires
- MAP sensor

It's been so long since I had something with a distributor I completely forgot about the cap and rotor. Thank you!

Is there any particular order you would start in?

[Jonny Quest]

Cap, rotor, wires and plugs first. Be sure to change out the filter inside the FCC. Then look at injectors.

Have you been running regular pump gas with 10% ethanol? If so, try to use ethanol free whenever possible. Ethanol attracts water/moisture into the fuel system and that's bad ju ju.

JQ

[Spessx]

Cap, rotor, wires and plugs first. Be sure to change out the filter inside the FCC. Then look at injectors.

Have you been running regular pump gas with 10% ethanol? If so, try to use ethanol free whenever possible. Ethanol attracts water/moisture into the fuel system and that's bad ju ju.

JQ

Hi - Thanks for the response. What is the FCC?

Also, I just bought the boat a few week's back and have less than an hour of run time on it - so I'm not sure what type of gas was in it. I'm in Texas and it seems like ethanol free fuel is hard to find.

[Jonny Quest]

FCC = Fuel Control Cell. Black vertical canister thing in the front of the engine (port side). PCM uses the FCC to control fuel to the high pressure pump and to eliminate vapor locking.


This photo came from this thread:
https://www.planetnautique.com/vb5/f...eshooting-info

JQ

[mpost]

Hi All

My boat has about 400 hours on it and it's running rough at idle and various places in the rev range when driving. Its running rough enough the the boat is shaking and vibrating a bit. The boat is 100% stock. Oddly enough, when I put the button to rev the motor without the prop engaged it seemed to smooth out a little.?

After youor last post saying you just purchased the boat, you might want to inspect the prop for dings and drive shaft for being straight. JQ is right on for engine stuff for a vibration, but your statement about if vibrating "when driving" and "smothing out when prop is not engaged" could becausing some or all of this.

[jpwhit]

Sounds to me like it is missing on one or more cylinder. You'll feel a miss under load, not so much just revving the engine out of gear. Before changing a lot of parts, I'd try to determine if you do indeed have a cylinder(s) not firing or not developing proper power, so you know what you're dealing with. I'd start with a compression test. Then check for spark for each cylinder.

My preferred way to do that is what an extra spark plug. Unplug the wire from each cylinder one at a time and connect it to the extra plug. You can either hold the metal body of the extra plug to ground somewhere, but my preferred method is to have a piece of bare copper electrical ground wire with a large alligator clip on the end. Then you can wrap the stiff ground wire around the plug and twist it tight. Then use the alligator clip on the other end of the wire to both ground the plug and hold it into a convenient place to see.

Drive the boat with each plug connected to the extra plug. You want to pay attention to two things. Are you seeing a solid spark on the external plug that looks the same as each other cylinder, and does the engine feel even worse with that cylinder disabled? Even if you see a good spark on every cylinder, but there is one cylinder when disconnected that doesn't run a badly as when other cylinders are disconnected, then that cylinder may not be firing properly. For example, if the injector on that cylinder is clogged, broken, or not getting power from the ECU, then the spark will be good, but the cylinder is not developing power in this case due to lack of fuel. A cylinder also may not develop power if the compression is low.

[JayG80]

My Bucee’s has 2 grades of ethanol free fuel

https://www.pure-gas.org has available sources.

[SilentSeven]

Spessx - helpful if you can post a picture of your engine. This will let us see general condition.

Consider that engine is 20+ years old and if it hasn't had a recent tune-up, that's the place to start. A good list was provided above and I would do all of the following.

- New plugs
- New cap and rotor
- New plug wires
- FCC filter change
- Check engine timing (not listed above but something I would also do)

Just do them all as it will increase reliability and eliminate questions.

If that doesn't correct the problem, then move to additional candidates like dirty injectors, faulty MAP sensor, faulty hall sensor in the distributor, etc.

Before I sold it, my '97 Ski with a GT-40 had 1600 hours and ran as new. But when I purchased it, the engine required the maintenance items above and I also put in a remanufactured distributor as I had a problem with my hall sensor. GT-40s are tough engines...based on a Ford truck motor.