RPM fluctuations

[goroski]

First time posting on this forum, or any boating forum. I am in Eastern NC and have an 1998 SNOB. PCM GT-40 engine. I bought it used in 2012. Had a new transmission put in shortly after I purchased it. Since then boat ran great until summer of 2013.

Symptom: RPM’s bounce around. The tachometer needled looks like it is dancing to music. I have a short video of it here. http://youtu.be/8Lb1XFdHG00 I am not moving the throttle, all that fluctuation is happening on its own. Whenever I charge the battery there are absolutely no fluctuations, the boat runs smooth and perfect. However the next time I take it out, same thing. If I charge battery again, it will run smoothly. The problem only happens when the battery is drained a bit.

Probable cause: The stuffing box had loosened enough to allow water in, which drained the battery trying to pump it out. In the morning the boat was sunken quite a bit and had water up to about 6-8 inches above the floor. I was out of town when it happened, so not exactly certain. Drained the boat, learned about stuffing box, fixed it, dried it out, charged battery and things seemed to work ok. Except for the symptom described above. Thought it was the battery, so i bought a new one and it worked great for the first two outings, then the same thing started again. Only runs smooth with a new charge.

My uneducated guess: The battery is not being charged properly or some electrical component has been affected by the water and is messing with the electrical system.

My Skills: I am fairly handy with tools, although new to boat mechanics. I can change my oil and transmission fluid, inspect spark plugs, and have installed a new blower motor. I have a multi meter and can perform some electronic tests. Any help would be appreciated.

My boat currently is winterized and I am trying to figure out if I can do what needs to be done or if I need to take it in somewhere.
Darren

[DanielC]

The voltage at the battery should be 13.5 to 14.5, with the engine running. In order to run the engine, you must supply water to the raw water pump.
The alternator is internally regulated, if the voltage is not in the above range, at least two possible causes. Bad alternator, or bad connections somewhere in the wiring between the alternator, and the battery.
Lead acid batteries do not like being fully discharged. You may need a new battery.

You can also go to the downloads section of thei site, and download the GT-40 service manual.

[TX-Foilhead]

Running a bilge pump should pull down the motor, it's not that big of a draw. Check the health of the battery and see if the alternator is doing its job first. If there is still a problem go through the manual and figure out how to check the idle air control valve and try a new battery. My guess is the low voltage is causing the IAC to behave funny which causes the surge. It controls the idle by letting in a little air which causes a small lean surge when the load changes at idle.

[NCH2oSki]

The 97-98 gt-40s had a known issue with idle flair at start up. This may not be what you are asking about, but could be related. When starting the engine it revs way above idle and stays there for a few seconds, but will drop to standard idle after a few seconds. My regular ski bud has a 97 ski, and it always hits around 1700rpms or so at start up, then if you are patient it will drop to 750-800 rpms till you engage. Rpms stay constant as long as you don't shut it down and restart.

[nyryan2001]

Darren-

need to further define 6-8" above the floor....

Was the alternator submerged? ECU? Batteries?

a relatively easy check would be to double check your negative engine ground... usually a braded strap to the bottom of the engine??

Unhook, clean, wirebrush, inspect, reattach.

I would get into any low laying wire bundles that may have been submerged and unhook and of the exposed butt connectors, spray with WD40, wire brush and reattach.

[goroski]

Thank you so far.

I will get a digital multimeter and check battery volts exactly. Are there some other ways to test if the alternator is the problem?

I will look for the idle air control valve and see what i can figure out about that.

I don't know exactly how deep water was, I was out of town when it happened. My wife said it was halfway up the engine cover, neighbors came to the rescue. Battery for sure was submerged. New one since then. Alternator I'm pretty sure was not. Don't know where the ECU is, sorry. I know where the engine ground is, I will check and clean that. I will also unhook wires that are low lying and clean them.

may not be for a few more weeks, when i don't have any more freezing nights.

[s_kelley2000]

If the battery was submerged you are definitely going to want to check your battery cables too. Might be a good idea to peal back the coating a little bit and look for corrosion. If there is I am guessing it only goes an inch or two. If that is the case and you have the slack I would cut out the bad section and reconnect your terminals to a good section. I would start with cleaning up the engine ground since that is pretty easy and then work your way through the wiring from there. Good luck.

[flipflop]

Here's a picture of the GT-40. Halfway up the engine cover cuts it close to the ECM. You can unscrew the wiring harness to the ECM. I would check for corrosion and ohm your grounds to the ECM. A good place to start is to download the GT-40 manual and review the diagrams (Pin 20, 40, and 60). Also, check the main engine grounds. They are easy to identify, the big/medium black wires connecting to metal.

x2 on check your battery cables.

[goroski]

Update.
Cleaned ground bolt on engine block and wires attached to it.
Cleaned battery connections and trimmed back negative cable and reattached, it was fairly rusty. Positive was in good shape.
Hooked up a water line, attached multimeter to battery.
Engine off 12.86v
Idle 14.12v
Idle with blower and pump on would drop slightly to 14.09v
1000-2000 rpm 14.19v
When switched off went very slowly from 13.3v down to 12.86v
Does this mean my battery and alternator are working ok?

FlipFlop your pictures didn't work and I'm not certain how to unscrew the wiring harness to check for corrosion.
TX-Foilhead I can't find the IAC valve and not sure how I would clean it.
Another forum suggested it could be the throttle position sensor.

You guys have been great and I appreciate your help.
Darren

[goroski]

forgot to mention. symptoms still exist. same fluctuations.

[east tx skier]

These aren't identical symptoms to what I had on my 98 Ski, but I had some weird tach issues that were easily resolved.

Get access to the rear of the tach. There is a little switch. It should be set to 8V or V8 (I can't remember what the notation looks like). Rotate the switch all the way back and forth through the other settings two or three times. Then put it back on the original setting.

Apparently, as these things age, they get some deposits and what not in the mechanism, and this clears it out. It did in my case.

[ffmedic74]

Had the same throttle issues on my last boat and ended up being the ECM.

[2001SAN]

Battery and alternator sound ok. You could swap the ECM if you knew someone with the same engine? That would rule it out. Easier said than done though.

Darren (as well).

[Orlandoguy82]

The trick of turning the switch from 8 to 6 cly on the back of the tach fixed my tach jumping issue!


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