'87 Merc 3.0 stutter at ~4000 rpm

[Jns1981]

Boat was running very well all day Sunday, until the last ride to the mooring. I gunned it and it started to stutter approaching WOT. Tach was bouncing before it started to stutter. I throttled back and it stopped... throttle up=same issue. When i finally got it to the mooring i turned it off and when i tried starting it again, no dials jumped- turned the key and nothing happened. Bilge and trim worked... tried the key a few minutes later and it started right up... originally was thinking a fuel issue, but could a faulty ignition switch cause these symptoms? Grounds?

 

[alldodge]

could a faulty ignition switch cause these symptoms? Grounds?

Yes, and since shutting it off, and it wouldn't come back on, more then likely it is electrical

 

[Bondo]

Ayuh,..... Start at the batteries with a test light to see where the power stops,.....

Probably wouldn't hurt to clean all the connections with a wire brush til shiny metal clean while yer lookin',....
I then slather 'em with grease so they can't corrode again,....

 

[Scott Danforth]

when was the last time you serviced the points? set the dwell? and adjusted timing?

tach bouncing and engine stuttering is related.

troubleshooting and setting breaker point ignition covered in the stickies

 

[Jns1981]

where are the stickies?

 

[alldodge]

where are the stickies?

Adults Only section at the top of this section or a link in my sig

 

[Jns1981]

Thanks! Thats a great post! Not sure how that relates to the electrical issue at ignition: turn key=no power to dash or starter but blower/trim etc working properly...could it be two issues at once? Tune up causing engine stutter at WOT, AND a random ignition/electrical/ground issue causing the no power at the dash? could need of a tune up cause a no power issue at the ignition switch?! the jumpy tach happened with the engine running smooth.

 

[alldodge]

Think you have an electrical issue because:

Tach was bouncing before it started to stutter

If your loosing connection the coil starts misfiring.

turned it off and when i tried starting it again, no dials jumped- turned the key and nothing happened.

No power from key

Bilge and trim worked...

They do not run from the key, they are powered separately

tried the key a few minutes later and it started right up

Fires back up after making connection again

So start with the bat cables both ends

Then start checking voltages, might be the key switch breaking connection. Just had to change my ignition key last month, just died

Doing a tune up, you might have made a weak connection worst

 

[Jns1981]

oh wow thank you so much AllDodge!

 

[Jns1981]

update: i cleaned connections at engine and under dash-- ran beautifully for a few days, no fluttering tach, engine ran great-- after a rain, it happened all over again.

 

[alldodge]

You sure your key switch is OK?

 

[Jns1981]

im replacing it tonight/tomorrow. i first wanted to test wiring (cleaning worked at first!) before i tried everything at once you know?
Two additional things:
first-the coil mounting bolt was loose when i checked everything.. meaning the coil bounced around a bit. I tightened it back up. could that have wrecked the coil?

second- read similar symptoms (jumpy tach & rough engine at higher RPM) were caused by the shift interrupter switch. Is there a way to test the switch itself?

will first do wiring checks, then replace ignition switch- then move to other possibilities.

 

[alldodge]

Don't see something happening to the coil

The interrupter can cause the engine to miss, stall or die. Disconnect the ground wire off the switch for testing. You can also disconnect both wires, just don't let them come in contact with each other or the one coming from the distributor touch ground.

When the switch is not connected, you will not be able to shift out of gear in to neutral without shutting the motor off. So only disconnect it a safe place

 

[Jns1981]

Thanks! Will try that if my wire work and new ignition switch doesn’t solve it. Much appreciated!